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Alveolar adenoma: A histochemical, immunohistochemical, and ultrastructural analysis of 17 cases
Alveolar Adenoma: A Histochemical, Immunohistochemical, and Ultrastructural Analysis of 17 Cases LOUISE M. BURKE, MB, BCH, BAO, MRCPATH,WALTER I. RUSH, MD, ANDRAS KHOOR, MD, BRUCE MACKAY, MD, PHD, P. OLIVEIRA, MD, JEFFREY A. WHITSETT, MD, G. SINGH, MD, RONALD TURNICKY, MD, MARIAN V. FLEMING, MD, JD, MICHAEL N. KOSS, MD, AND WILLIAM D. TRAVIS, MD A l v e o l a r a d e n o m a o f l u n g is a rare b e n i g n n e o p l a s m o f u n c e r t a i n h i s t o g e n e s i s . Its rarity hampers characterization of its epithelial a n d
mesenchymal elements. Clinical and histopathologic features of 17 alveolar adenomas were reviewed. Histochemistry w a s p e r f o r m e d o n 10 cases, ultrastrucatral analysis on two, and immunohistochemistry o n s i x c a s e s for pnettmocyte markers, thyroid transcription f a c t o r (TTF-1), s u r f a c t a n t p r o t e i n markers pro-SP-B and pro-SP-C, and the Clara cell marker, CC10. Immunohistochemistry was performed in nine cases for desmin, smooth muscle actin, muscle-specific actin, cytokerathl, proliferating cell nuclear antigen (PCNA), factor VIII, a n d carcinoembryonic antigen. The mean age w a s 53 years. Seven c a s e s o c c u r r e d in men, and nine occurred in women. The a g e a n d s e x were not known for one patient. The tumors were coin lesions on c h e s t r a d i o g r a p h s in asymptomatic patients except for one (cough). The mean size was 2.2 cm. The tumors were well demarcated with multiple cystic spaces containing granular material. Mostly type 2 pneumocytes lined the cystic spaces with fewer type 1 cells a n d n o Clara cells. This was confirmed by staining for TTF-1, pro-SP-B, a n d
Alveolar adenoma is a rare lung parenchymal lesion initially described by Yousem and Hochholzer in 1986.1 It represents one of several types of pulmonary adenomas recognized by the revised World Health Organization/International Association for the Study of Lung Cancer (IASLC) classification. Other pulmonary adenomas are papillary adenoma, mucinous cystadenoma, and salivary gland adenomas such as pleomorphic adenoma and mucous gland adenoma. 2 Alveolar adenoma consists of a solitary, circumscribed, peripheral pulmonary nodule composed of a network of spaces lined by simple cuboidal epithelium with stroma that can range from thin inconspicuous strands of connective tissue to broad accumulations of spindle
From the Department of Pulmonary and Mediastinal Pathology, Armed Forces Institute of Pathology, Washington, DC; the Department of Pathology, University of Maryland, Baltimore, MD; the Department of Pathology, University of South Florida; the Department of Pathology, University of Texas MD Anderson Cancer Center, Houston, TX; Instituto Portugues de Oncologia, Lisbon, Portugal; Childrens Hospital, Cincinnati, OH; the Department of Pathology, Veterans Affairs Medical Center, Pittsburgh, PA; and the Department of Pathology, Walter Reed Army Medical Center, Washington, DC. Accepted for publication September 1, 1998. Address correspondence and reprint requests to William D. Travis, MD, Department of Pulmonary and Mediastinal Pathology, Armed Forces Institute of Pathology, 6825 NW 16th St, Bld 54, Rm M003B, Washington, DC 20306-6000. This is a US government work. There are no restrictions on its use. 0046-8177/99/3002-0008500.00/0
pro-SP-C and by ultrastructure. CC10 was n e g a t i v e in all cases. The stroma varied from prominent spindle cells with a myxoid matrix to thin alveolar septa. The interstitial spindle cells r e s e m b l e d fibroblasts by immunohistochemistry and ultrastructure. Follow-up data available in five c a s e s s h o w e d n o r e c u r r e n c e at 2, 2, 5, 8, and 13 years. In summary, alveolar a d e n o m a is a benign neoplasm consisting of a n intimate admixatre of alveolar epithelial and septal m e s e n c h y m a l tissue. M o s t of the epithelial cells are type 2 pneumocytes, and the interstitial stromal cells are fibroblasts or fibroblast-like cells. Recognition of its characteristic morphological appearance allows for its distinction from other benign lesions of the lung. H u M PATHOL 30:158-167. This is a US government work. T h e r e are n o restrictions o n its use.
Key words: alveolar a d e n o m a , a d e n o m a , surfactant, thyroid transcription factor, type 2 pneumocyte, Clara cell, lung, papillary adenoma, immunohistochemistry, electron microscopy. Abbreviations: TTF-1, thyroid transcription factor-l; CC-10, Clara cell s p e c i f i c p r o t e i n ; AAH, atypical adenomatous hyperplasia.
cells with a myxoid matrix. 2 The epithelial component is generally recognized to consist mostly of type 2 pneumocytes, but the interstitial cells are not well characterized. Only a small number of cases published in the English literature have been studied by immunohistochemical and electron microscopic techniques, 1,3,4 and these studies have focused primarily on characterizing the epithelial rather than the mesenchymal component. Several recent studies published in other languages also have examined these tumors by electron microscopy and immunohistochemistryY Our report of 17 cases includes a review of the current knowledge about the clinical, gross, and histological features of this tumor. Using a combination of histochemical, immunohistochemical, and electron microscopic techniques, we have further characterized both the epithelial and mesenchymal components of the lesion. In particular, we have used a panel of antibodies to pneumocyte markers, including thyroid transcription factor-1 (TTF1), surfactant, and Clara cell proteins, to study the epithelial cells and several mesenchymal markers to investigate the stromal cells.
MATERIALS AND METHODS S e v e n t e e n cases were i n c l u d e d in t h e study. Sixteen were r e t r i e v e d f r o m t h e files o f t h e D e p a r t m e n t of P u l m o n a r y a n d Mediastinal P a t h o l o g y at t h e A r m e d Forces Institute of Pathology (1951 to 1995); case 17 was previously p u b l i s h e d in 1996
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ALVEOLAR ADENOMA OF THE LUNG (Burke et al) and was submitted by Dr E Oliveira of the Department of Pathology, Portuguese Cancer Institute, Lisbon. 4,9 Cases 8, 9, 10, 12, 15, and 16 were included in the study by Yousem and Hochholzer in 1986 (Table 1). 1 Clinical information and follow-up was obtained from the patient records and contributing physicians. In all 17 cases, hematoxylin and eosin-stained sections were assessed. Histochemical and immunohistochemical stains were analyzed in cases for which paraffin-embedded tissue was available.
specificity of all these antibodies have been described previously. 1°-12Pro-SP-B and TTF-1 immunoreactivity was visualized by a biotin-streptavidin detection system (Super Sensitive, Biogenix, San Ramon, CA), whereas localization ofimmunoreactive CC10 was p e r f o r m e d with the peroxidase-antiperoxidase method. All antibodies applied 3,3'-diaminobenzidine as the detection chromogen. Nuclear localization is the standard for TTF-1, whereas cytoplasmic staining is observed with the other three antibodies.
Histochemistry
Electron M i c r o s c o p y
Ten cases were stained with one or more of the following: alcian blue with and without hyaluronidase digestion, periodic acid-Schiff with and without diastase predigestion, mucicarmine, Movat and Masson trichrome stains, elastic van Gieson, and Wilder's reticulin.
Specimens from two cases were fixed in buffered glutaraldehyde and processed for electron microscopy. Thin sections were stained with lead citrate and uranyl acetate, e m b e d d e d in epoxy resin, and examined using a JEOL 1200-EX electron microscope (Tokyo,Japan).
Immunohistochemistry Immunohistochemical analysis was p e r f o r m e d in nine cases using the standard avidin-biotin complex and the peroxidase-antiperoxidase technique. Combinations of antibodies using standard dilutions were used: Keratin (Pharmacia Chemical Co, Piscataway, NJ), Kermix (AE1/AE3 & CK1)(1:2, Boehringer Mannheim, Indianapolis, IN; Dako, Carpenteria, CA), CAM 5.2 (1:100, Becton Dickinson, San Jose, CA), epithelial membrane antigen (1:800, Dako), Ber EP4 (1:100, Dako), carcinoembryonic antigen (1:800, Dako), desmin (1:200, Ventana, Tucson, AZ), smooth muscle actin (SMA, 1:8,000, Sigma, St. Louis, MO), muscle-specific actin (MSA, 1:40, Enzo, Farmingdale, NY), factor VIII-related antigen (FVIIIRA, 1:800, Dako), proliferating cell nuclear antigen (1:1600, Dako), and CD 68 or KP-1 (1:200, Dako). Immunohistochemical analysis for surfactant proteins B (pro-SP-B and pro-SP-C, Clara cell specific protein (CC-10) and thyroid transcription factor-1 (TTF-1) was performed under optimal conditions on six cases. Pro-SP-B (1:1,600) and pro-SP-C (1:100) immunoreactivity was detected using rabbit polyclonal antisera, and TTF-1 (1:250 and 1:1,000) was localized using a monoclonal antibody of clone 8G7G3/1. Dr Jeffrey A. Whitsett, Children's Hospital, Cincinnati, OH, kindly provided these antibodies. The CC10 antibody (1: 1,000) was provided by Dr Gurmukh Singh, Veterans Affairs Medical Center, Pittsburgh, Pennsylvania. Preparation and
RESULTS Clinical Features Clinical details o f t h e 17 cases a r e o u t l i n e d in T a b l e 1. Seven cases o c c u r r e d in m e n , a n d n i n e cases in w o m e n . I n o n e case, t h e age a n d sex o f t h e p a t i e n t w e r e n o t k n o w n . T h e m e a n a g e at p r e s e n t a t i o n was 53 years ( r a n g e , 39 to 74 years). Twelve cases o c c u r r e d in whites a n d f o u r in blacks. O n e p a t i e n t p r e s e n t e d with c h r o n i c i n s i d i o u s n o n p r o d u c t i v e c o u g h , b u t in 13 p a t i e n t s p u l m o n a r y sympt o m s w e r e a b s e n t , a n d a s o l i t a r y n o d u l e , mass, shadow, o r c o i n l e s i o n was d e t e c t e d i n c i d e n t a l l y o n a c h e s t r a d i o g r a p h . I n f o u r cases, t h e p a t i e n t s h a d b e e n foll o w e d - u p f o r 2 to 10 years after i n c i d e n t a l d i s c o v e r y o f p u l m o n a r y n o d u l e s o n c h e s t r a d i o g r a p h s ; two o f t h e s e lesions h a d i n c r e a s e d in size d u r i n g t h e follow-up interval. T h e t u m o r a r o s e in t h e left l o w e r l o b e in h a l f (8 o f 16) o f t h e cases. Seven t u m o r s w e r e l o c a t e d in t h e r i g h t lung; t h r e e o c c u r r e d in t h e u p p e r lobe, two in t h e l o w e r l o b e , a n d o n e in t h e m i d d l e lobe. T h e e x a c t site w i t h i n t h e r i g h t l u n g was n o t s p e c i f i e d in t h e r e m a i n i n g case.
