A Contemporary View of Neoplasms in Children: The Pathologist’s Perspective

Selected Topics in Pediatric Pathology

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A Contemporary View of Neoplasms in Children The Pathologist's Perspective

Louis P. Dehner, MD*

In the past 25 to 30 years, the Pediatric Clinics has periodically considered the topic of pediatric neoplasms from the clinical and pathologic perspectives. In the symposium on neoplastic diseases edited by Arey in 1959, there were several excellent discussions on the pathologic anatomy of benign and malignant tumors in children. 2 At that time, the emphasis was principally upon the light microscopic interpretation, because electron microscopy as a diagnostic technique was in its infancy. However, a volume on pediatric oncology 17 years later had a discussion by Favara32 on the roles of immunophenotyping of hematolymphoid malignancies and of electron microscopy in the fine structural characterization of "undifferentiated" small cell tumors and histiocytosis. Today both of these methodologies have been completely incorporated almost to the point of routine into the laboratory examination of malignant lymphomas and leukemias and in a number of nonhematologic solid neoplasms. Because progress is perceived as movement from one point in time to another in understanding, newer techniques have joined older ones in the study and diagnosis of tumors in children and adults. Excellence in histopathologic interpretation was not superseded by electron microscopy or cell marker studies, nor is it anticipated that immunohistochemistry, flow cytometry, or molecular probes will render light microscopy obsolete. The latter three techniques have unquestionably amplified and refined the power of the diagnosis in this era of treatment protocols for specific sets and even subsets of tumor types. Through the process of trial and error in the past two decades, the pediatric oncologist has available effective therapeutic regimens that are predicated to a major extent upon an accurate pathologic diagnosis