TABLE 1. Clinical Features of Alveolar Adenomas Case No.
Patient's Age/Race/Sex
Presenting Symptoms
Radiographic Findings
Location
Size (cm) Maximum Diameter
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
45/C/M 58/AA/M 50/AA/M 39/AA/M 41/C/F 52/C/F 41/C/F 54/C/F 64/C/M 58/C/M 45/C/F 68/AA/M 46/C/F N/A 74/C/F 59/C/A 55/C/F
N/A Asymptomadc Asymptomatic Asymptomatic Asymptomadc hx-melanoma Asymptomatic Asymptomatic Asymptomadc N/A Asymptomatic hx-acromegaly Asymptomadc N/A Asymptomatic I-Ix-uterine leiomyoma Lack of appetite Weakness, Rash Asymptomatic Chronic cough
N/A Solitary nodule Solitary lesion Solitary nodule Solitary nodule Solitary lesion Solitary lesion Hilar mass Solitary nodule Solitary nodule Coin lesion Coin lesion Round shadow Shadow Solitary nodule Solitary nodule Solitary mass
LUL LLL LLL RLL LLL LLL LLL RUL RUL LLL LLL LLL N/A Right lung RML RUL RLL
1.5 1.9 N/A 2 1.l 3 2.5 2.5 1.2 1.5 2 1.8 N/A 3 2.5 1.3 6
Abbreviations: M, male; F, female; C, Caucasian; AA, African-American; N/A, not available; LUL, Left upper lobe; LLL, Left lower lobe; RLL, Right lower lobe; RUL, Right upper lobe; Max = maximum; Hx = history of.
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One t u m o r was located in the left u p p e r lobe, and the site was not specified in one case. Absence of hilar or mediastinal lymphadenopathy and lack of a history of smoking was m e n t i o n e d in three cases each; however, these issues were not addressed in all cases. In five cases, bronchoscopy, transbronchial biopsy (n = 3), fine-needle aspiration biopsy (n = 1), bronchial washings/brushings (n = 2), and cytology cell blocks and smears (n = 3) were r e p o r t e d as benign or nondiagnostic. All tumors were surgically excised. Lobectomy was p e r f o r m e d in two cases, wedge resection, biopsy, or excision in eight cases, and in the remaining cases tile p r o c e d u r e was not specified. T h e contributors' diagnoses included congenital cyst, acute and chronic inflammatory benign reactive lesion, hamartoma, intralobar sequestration, lymphangioma, atypical mesenchymal lesion, and alveolar adenoma. In one case, a frozen section diagnosis of adenocarcinoma was made; however, this was a m e n d e d to a diagnosis of alveolar a d e n o m a after review of p e r m a n e n t sections f r o m formalin-fixed, paraffine m b e d d e d material. Gross Findings
The m e a n t u m o r size was 2.2 cm (range, 1.1 to 6 cm). T h e tumors were well demarcated, circumscribed, and on occasion shelled out from the surrounding lung parenchyma. Several tumors appeared polypoid or loosely a d h e r e n t to a cyst wall. Most of the lesions lay in a subpleural location, but one was e m b e d d e d within the hilar structures. In one patient, the t u m o r caused external compression of a basal medial bronchus, resulting in p u l m o n a r y symptoms. No direct communication with a bronchus or association with major vessels was noted. T h e cut surface was smooth, lobulated, multicystic, glistening, and partly mucoid or fish-flesh in appearance. T h e texture ranged from being soft, gelatinous, or m u c o i d in consistency to firm, rubbery, elastic, or spongy. T h e color was pale yellow, gray, white, or tan, and focal areas of h e m o r r h a g e and necrosis were described.
FIGURE 1. Low power shows a circumscribed nodule with several large cysts and multiple small cysts. A thin layer of connective tissue is seen focally around the edge of the tumor. (Hematoxylin and eosin, original magnification x9.5.)
large to small. Large cysts were readily visible on gross examination and on low-power histological examination, and smaller cysts resembled alveolar spaces (Figs 3 and 4). Papillary formations were not seen. In four cases, there was a tendency for the large cystic spaces to be located centrally rather than peripherally; however, in other tumors, the large cysts were seen peripherally. In 12 of the 16 cases, eosinophilic proteinaceous granular material was identified in the cystic spaces within the tumors. This material was not seen in the adjacent lung parenchyma. In nine of these tumors, complete lamellar bodies or fragments of lamellar material were seen. In 3 of the 12 cases, the eosinophilic material had a globular appearance. Macrophages were seen within the cystic spaces in 14 cases. They had an eosinophilic, foamy, vacuolated cytoplasm and occasionally f o r m e d multinucleated giant cells. Intracytoplasmic inclusions of cholesterol esters were occasionally noted. A single layer of epithelial cells lined the cystic spaces (Figs 4 and 5). Most of the epithelial cells had the appearance of hyperplastic type 2 pneumocytes with a cuboidal or hobnailed appearance and eosinophilic, finely vacuolated or foamy cytoplasm. Nuclear pseudoinclusions were occasionally seen (Fig 5). A few flattened or squamoid type 1 pneumocytes lined cysts, surr o u n d e d by a thin wall resembling that of a normal
Histological Features
In all cases, low-power magnification showed a well-demarcated, multicystic lesion easily distinguished from the surrounding lung parenchyma (Fig 1). Alt h o u g h a complete fibrous capsule or pseudocapsule was not present, focal condensation of fibrous connective tissue was noticed at the periphery, particularly in areas where the lesion abutted bronchioles, blood vessels, or lymph nodes. Although low-power views suggested a sharp demarcation from the surrounding lung parenchyma, closer examination in 12 of 13 cases with adjacent lung parenchyma in the section showed a focal gradual transition between alveolar wails of the alveolar a d e n o m a and those of the surrounding normal lung parenchyma (Fig 2). All tumors were multicystic, with cysts varying from
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there a medium-sized p u l m o n a r y vessel within the lesion; however, this vessel was probably entrapped, because the lesion was e m b e d d e d within the hilar structures. Bronchioles were not identified within the lesion, n o r were cartilage, adipose tissue, or smooth muscle. No cellular dysplasia, necrosis, invasion of adjacent parenchyma, or vascular invasion was seen. The adjacent lung tissue showed a variety of nonspecific features, including focal atelectasis, intra-alveolar fresh hemorrhage, or compression of lung parenchyma. Occasional lymphoid aggregates also were noted, and, in one case, there were histological features in the adjacent lung highly suggestive of hypersensitivity pneumonitis. One lesion was associated with an incidental 0.4-cm focus of well-differentiated adenocarcinoma. Morphologically and cytologically, however, the two lesions were distinct and separate. Histochemistry
Histochemical analysis by periodic acid-Schiff and periodic acid-Schiffwith diastase predigestion and mucicarmine stains in seven cases confirmed the absence of cytoplasmic mucin in epithelial cells. The interstitial matrix in five cases displayed more p r o m i n e n t positivity with the alcian blue stain c o m p a r e d with the adjacent lung parenchyma, highlighting the presence of extracelFIGURE 2. At the edge of this tumor, some of the alveolar walls of the surrounding lung parenchyma merge imperceptibly with those of the alveolar adenoma (right). The alveolar adenoma consists of a nodule of tissue closely resembling alveolar lung parenchyma. (Hematoxylin and eosin, original magnification ×75,)
alveolus. Neither ciliated cells n o r cells with Clara cell morphology were seen. The interstitial c o m p o n e n t varied in appearance from a thin connective tissue layer resembling normal alveolar septa (Fig 4) to markedly thickened alveolar walls with myxoid connective tissue similar to that of a h a m a r t o m a or mesenchymal lesion (Fig 6). In two cases, the interstitial matrix was so p r o m i n e n t that the alveolar architecture was lost, and the air spaces were compressed in a slit-like fashion (Fig 6). O t h e r areas displayed hypercellular thickened septa (Fig 7) with p r o m i n e n t spindle-/oval-shaped cells (Fig 8). These cells had a fine to o p e n nuclear chromatin and mostly inconspicuous nucleoli. Often small processes emanated from their cytoplasm. In some areas, the nuclei were "cigar shaped," resembling those of smooth muscle cells. Inflammatory cells were mostly inconspicuous. However, mild to moderate numbers of interstitial lymphocytes, plasma cells, and eosinophils were seen, sometimes in clusters. Mast cells were frequent. Interstitial macrophages, often with vacuolated cytoplasm, also were present, occasionally forming multinucleated giant cells containing cholesterol clefts. Foci of recent and old h e m o r r h a g e with hemosiderin deposition were frequent. A well-developed fine capillary network traversed the interstitial mesenchyme. In only one case was
FIGURE 3. The alveolus-like spaces of this tumor are filled with a finely granular proteinaceous material and lined by thin walls. At this magnification, it is difficult to discern an epithelial lining. At the edge of this tumor (upper right corner), there is a condensed layer of epithelial cells. (Hematoxylin and eosin, original magnification x400.) 161
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stained macrophages within the interstitium. The interstitial stromal cells did not stain for desmin in six cases. Focal positivity for smooth muscle actin was observed in stromal cells in one of seven cases, and focal positivity for muscle-specific actin in one of six cases. The proliferative nature of the lesion in both the epithelial and interstitial c o m p o n e n t s was shown in four cases by the more intense and widespread staining of t u m o r nuclei by the proliferating cell nuclear antigen as c o m p a r e d with the adjacent normal lung parenchyma. Positive cytoplasmic staining for Pro-SP-B (Fig 9) and Pro-SP-C was observed in most of the epithelial cells in all six cases analyzed, confirming the presence of type 2 pneumocytes (Table 3). T h e type 2 pneumocytes also showed positive nuclear staining with the TTF-1 antibody in five of six cases (Fig 10, Table 3). In three of these cases there was cytoplasmic staining, the reason for which is unclear. T h e flatter, more squamoid, type 1 cells were negative with these pneumocyte markers (Pro-SP-B, Pro-SP-C, and TTF-1). Clara cells were not identified, because the CC10 immunostain was consistently negative. T h e interstitial cells were negative for all four pneumocyte markers. T h e cyst contents showed focal positivity for Pro-SP-B, as did some of the macrophages within the cystic alveolar spaces. Electron M i c r o s c o p y
Electron microscopy confirmed that most of the epithelial cells were type 2 pneumocytes containing
FIGURE 4. Many of the alveolus-like spaces of this tumor are lined by hyperplastic, cuboidal type 2 pneumocytes. The connective tissue within the walls of these spaces is very thin, similar to that seen in normal alveoli, (Hematoxylin and eosin, original magnification x400.)
lular acid mucopolysaccharides. In the one case analyzed, this material was sensitive to predigestion with hyaluronidase. Glycogen also was n o t e d in the interstitium, and, in many areas, it appeared predominantly intracellular in location. T h e interstitium also contained collagen fibrils, as highlighted by the Movat/Masson trichrome stain in the six cases examined, and in one case a reticulin stain showed reticulin fibers. In four cases examined with the van Gieson stain, there was an absence of or a reduction in the n u m b e r of elastic fibrils in the t u m o r as compared with the normal lung parenchyma, and any elastic fiber fragments present lay at the p e r i p h e r y of the lesion. Immunohistochemistry
The results of the immunohistochemical assays of the nine cases available for assessment are tabulated in Table 2. T h e epithelial phenotype of the cells lining the cystic spaces was confirmed with positive staining for at least one cytokeratin antibody in eight of nine cases. Negative staining for factor VIII was seen in two of these cases. Positive staining for carcinoembryonic antigen was observed in four of four interpretable, keratinpositive cases. T h e interstitial cellular c o m p o n e n t was consistently negative for cytokeratin. Staining for factor VIII highlighted the vascular network. In one case, CD68
FIGURE 5. Virtually all of these pneumocytes lining the wall of this alveolar space are hyperplastic type 2 cells with cuboidal morphology. A cytoplasmic nuclear inclusion is present (arrow). (Hematoxylin and eosin, original magnification x 1,000.)
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interstitial component is very prominent, exceeding that seen in normal alveolar parenchyma. Our ultrastructural and immunohistochemical studies showed that most of the epithelial cells lining the cystic spaces of alveolar adenomas are type 2 pneumocytes, with a minority of type 1 cells and no Clara cells. Positive staining of type 2 cells for TTF-1, pro-SP-B, and pro-SP-C, and negative staining for CC-10 supported this impression. The capability of the immunohistochemical stains to surfactant proteins, TTF-1, and CC-10 to differentiate cell types is supported by specificity studies previously performed on these reagents. 1°-12 TTF-1 is a homeodomain-containing transcription factor that plays critical roles in lung-specific expression of surfactant proteins and CC-10. It is expressed selectively in thyroid, diencephalon, and lung tissue. 13In the lung, nuclear localization of TTF-1 is confined to alveolar type 2 pneumocytes and nonciliated bronchiolar and bronchioloalveolar portal epithelial cells. ~4 Immunoreactivity for TTF-1 together with a surfactant precursor protein pro-SP-B also has been shown in adenocarcinomas of the lung. 14,15 Pro-SP-B and Pro-SP-C are the precursor proteins for surfactant proteins B and C, respectively. Pro-SP-B cytoplasmic localization has been detected in alveolar Type 2 cells 15'16 and in nonciliated bronchial and bronchiolar epithelial cells, s° Immunoreactivity for proFIGURE 6. The interstitial stroma is very prominent in this area, consisting of myxoid connective tissue. The alveolar spaces are narrow, slit-like, and lined by cuboidal pneumocytes. (Hematoxylin and eosin, original magnification ×200.)
varying numbers of lamellar bodies in their apical cytoplasm (Fig 11). Interspersed among these type 2 cells, small numbers of cells with markedly attenuated cytoplasm characteristic of type 1 pneumocytes were identified (Fig 12). The epithelial cells typically formed a single layer encircling a central space that was often small with an irregular shape, though many were large and round. Clara cells were not seen. The interstitium contained fibroblasts, fibroblastlike ceils, macrophages, and a few mast cells and lymphocytes (Fig 13) embedded in a moderately collagenous stroma. Most of the stromal vessels were capillaries.
DISCUSSION Alveolar a d e n o m a is a rare benign neoplasm with distinctive gross and microscopic findings. The radiographic presentation as a solitary lung nodule is nonspecific; therefore, correct diagnosis requires accurate recognition of the characteristic histological features. Our study of 17 cases not only reviews the distinctive pathological features of the neoplasm but also provides additional information regarding the histogenesis of this lesion. Our findings support the use of the term alveolar adenoma for this lesion, because we have shown an intimate association of both native alveolar epithelial and interstitial elements, although in some cases the
FIGURE 7. Thistumor consists of large cystic spaces with thick walls composed of a cellular spindle cell and connective tissue stroma. There are hyperplastic cuboidal epithelial cells lining the walls of the cysts. (Hematoxylin and eosin, original magnification × 75.)
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FIGURE 9. The cytoplasm of the hyperplastic type II pneumocytes lining the walls of the alveolus-like spaces is stained strongly with the antibody to surfactant pro-SPB. (Immunohistochemistry for surfactant pro-SPB, original magnification x300.)
FIGURE 8. This stroma consists of prominent spindle-shaped cells embedded in a collagenous stroma. Hyperplastic cuboidal type 2 pneumocytes line the surface of this cyst wall. (Hematoxylin and eosin, original magnification x300.) SP-C a p p e a r s l i m i t e d to alveolar type 2 cells p r e d o m i nantly, with m u c h less f r e q u e n t s t a i n i n g o f the n o n c i l i a t e d b r o n c h i o l o a l v e o l a r p o r t a l cells t h a n t h a t s e e n with pro-SP-B. 1° T h e a n t i b o d y CC-10 detects the Clara c e l l specific 10-kD p r o t e i n , a n a b u n d a n t s e c r e t o r y p r o d u c t o f n o n c i l i a t e d b r o n c h i o l a r epithelial (Clara) cells. T h e r e fore, this a n t i b o d y stains Clara cells b u t n o t type 2 p n e u m o c y t e s . 17 O u r i m m u n o h i s t o c h e m i c a l a n d u l t r a s t r u c t u r a l findings also i n d i c a t e d t h a t the s p i n d l e cells i n the interstit i u m were fibroblasts o r fibroblast-like cells, s u p p o r t e d
TABLE 2.
Immunohistochemical Results of Cases
Epithelial
Interstitium
Both
Case Keratin CEA FactorVlll Desmin Actin SM Actin MS PCNA 1 2 3 6 10 11 12
++ ++ ++ ++ ++ ++ -
NE N/I NE + + NE NE
16
+ +
+
17
++
+
NE NE . NE NE NE NE NE
.
NE . NE NE -
+ -
+ NE
NE NE -
NE NE -
.
++
by the p r e s e n c e o f c o l l a g e n fibrils by e l e c t r o n microscopy. A b s e n c e o f d e s m i n s t a i n i n g essentially e x c l u d e d m u s c l e d e r i v a t i o n for these cells. T h e p r o m i n e n t m a s t cells are a f e a t u r e n o t previously e m p h a s i z e d i n these lesions. A l t h o u g h alveolar a d e n o m a s have traditionally b e e n r e g a r d e d as n e o p l a s t i c lesions, 1 it is n o t clear w h e t h e r b o t h the e p i t h e l i a l a n d m e s e n c h y m a l c o m p o n e n t s are neoplastic. S o m e believe t h a t the p r o l i f e r a t i o n is pred o m i n a n t l y m e s e n c h y m a l with alveolar e n t r a p m e n t , b u t it is possible t h a t the e p i t h e l i a l c o m p o n e n t is also neoplastic. R o q u e et al 9 o b s e r v e d a c l o n a l cytogenetic t r a n s l o c a t i o n o f d e r ( 1 6 ) t ( 1 0 ; 1 6 ) ( q 2 3 : 2 4 ) i n 19% o f cells i n these lesions, 9 s u g g e s t i n g c l o n a l e x p a n s i o n a n d n e o p l a s i a i n these lesions. U l t i m a t e l y the issue o f cellular n e o p l a s i a n e e d s to b e a d d r e s s e d by clonality studies i n b o t h e p i t h e l i a l a n d m e s e n c h y m a l cells. O u r
TABLE 3. Surfactant, Clara Cell, a n d 1-rF-1 Immunostain Results on the Lining Epithelial Cells
++ ++ ++
Abbreviations: - , Negative; +, Focal positivity; ++, Strong positivity;NE, Not evaluated; NI, Not interpretable; Actin SM, actin, smooth muscle; Actin, MS, Actin, muscle specific; PCNA, proliferating nuclear cell antigen.
Case No.
TTF-1
CC10
PRO-SP-B
PRO-SP-C
1 6
Positive Positive
Negative Negative Negative Negative Negative Negative
Positive Positive Positive Positive Positive Positive
Positive Positive Positive Positive Positive Positive
10
N/A
11 16 17
Positive Positive Positive
N/A = Not assessed.
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finding of increased proliferating cell nuclear antigen activity in both epithelial and interstitial cells suggests that both components are proliferating. The differential diagnoses to be considered include papillary adenoma, sclerosing hemangioma, atypical adenomatous hyperplasia (AAH), bronchioloalveolar carcinoma, l y m p h a n g i o m a , a n d h a m a r t o m a . Congenital cystic adenomatoid malformation and honeycomb fibrosis are also considerations. Papillary adenoma is similar to alveolar adenoma in that it presents as a peripheral, circumscribed nodule that radiologically can present as a solitary coin lesion in an otherwise asymptomatic patient, ls-~4 The tumors are also circumscribed but differ from alveolar adenomas by showing a prominent papillary pattern with a heterogeneous epithelial population including type 2 cells, Clara cells, and, sometime, ciliated cells. Sclerosing hemangiomas differ from alveolar adenomas in that they show a mixture of solid, vascular, sclerotic, and papillary patterns. The solid epithelioid cells, which appear to represent the neoplastic cells, usually stain for epithelial membrane antigen. 2~,26 Recently, Niho et a127 showed the X-chromosome-linked polymorphic markers of the h u m a n androgen receptor gene and the phosphoglycerate kinase gene were amplified in five of six sclerosing hemangiomas, indicating a clonal o r i g i n s The female predominance of sclerosing
FIGURE I1. Electron microscopy shows type 2 pneumocytes (arrow) lining the walls of the alveolus-like spaces and many extruded lamellar bodies (arrowhead), Type 1 pneumocytes are also present. (Original magnification x2,500.)
hemangioma is stronger than the slight female predilection we observed for alveolar adenomas. Alveolar adenoma should not be confused with the entity known as AAH, which represents an atypical bronchioloalveolar proliferation that falls short of criteria for bronchioloalveolar carcinoma. These lesions are usually incidental findings in lung cancer resection specimens and are thought to be precursor lesions to a d e n o c a r c i n o m a . 28-3°Miller 3° used the term bronchioloalveolar adenoma for these lesions, 3° but the preferred term is AAH, because use of the term adenomashould be restricted to the group of alveolar adenoma, papillary adenoma, mucinous cystadenoma, and salivary gland adenomas. 2 These lesions are benign and are not thought to be preneoplastic. Indeed, the lack of cellular atypia, necrosis, and invasion in alveolar adenomas differentiates them from a malignant process, and the lack of deaths, metastases, or recurrences in these patients attests to its benignity. For many years, alveolar adenomas were thought to be lymphangiomas because of their multicystic appearance, the thin-walled nature of many of the cysts, and the sometimes flat lining cells resembling endothelial cells. 3a,32The presence, however, of keratin-positive and factor VIII-negative cells lining the cystic spaces as well as the ultrastructure confirms its epithelial rather than vascular origin. A hamartoma also may be a consideration, espe-
FIGURE 10. The nuclei of the hyperplastic type 2 pneumocytes lining the alveolus-like spaces are stained strongly with an antibody to thyroid transcription factor (TTF-1). (Immunohistochemistry for surfactant pro-SPB, original magnification x300.)
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cially when an extensive mesenchymal proliferation is very prominent, as was seen in two of our cases. 25,s3 However, the alveolar appearance in addition to the lack of an intimate association of bronchiolar, adipose, and c h o n d r o i d tissue would militate against a diagnosis of hamartoma. Congenital cystic a d e n o m a t o i d malformation differs from alveolar a d e n o m a in that it is rarely seen in adults. 25 Grossly, it also tends to be larger than alveolar adenomas, and histologically the lung appears immature and malformed, often with a bronchiolar and papillary epithelial proliferation. Interstitial fibrosis with h o n e y c o m b is very different from an alveolar a d e n o m a in that it is not grossly circumscribed, and the cystic spaces are mostly lined by a metaplastic bronchiolar epithelium. Alveolar a d e n o m a is a difficult lesion to diagnose on small biopsy specimens and on frozen section, because the tissue may look exactly like normal lung parenchyma or could simulate a malignancy with small glandular spaces lined by regular glandular epithelium. This difficulty is reflected in the nondiagnostic nature of the small biopsy or cytology specimens in five of our cases. Given the morphological resemblance of alveolar a d e n o m a to normal lung, it is understandable how a small tissue sample could look like unremarkable lung parenchyma. Awareness of this possibility may be of particular importance in those patients who have a history of malignancy elsewhere, and where metastatic FIGURE 13. Electron microscopy shows fibroblasts, fibroblastlike cells, and mast cells within the interstitium. (Original magnification x2,500,)
t u m o r is the prime consideration. Examination of the resected specimen should indicate the correct diagnosis. In conclusion, alveolar a d e n o m a is a rare, benign neoplasm that usually presents in asymptomatic patients as a coin lesion on chest radiograph and has a characteristic multicystic histological appearance that often resembles normal alveolar lung tissue. Immunohistochemically and ultrastructurally, our study indicates that most of the epithelial cells are type 2 pneumocytes and that the interstitial stromal cells are fibroblasts or fibroblast-like cells. Our follow-up data also confirm that this t u m o r is benign.
REFERENCES
FIGURE 12.
1. Yousem SA, Hochholzer L: Alveolar adenoma. HUM PATHOL 17:1066-1071, 1986 2. Travis WD, Colby TV, Corrin B, et al: Histological Typing of Lung and Pleural Tumors (ed 3). Berlin, Springer-Verlag, 1999 3. BohmJ, Fellbaum C, Bautz W, et al: Pulmonary nodule caused by an alveolar adenoma of the lung. Virchows Arch 430:181-184, 1997 4. Oliveira P, Moura NunesJF, Clode AL, et al: Alveolar adenoma of the lung: Further characterization of this uncommon turnout. Virchows Arch 429:101-108, 1996 5. Haskovcova I, Povysil C, Pafko P: Alveolar adenoma of the lung (case report). Cesk Patol 33:49-52, 1997 6. Nussle K, Kolokythas O, Karhausen J, et al: Alveolar cell adenoma of the lung. Rontgenpraxis 50:29-30, 1997
Electron microscopy shows a fiat type I pneumo-
cyte lining the wall of this alveolus-like space. (Original magnification x2,500.)
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ALVEOLAR ADENOMA OF THE LUNG (Burke et al) 7. Koppl H, Freudenberg N, Berwanger I, et al: Alveolar adenoma of the lung: Immunohistochemical characterization of type II pneumocytes. Pathologe 17:150-153, 1996 8. Siebenmann RE, Odermatt B, Hegglin J, et al: Alveolar cell adenoma: A newly defined benign lung tumor. Pathologe 11:48-54, 1990 9. Roque L, Oliveira P, Martins C, et al: A nonbalanced translocation (10;16) demonstrated by FISH analysis in a case of alveolar adenoma of the lung. Cancer Genet Cytogenet 89:34-37, 1996 10. Khoor A, Stahlman MT, Gray ME, et ah Temporal-spatial distribution of SP-B and SP-C proteins and mP,aNAs in developing respiratory epithelium of human lung.J Histochem Cytochem 42:11871199, 1994 11. Singh G, Singh J, Katyal SL, et al: Identification, cellular localization, isolation, and characterization of human Clara cellspecific 10 KD protein. J Histochem Cytochem 36:73-80, 1988 12. Holzinger A, Dingle S, Bejarano PA, et ah Monoclonal antibody to thyroid transcription factor-l: Production, characterization, and usefulness in tumor diagnosis. Hybridoma 15:49-53, 1996 13. Lazzaro D, Price M, de Felice M, et al: The transcription factor TTF-1 is expressed at the onset of thyroid and lung morphogenesis and in restricted regions of the foetal brain. Development 113:1093-1104, 1991 14. Stahlman MT, Gray ME, Whitsett JA: Expression of thyroid transcription factor-1 (TTF-I) in fetal and neonatal human lung. J Histochem Cytochem 44:673-678, 1996 15. Khoor A, Whitsett JA, Stahlman MT, et al: Expression of surfactant protein B precursor and surfactant protein B mRNA in adenocarcinoma of the lung. Mod Pathol 10:62-67, 1997 16. Bejarano PA, Baughman RP, Biddinger PW, et ah Surfactant proteins and thyroid transcription factor-1 in pulmonary and breast carcinomas. Mod Pathol 9:445-452, 1996 17. Khoor A, Gray ME, Singh G, et al: Ontogeuy of Clara cell-specific protein and its mRNA: Their association with neuroepithelial bodies in human fetal lung and in bronchopulmonary dysplasia.J Histochem Cytochem 44:1429-1438, 1996 18. Mori M, Chiba R, Tezuka E et ah Papillary adenoma of type II pneumocytes might have malignant potential. Virchows Arch 428:195200, 1996 19. Sanchez-JimenezJ, Ballester-Martinez A, Lodo-Besse J, et al:
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Papillary adenoma of type 2 pneumocytes. Pediatr Pulmonol 17:396400, 1994 20. Fukuda T, Ohnishi Y, Kanai I, et al: Papillary adenoma of the lung: Histological and ultrastructural findings in two cases. Acta PatholJpn 42:56-61, 1992 21. Hegg CA, Flint A, Singh G: Papillary adenoma of the lung. AmJ Clin Patho197:393-397, 1992 22. Xu Y: Papillary adenoma of the lung: Two cases report. Chung Hua Chieh Ho Ho Hu Hsi Tsa Chih 15:296-297, 320, 1992 23. Noguchi M, Kodama T, Shimosato 11, et al: Papillary adenoma of type 2 pneumocytes. AmJ Surg Patho110:134-139, 1986 24. Fantone JC, Geisinger KR, Appelman HD: Papillary adenoma of the lung with lamellar and electron dense granules: An ultrastructural study. Cancer 50:2839-2844, 1982 25. Colby TV, Koss MN, Travis WD: Tumors of the lower respiratory tract. Armed Forces Institute of Pathology Fascicle, Third Series. Washington, DC, Armed Forces Institute of Pathology, 1995 26. Yousem SA, Wick MR, Singh G, et al: So-called sclerosing hemangiomas of lung: An immunohistochemical study supporting a respiratory epithelial origin [published erratum appears in AmJ Surg Patho11989 Apr;13(4):337]. AmJ Surg Pathol 12:582-590, 1988 27. Niho S, Suzuki K, Yokose T, et ah Monoclonality of both pale cells and cuboidal cells of sclerosing hemangioma of the lung. Am J Patho1152:1065-1069, 1998 28. Kitamura H, KamedaY, Nakamura N, et al: Atypical adenomatous hyperplasia and bronchoalveolar lung carcinoma: Analysis by morphometry and the expressions of p53 and carcinoembryonic antigen. AmJ Surg Patho120:553-562, 1996 29. Mori M, Tezuka F, Chiba R, et ah Atypical adenomatous hyperplasia and adenocarcinoma of the human lung: Their heterology in form and analogy in immunohistochemical characteristics. Cancer 77:665-674, 1996 30. Miller RR: Bronchioloalveolar cell adenomas. Am J Surg Pathol 14:904-912, 1990 31. Wada A, Tateishi R, Terazawa T, et al: Case report: Lymphangioma of the lung. Arch Pathol 98:211-213, 1974 32. A1-Hilli F: Lymphangioma (or alveolar adenoma?) of the lung. Histopathology 11:979-980, 1987 33. Gjevre JA, Myers JL, Prakash UB: Pulmonary hamartomas. Mayo Clin Proc 71:14-20, 1996
mesenchymal elements. Clinical and histopathologic features of 17 alveolar adenomas were reviewed. Histochemistry w a s p e r f o r m e d o n 10 cases, ultrastrucatral analysis on two, and immunohistochemistry o n s i x c a s e s for pnettmocyte markers, thyroid transcription f a c t o r (TTF-1), s u r f a c t a n t p r o t e i n markers pro-SP-B and pro-SP-C, and the Clara cell marker, CC10. Immunohistochemistry was performed in nine cases for desmin, smooth muscle actin, muscle-specific actin, cytokerathl, proliferating cell nuclear antigen (PCNA), factor VIII, a n d carcinoembryonic antigen. The mean age w a s 53 years. Seven c a s e s o c c u r r e d in men, and nine occurred in women. The a g e a n d s e x were not known for one patient. The tumors were coin lesions on c h e s t r a d i o g r a p h s in asymptomatic patients except for one (cough). The mean size was 2.2 cm. The tumors were well demarcated with multiple cystic spaces containing granular material. Mostly type 2 pneumocytes lined the cystic spaces with fewer type 1 cells a n d n o Clara cells. This was confirmed by staining for TTF-1, pro-SP-B, a n d
Alveolar adenoma is a rare lung parenchymal lesion initially described by Yousem and Hochholzer in 1986.1 It represents one of several types of pulmonary adenomas recognized by the revised World Health Organization/International Association for the Study of Lung Cancer (IASLC) classification. Other pulmonary adenomas are papillary adenoma, mucinous cystadenoma, and salivary gland adenomas such as pleomorphic adenoma and mucous gland adenoma. 2 Alveolar adenoma consists of a solitary, circumscribed, peripheral pulmonary nodule composed of a network of spaces lined by simple cuboidal epithelium with stroma that can range from thin inconspicuous strands of connective tissue to broad accumulations of spindle
From the Department of Pulmonary and Mediastinal Pathology, Armed Forces Institute of Pathology, Washington, DC; the Department of Pathology, University of Maryland, Baltimore, MD; the Department of Pathology, University of South Florida; the Department of Pathology, University of Texas MD Anderson Cancer Center, Houston, TX; Instituto Portugues de Oncologia, Lisbon, Portugal; Childrens Hospital, Cincinnati, OH; the Department of Pathology, Veterans Affairs Medical Center, Pittsburgh, PA; and the Department of Pathology, Walter Reed Army Medical Center, Washington, DC. Accepted for publication September 1, 1998. Address correspondence and reprint requests to William D. Travis, MD, Department of Pulmonary and Mediastinal Pathology, Armed Forces Institute of Pathology, 6825 NW 16th St, Bld 54, Rm M003B, Washington, DC 20306-6000. This is a US government work. There are no restrictions on its use. 0046-8177/99/3002-0008500.00/0
pro-SP-C and by ultrastructure. CC10 was n e g a t i v e in all cases. The stroma varied from prominent spindle cells with a myxoid matrix to thin alveolar septa. The interstitial spindle cells r e s e m b l e d fibroblasts by immunohistochemistry and ultrastructure. Follow-up data available in five c a s e s s h o w e d n o r e c u r r e n c e at 2, 2, 5, 8, and 13 years. In summary, alveolar a d e n o m a is a benign neoplasm consisting of a n intimate admixatre of alveolar epithelial and septal m e s e n c h y m a l tissue. M o s t of the epithelial cells are type 2 pneumocytes, and the interstitial stromal cells are fibroblasts or fibroblast-like cells. Recognition of its characteristic morphological appearance allows for its distinction from other benign lesions of the lung. H u M PATHOL 30:158-167. This is a US government work. T h e r e are n o restrictions o n its use.
Key words: alveolar a d e n o m a , a d e n o m a , surfactant, thyroid transcription factor, type 2 pneumocyte, Clara cell, lung, papillary adenoma, immunohistochemistry, electron microscopy. Abbreviations: TTF-1, thyroid transcription factor-l; CC-10, Clara cell s p e c i f i c p r o t e i n ; AAH, atypical adenomatous hyperplasia.
cells with a myxoid matrix. 2 The epithelial component is generally recognized to consist mostly of type 2 pneumocytes, but the interstitial cells are not well characterized. Only a small number of cases published in the English literature have been studied by immunohistochemical and electron microscopic techniques, 1,3,4 and these studies have focused primarily on characterizing the epithelial rather than the mesenchymal component. Several recent studies published in other languages also have examined these tumors by electron microscopy and immunohistochemistryY Our report of 17 cases includes a review of the current knowledge about the clinical, gross, and histological features of this tumor. Using a combination of histochemical, immunohistochemical, and electron microscopic techniques, we have further characterized both the epithelial and mesenchymal components of the lesion. In particular, we have used a panel of antibodies to pneumocyte markers, including thyroid transcription factor-1 (TTF1), surfactant, and Clara cell proteins, to study the epithelial cells and several mesenchymal markers to investigate the stromal cells.
MATERIALS AND METHODS S e v e n t e e n cases were i n c l u d e d in t h e study. Sixteen were r e t r i e v e d f r o m t h e files o f t h e D e p a r t m e n t of P u l m o n a r y a n d Mediastinal P a t h o l o g y at t h e A r m e d Forces Institute of Pathology (1951 to 1995); case 17 was previously p u b l i s h e d in 1996
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ALVEOLAR ADENOMA OF THE LUNG (Burke et al) and was submitted by Dr E Oliveira of the Department of Pathology, Portuguese Cancer Institute, Lisbon. 4,9 Cases 8, 9, 10, 12, 15, and 16 were included in the study by Yousem and Hochholzer in 1986 (Table 1). 1 Clinical information and follow-up was obtained from the patient records and contributing physicians. In all 17 cases, hematoxylin and eosin-stained sections were assessed. Histochemical and immunohistochemical stains were analyzed in cases for which paraffin-embedded tissue was available.
specificity of all these antibodies have been described previously. 1°-12Pro-SP-B and TTF-1 immunoreactivity was visualized by a biotin-streptavidin detection system (Super Sensitive, Biogenix, San Ramon, CA), whereas localization ofimmunoreactive CC10 was p e r f o r m e d with the peroxidase-antiperoxidase method. All antibodies applied 3,3'-diaminobenzidine as the detection chromogen. Nuclear localization is the standard for TTF-1, whereas cytoplasmic staining is observed with the other three antibodies.
Histochemistry
Electron M i c r o s c o p y
Ten cases were stained with one or more of the following: alcian blue with and without hyaluronidase digestion, periodic acid-Schiff with and without diastase predigestion, mucicarmine, Movat and Masson trichrome stains, elastic van Gieson, and Wilder's reticulin.
Specimens from two cases were fixed in buffered glutaraldehyde and processed for electron microscopy. Thin sections were stained with lead citrate and uranyl acetate, e m b e d d e d in epoxy resin, and examined using a JEOL 1200-EX electron microscope (Tokyo,Japan).
Immunohistochemistry Immunohistochemical analysis was p e r f o r m e d in nine cases using the standard avidin-biotin complex and the peroxidase-antiperoxidase technique. Combinations of antibodies using standard dilutions were used: Keratin (Pharmacia Chemical Co, Piscataway, NJ), Kermix (AE1/AE3 & CK1)(1:2, Boehringer Mannheim, Indianapolis, IN; Dako, Carpenteria, CA), CAM 5.2 (1:100, Becton Dickinson, San Jose, CA), epithelial membrane antigen (1:800, Dako), Ber EP4 (1:100, Dako), carcinoembryonic antigen (1:800, Dako), desmin (1:200, Ventana, Tucson, AZ), smooth muscle actin (SMA, 1:8,000, Sigma, St. Louis, MO), muscle-specific actin (MSA, 1:40, Enzo, Farmingdale, NY), factor VIII-related antigen (FVIIIRA, 1:800, Dako), proliferating cell nuclear antigen (1:1600, Dako), and CD 68 or KP-1 (1:200, Dako). Immunohistochemical analysis for surfactant proteins B (pro-SP-B and pro-SP-C, Clara cell specific protein (CC-10) and thyroid transcription factor-1 (TTF-1) was performed under optimal conditions on six cases. Pro-SP-B (1:1,600) and pro-SP-C (1:100) immunoreactivity was detected using rabbit polyclonal antisera, and TTF-1 (1:250 and 1:1,000) was localized using a monoclonal antibody of clone 8G7G3/1. Dr Jeffrey A. Whitsett, Children's Hospital, Cincinnati, OH, kindly provided these antibodies. The CC10 antibody (1: 1,000) was provided by Dr Gurmukh Singh, Veterans Affairs Medical Center, Pittsburgh, Pennsylvania. Preparation and
RESULTS Clinical Features Clinical details o f t h e 17 cases a r e o u t l i n e d in T a b l e 1. Seven cases o c c u r r e d in m e n , a n d n i n e cases in w o m e n . I n o n e case, t h e age a n d sex o f t h e p a t i e n t w e r e n o t k n o w n . T h e m e a n a g e at p r e s e n t a t i o n was 53 years ( r a n g e , 39 to 74 years). Twelve cases o c c u r r e d in whites a n d f o u r in blacks. O n e p a t i e n t p r e s e n t e d with c h r o n i c i n s i d i o u s n o n p r o d u c t i v e c o u g h , b u t in 13 p a t i e n t s p u l m o n a r y sympt o m s w e r e a b s e n t , a n d a s o l i t a r y n o d u l e , mass, shadow, o r c o i n l e s i o n was d e t e c t e d i n c i d e n t a l l y o n a c h e s t r a d i o g r a p h . I n f o u r cases, t h e p a t i e n t s h a d b e e n foll o w e d - u p f o r 2 to 10 years after i n c i d e n t a l d i s c o v e r y o f p u l m o n a r y n o d u l e s o n c h e s t r a d i o g r a p h s ; two o f t h e s e lesions h a d i n c r e a s e d in size d u r i n g t h e follow-up interval. T h e t u m o r a r o s e in t h e left l o w e r l o b e in h a l f (8 o f 16) o f t h e cases. Seven t u m o r s w e r e l o c a t e d in t h e r i g h t lung; t h r e e o c c u r r e d in t h e u p p e r lobe, two in t h e l o w e r l o b e , a n d o n e in t h e m i d d l e lobe. T h e e x a c t site w i t h i n t h e r i g h t l u n g was n o t s p e c i f i e d in t h e r e m a i n i n g case.
TABLE 1. Clinical Features of Alveolar Adenomas Case No.
Patient's Age/Race/Sex
Presenting Symptoms
Radiographic Findings
Location
Size (cm) Maximum Diameter
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
45/C/M 58/AA/M 50/AA/M 39/AA/M 41/C/F 52/C/F 41/C/F 54/C/F 64/C/M 58/C/M 45/C/F 68/AA/M 46/C/F N/A 74/C/F 59/C/A 55/C/F
N/A Asymptomadc Asymptomatic Asymptomatic Asymptomadc hx-melanoma Asymptomatic Asymptomatic Asymptomadc N/A Asymptomatic hx-acromegaly Asymptomadc N/A Asymptomatic I-Ix-uterine leiomyoma Lack of appetite Weakness, Rash Asymptomatic Chronic cough
N/A Solitary nodule Solitary lesion Solitary nodule Solitary nodule Solitary lesion Solitary lesion Hilar mass Solitary nodule Solitary nodule Coin lesion Coin lesion Round shadow Shadow Solitary nodule Solitary nodule Solitary mass
LUL LLL LLL RLL LLL LLL LLL RUL RUL LLL LLL LLL N/A Right lung RML RUL RLL
1.5 1.9 N/A 2 1.l 3 2.5 2.5 1.2 1.5 2 1.8 N/A 3 2.5 1.3 6
Abbreviations: M, male; F, female; C, Caucasian; AA, African-American; N/A, not available; LUL, Left upper lobe; LLL, Left lower lobe; RLL, Right lower lobe; RUL, Right upper lobe; Max = maximum; Hx = history of.
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One t u m o r was located in the left u p p e r lobe, and the site was not specified in one case. Absence of hilar or mediastinal lymphadenopathy and lack of a history of smoking was m e n t i o n e d in three cases each; however, these issues were not addressed in all cases. In five cases, bronchoscopy, transbronchial biopsy (n = 3), fine-needle aspiration biopsy (n = 1), bronchial washings/brushings (n = 2), and cytology cell blocks and smears (n = 3) were r e p o r t e d as benign or nondiagnostic. All tumors were surgically excised. Lobectomy was p e r f o r m e d in two cases, wedge resection, biopsy, or excision in eight cases, and in the remaining cases tile p r o c e d u r e was not specified. T h e contributors' diagnoses included congenital cyst, acute and chronic inflammatory benign reactive lesion, hamartoma, intralobar sequestration, lymphangioma, atypical mesenchymal lesion, and alveolar adenoma. In one case, a frozen section diagnosis of adenocarcinoma was made; however, this was a m e n d e d to a diagnosis of alveolar a d e n o m a after review of p e r m a n e n t sections f r o m formalin-fixed, paraffine m b e d d e d material. Gross Findings
The m e a n t u m o r size was 2.2 cm (range, 1.1 to 6 cm). T h e tumors were well demarcated, circumscribed, and on occasion shelled out from the surrounding lung parenchyma. Several tumors appeared polypoid or loosely a d h e r e n t to a cyst wall. Most of the lesions lay in a subpleural location, but one was e m b e d d e d within the hilar structures. In one patient, the t u m o r caused external compression of a basal medial bronchus, resulting in p u l m o n a r y symptoms. No direct communication with a bronchus or association with major vessels was noted. T h e cut surface was smooth, lobulated, multicystic, glistening, and partly mucoid or fish-flesh in appearance. T h e texture ranged from being soft, gelatinous, or m u c o i d in consistency to firm, rubbery, elastic, or spongy. T h e color was pale yellow, gray, white, or tan, and focal areas of h e m o r r h a g e and necrosis were described.
FIGURE 1. Low power shows a circumscribed nodule with several large cysts and multiple small cysts. A thin layer of connective tissue is seen focally around the edge of the tumor. (Hematoxylin and eosin, original magnification x9.5.)
large to small. Large cysts were readily visible on gross examination and on low-power histological examination, and smaller cysts resembled alveolar spaces (Figs 3 and 4). Papillary formations were not seen. In four cases, there was a tendency for the large cystic spaces to be located centrally rather than peripherally; however, in other tumors, the large cysts were seen peripherally. In 12 of the 16 cases, eosinophilic proteinaceous granular material was identified in the cystic spaces within the tumors. This material was not seen in the adjacent lung parenchyma. In nine of these tumors, complete lamellar bodies or fragments of lamellar material were seen. In 3 of the 12 cases, the eosinophilic material had a globular appearance. Macrophages were seen within the cystic spaces in 14 cases. They had an eosinophilic, foamy, vacuolated cytoplasm and occasionally f o r m e d multinucleated giant cells. Intracytoplasmic inclusions of cholesterol esters were occasionally noted. A single layer of epithelial cells lined the cystic spaces (Figs 4 and 5). Most of the epithelial cells had the appearance of hyperplastic type 2 pneumocytes with a cuboidal or hobnailed appearance and eosinophilic, finely vacuolated or foamy cytoplasm. Nuclear pseudoinclusions were occasionally seen (Fig 5). A few flattened or squamoid type 1 pneumocytes lined cysts, surr o u n d e d by a thin wall resembling that of a normal
Histological Features
In all cases, low-power magnification showed a well-demarcated, multicystic lesion easily distinguished from the surrounding lung parenchyma (Fig 1). Alt h o u g h a complete fibrous capsule or pseudocapsule was not present, focal condensation of fibrous connective tissue was noticed at the periphery, particularly in areas where the lesion abutted bronchioles, blood vessels, or lymph nodes. Although low-power views suggested a sharp demarcation from the surrounding lung parenchyma, closer examination in 12 of 13 cases with adjacent lung parenchyma in the section showed a focal gradual transition between alveolar wails of the alveolar a d e n o m a and those of the surrounding normal lung parenchyma (Fig 2). All tumors were multicystic, with cysts varying from
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there a medium-sized p u l m o n a r y vessel within the lesion; however, this vessel was probably entrapped, because the lesion was e m b e d d e d within the hilar structures. Bronchioles were not identified within the lesion, n o r were cartilage, adipose tissue, or smooth muscle. No cellular dysplasia, necrosis, invasion of adjacent parenchyma, or vascular invasion was seen. The adjacent lung tissue showed a variety of nonspecific features, including focal atelectasis, intra-alveolar fresh hemorrhage, or compression of lung parenchyma. Occasional lymphoid aggregates also were noted, and, in one case, there were histological features in the adjacent lung highly suggestive of hypersensitivity pneumonitis. One lesion was associated with an incidental 0.4-cm focus of well-differentiated adenocarcinoma. Morphologically and cytologically, however, the two lesions were distinct and separate. Histochemistry
Histochemical analysis by periodic acid-Schiff and periodic acid-Schiffwith diastase predigestion and mucicarmine stains in seven cases confirmed the absence of cytoplasmic mucin in epithelial cells. The interstitial matrix in five cases displayed more p r o m i n e n t positivity with the alcian blue stain c o m p a r e d with the adjacent lung parenchyma, highlighting the presence of extracelFIGURE 2. At the edge of this tumor, some of the alveolar walls of the surrounding lung parenchyma merge imperceptibly with those of the alveolar adenoma (right). The alveolar adenoma consists of a nodule of tissue closely resembling alveolar lung parenchyma. (Hematoxylin and eosin, original magnification ×75,)
alveolus. Neither ciliated cells n o r cells with Clara cell morphology were seen. The interstitial c o m p o n e n t varied in appearance from a thin connective tissue layer resembling normal alveolar septa (Fig 4) to markedly thickened alveolar walls with myxoid connective tissue similar to that of a h a m a r t o m a or mesenchymal lesion (Fig 6). In two cases, the interstitial matrix was so p r o m i n e n t that the alveolar architecture was lost, and the air spaces were compressed in a slit-like fashion (Fig 6). O t h e r areas displayed hypercellular thickened septa (Fig 7) with p r o m i n e n t spindle-/oval-shaped cells (Fig 8). These cells had a fine to o p e n nuclear chromatin and mostly inconspicuous nucleoli. Often small processes emanated from their cytoplasm. In some areas, the nuclei were "cigar shaped," resembling those of smooth muscle cells. Inflammatory cells were mostly inconspicuous. However, mild to moderate numbers of interstitial lymphocytes, plasma cells, and eosinophils were seen, sometimes in clusters. Mast cells were frequent. Interstitial macrophages, often with vacuolated cytoplasm, also were present, occasionally forming multinucleated giant cells containing cholesterol clefts. Foci of recent and old h e m o r r h a g e with hemosiderin deposition were frequent. A well-developed fine capillary network traversed the interstitial mesenchyme. In only one case was
FIGURE 3. The alveolus-like spaces of this tumor are filled with a finely granular proteinaceous material and lined by thin walls. At this magnification, it is difficult to discern an epithelial lining. At the edge of this tumor (upper right corner), there is a condensed layer of epithelial cells. (Hematoxylin and eosin, original magnification x400.) 161
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stained macrophages within the interstitium. The interstitial stromal cells did not stain for desmin in six cases. Focal positivity for smooth muscle actin was observed in stromal cells in one of seven cases, and focal positivity for muscle-specific actin in one of six cases. The proliferative nature of the lesion in both the epithelial and interstitial c o m p o n e n t s was shown in four cases by the more intense and widespread staining of t u m o r nuclei by the proliferating cell nuclear antigen as c o m p a r e d with the adjacent normal lung parenchyma. Positive cytoplasmic staining for Pro-SP-B (Fig 9) and Pro-SP-C was observed in most of the epithelial cells in all six cases analyzed, confirming the presence of type 2 pneumocytes (Table 3). T h e type 2 pneumocytes also showed positive nuclear staining with the TTF-1 antibody in five of six cases (Fig 10, Table 3). In three of these cases there was cytoplasmic staining, the reason for which is unclear. T h e flatter, more squamoid, type 1 cells were negative with these pneumocyte markers (Pro-SP-B, Pro-SP-C, and TTF-1). Clara cells were not identified, because the CC10 immunostain was consistently negative. T h e interstitial cells were negative for all four pneumocyte markers. T h e cyst contents showed focal positivity for Pro-SP-B, as did some of the macrophages within the cystic alveolar spaces. Electron M i c r o s c o p y
Electron microscopy confirmed that most of the epithelial cells were type 2 pneumocytes containing
FIGURE 4. Many of the alveolus-like spaces of this tumor are lined by hyperplastic, cuboidal type 2 pneumocytes. The connective tissue within the walls of these spaces is very thin, similar to that seen in normal alveoli, (Hematoxylin and eosin, original magnification x400.)
lular acid mucopolysaccharides. In the one case analyzed, this material was sensitive to predigestion with hyaluronidase. Glycogen also was n o t e d in the interstitium, and, in many areas, it appeared predominantly intracellular in location. T h e interstitium also contained collagen fibrils, as highlighted by the Movat/Masson trichrome stain in the six cases examined, and in one case a reticulin stain showed reticulin fibers. In four cases examined with the van Gieson stain, there was an absence of or a reduction in the n u m b e r of elastic fibrils in the t u m o r as compared with the normal lung parenchyma, and any elastic fiber fragments present lay at the p e r i p h e r y of the lesion. Immunohistochemistry
The results of the immunohistochemical assays of the nine cases available for assessment are tabulated in Table 2. T h e epithelial phenotype of the cells lining the cystic spaces was confirmed with positive staining for at least one cytokeratin antibody in eight of nine cases. Negative staining for factor VIII was seen in two of these cases. Positive staining for carcinoembryonic antigen was observed in four of four interpretable, keratinpositive cases. T h e interstitial cellular c o m p o n e n t was consistently negative for cytokeratin. Staining for factor VIII highlighted the vascular network. In one case, CD68
FIGURE 5. Virtually all of these pneumocytes lining the wall of this alveolar space are hyperplastic type 2 cells with cuboidal morphology. A cytoplasmic nuclear inclusion is present (arrow). (Hematoxylin and eosin, original magnification x 1,000.)
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interstitial component is very prominent, exceeding that seen in normal alveolar parenchyma. Our ultrastructural and immunohistochemical studies showed that most of the epithelial cells lining the cystic spaces of alveolar adenomas are type 2 pneumocytes, with a minority of type 1 cells and no Clara cells. Positive staining of type 2 cells for TTF-1, pro-SP-B, and pro-SP-C, and negative staining for CC-10 supported this impression. The capability of the immunohistochemical stains to surfactant proteins, TTF-1, and CC-10 to differentiate cell types is supported by specificity studies previously performed on these reagents. 1°-12 TTF-1 is a homeodomain-containing transcription factor that plays critical roles in lung-specific expression of surfactant proteins and CC-10. It is expressed selectively in thyroid, diencephalon, and lung tissue. 13In the lung, nuclear localization of TTF-1 is confined to alveolar type 2 pneumocytes and nonciliated bronchiolar and bronchioloalveolar portal epithelial cells. ~4 Immunoreactivity for TTF-1 together with a surfactant precursor protein pro-SP-B also has been shown in adenocarcinomas of the lung. 14,15 Pro-SP-B and Pro-SP-C are the precursor proteins for surfactant proteins B and C, respectively. Pro-SP-B cytoplasmic localization has been detected in alveolar Type 2 cells 15'16 and in nonciliated bronchial and bronchiolar epithelial cells, s° Immunoreactivity for proFIGURE 6. The interstitial stroma is very prominent in this area, consisting of myxoid connective tissue. The alveolar spaces are narrow, slit-like, and lined by cuboidal pneumocytes. (Hematoxylin and eosin, original magnification ×200.)
varying numbers of lamellar bodies in their apical cytoplasm (Fig 11). Interspersed among these type 2 cells, small numbers of cells with markedly attenuated cytoplasm characteristic of type 1 pneumocytes were identified (Fig 12). The epithelial cells typically formed a single layer encircling a central space that was often small with an irregular shape, though many were large and round. Clara cells were not seen. The interstitium contained fibroblasts, fibroblastlike ceils, macrophages, and a few mast cells and lymphocytes (Fig 13) embedded in a moderately collagenous stroma. Most of the stromal vessels were capillaries.
DISCUSSION Alveolar a d e n o m a is a rare benign neoplasm with distinctive gross and microscopic findings. The radiographic presentation as a solitary lung nodule is nonspecific; therefore, correct diagnosis requires accurate recognition of the characteristic histological features. Our study of 17 cases not only reviews the distinctive pathological features of the neoplasm but also provides additional information regarding the histogenesis of this lesion. Our findings support the use of the term alveolar adenoma for this lesion, because we have shown an intimate association of both native alveolar epithelial and interstitial elements, although in some cases the
FIGURE 7. Thistumor consists of large cystic spaces with thick walls composed of a cellular spindle cell and connective tissue stroma. There are hyperplastic cuboidal epithelial cells lining the walls of the cysts. (Hematoxylin and eosin, original magnification × 75.)
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FIGURE 9. The cytoplasm of the hyperplastic type II pneumocytes lining the walls of the alveolus-like spaces is stained strongly with the antibody to surfactant pro-SPB. (Immunohistochemistry for surfactant pro-SPB, original magnification x300.)
FIGURE 8. This stroma consists of prominent spindle-shaped cells embedded in a collagenous stroma. Hyperplastic cuboidal type 2 pneumocytes line the surface of this cyst wall. (Hematoxylin and eosin, original magnification x300.) SP-C a p p e a r s l i m i t e d to alveolar type 2 cells p r e d o m i nantly, with m u c h less f r e q u e n t s t a i n i n g o f the n o n c i l i a t e d b r o n c h i o l o a l v e o l a r p o r t a l cells t h a n t h a t s e e n with pro-SP-B. 1° T h e a n t i b o d y CC-10 detects the Clara c e l l specific 10-kD p r o t e i n , a n a b u n d a n t s e c r e t o r y p r o d u c t o f n o n c i l i a t e d b r o n c h i o l a r epithelial (Clara) cells. T h e r e fore, this a n t i b o d y stains Clara cells b u t n o t type 2 p n e u m o c y t e s . 17 O u r i m m u n o h i s t o c h e m i c a l a n d u l t r a s t r u c t u r a l findings also i n d i c a t e d t h a t the s p i n d l e cells i n the interstit i u m were fibroblasts o r fibroblast-like cells, s u p p o r t e d
TABLE 2.
Immunohistochemical Results of Cases
Epithelial
Interstitium
Both
Case Keratin CEA FactorVlll Desmin Actin SM Actin MS PCNA 1 2 3 6 10 11 12
++ ++ ++ ++ ++ ++ -
NE N/I NE + + NE NE
16
+ +
+
17
++
+
NE NE . NE NE NE NE NE
.
NE . NE NE -
+ -
+ NE
NE NE -
NE NE -
.
++
by the p r e s e n c e o f c o l l a g e n fibrils by e l e c t r o n microscopy. A b s e n c e o f d e s m i n s t a i n i n g essentially e x c l u d e d m u s c l e d e r i v a t i o n for these cells. T h e p r o m i n e n t m a s t cells are a f e a t u r e n o t previously e m p h a s i z e d i n these lesions. A l t h o u g h alveolar a d e n o m a s have traditionally b e e n r e g a r d e d as n e o p l a s t i c lesions, 1 it is n o t clear w h e t h e r b o t h the e p i t h e l i a l a n d m e s e n c h y m a l c o m p o n e n t s are neoplastic. S o m e believe t h a t the p r o l i f e r a t i o n is pred o m i n a n t l y m e s e n c h y m a l with alveolar e n t r a p m e n t , b u t it is possible t h a t the e p i t h e l i a l c o m p o n e n t is also neoplastic. R o q u e et al 9 o b s e r v e d a c l o n a l cytogenetic t r a n s l o c a t i o n o f d e r ( 1 6 ) t ( 1 0 ; 1 6 ) ( q 2 3 : 2 4 ) i n 19% o f cells i n these lesions, 9 s u g g e s t i n g c l o n a l e x p a n s i o n a n d n e o p l a s i a i n these lesions. U l t i m a t e l y the issue o f cellular n e o p l a s i a n e e d s to b e a d d r e s s e d by clonality studies i n b o t h e p i t h e l i a l a n d m e s e n c h y m a l cells. O u r
TABLE 3. Surfactant, Clara Cell, a n d 1-rF-1 Immunostain Results on the Lining Epithelial Cells
++ ++ ++
Abbreviations: - , Negative; +, Focal positivity; ++, Strong positivity;NE, Not evaluated; NI, Not interpretable; Actin SM, actin, smooth muscle; Actin, MS, Actin, muscle specific; PCNA, proliferating nuclear cell antigen.
Case No.
TTF-1
CC10
PRO-SP-B
PRO-SP-C
1 6
Positive Positive
Negative Negative Negative Negative Negative Negative
Positive Positive Positive Positive Positive Positive
Positive Positive Positive Positive Positive Positive
10
N/A
11 16 17
Positive Positive Positive
N/A = Not assessed.
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finding of increased proliferating cell nuclear antigen activity in both epithelial and interstitial cells suggests that both components are proliferating. The differential diagnoses to be considered include papillary adenoma, sclerosing hemangioma, atypical adenomatous hyperplasia (AAH), bronchioloalveolar carcinoma, l y m p h a n g i o m a , a n d h a m a r t o m a . Congenital cystic adenomatoid malformation and honeycomb fibrosis are also considerations. Papillary adenoma is similar to alveolar adenoma in that it presents as a peripheral, circumscribed nodule that radiologically can present as a solitary coin lesion in an otherwise asymptomatic patient, ls-~4 The tumors are also circumscribed but differ from alveolar adenomas by showing a prominent papillary pattern with a heterogeneous epithelial population including type 2 cells, Clara cells, and, sometime, ciliated cells. Sclerosing hemangiomas differ from alveolar adenomas in that they show a mixture of solid, vascular, sclerotic, and papillary patterns. The solid epithelioid cells, which appear to represent the neoplastic cells, usually stain for epithelial membrane antigen. 2~,26 Recently, Niho et a127 showed the X-chromosome-linked polymorphic markers of the h u m a n androgen receptor gene and the phosphoglycerate kinase gene were amplified in five of six sclerosing hemangiomas, indicating a clonal o r i g i n s The female predominance of sclerosing
FIGURE I1. Electron microscopy shows type 2 pneumocytes (arrow) lining the walls of the alveolus-like spaces and many extruded lamellar bodies (arrowhead), Type 1 pneumocytes are also present. (Original magnification x2,500.)
hemangioma is stronger than the slight female predilection we observed for alveolar adenomas. Alveolar adenoma should not be confused with the entity known as AAH, which represents an atypical bronchioloalveolar proliferation that falls short of criteria for bronchioloalveolar carcinoma. These lesions are usually incidental findings in lung cancer resection specimens and are thought to be precursor lesions to a d e n o c a r c i n o m a . 28-3°Miller 3° used the term bronchioloalveolar adenoma for these lesions, 3° but the preferred term is AAH, because use of the term adenomashould be restricted to the group of alveolar adenoma, papillary adenoma, mucinous cystadenoma, and salivary gland adenomas. 2 These lesions are benign and are not thought to be preneoplastic. Indeed, the lack of cellular atypia, necrosis, and invasion in alveolar adenomas differentiates them from a malignant process, and the lack of deaths, metastases, or recurrences in these patients attests to its benignity. For many years, alveolar adenomas were thought to be lymphangiomas because of their multicystic appearance, the thin-walled nature of many of the cysts, and the sometimes flat lining cells resembling endothelial cells. 3a,32The presence, however, of keratin-positive and factor VIII-negative cells lining the cystic spaces as well as the ultrastructure confirms its epithelial rather than vascular origin. A hamartoma also may be a consideration, espe-
FIGURE 10. The nuclei of the hyperplastic type 2 pneumocytes lining the alveolus-like spaces are stained strongly with an antibody to thyroid transcription factor (TTF-1). (Immunohistochemistry for surfactant pro-SPB, original magnification x300.)
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cially when an extensive mesenchymal proliferation is very prominent, as was seen in two of our cases. 25,s3 However, the alveolar appearance in addition to the lack of an intimate association of bronchiolar, adipose, and c h o n d r o i d tissue would militate against a diagnosis of hamartoma. Congenital cystic a d e n o m a t o i d malformation differs from alveolar a d e n o m a in that it is rarely seen in adults. 25 Grossly, it also tends to be larger than alveolar adenomas, and histologically the lung appears immature and malformed, often with a bronchiolar and papillary epithelial proliferation. Interstitial fibrosis with h o n e y c o m b is very different from an alveolar a d e n o m a in that it is not grossly circumscribed, and the cystic spaces are mostly lined by a metaplastic bronchiolar epithelium. Alveolar a d e n o m a is a difficult lesion to diagnose on small biopsy specimens and on frozen section, because the tissue may look exactly like normal lung parenchyma or could simulate a malignancy with small glandular spaces lined by regular glandular epithelium. This difficulty is reflected in the nondiagnostic nature of the small biopsy or cytology specimens in five of our cases. Given the morphological resemblance of alveolar a d e n o m a to normal lung, it is understandable how a small tissue sample could look like unremarkable lung parenchyma. Awareness of this possibility may be of particular importance in those patients who have a history of malignancy elsewhere, and where metastatic FIGURE 13. Electron microscopy shows fibroblasts, fibroblastlike cells, and mast cells within the interstitium. (Original magnification x2,500,)
t u m o r is the prime consideration. Examination of the resected specimen should indicate the correct diagnosis. In conclusion, alveolar a d e n o m a is a rare, benign neoplasm that usually presents in asymptomatic patients as a coin lesion on chest radiograph and has a characteristic multicystic histological appearance that often resembles normal alveolar lung tissue. Immunohistochemically and ultrastructurally, our study indicates that most of the epithelial cells are type 2 pneumocytes and that the interstitial stromal cells are fibroblasts or fibroblast-like cells. Our follow-up data also confirm that this t u m o r is benign.
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FIGURE 12.
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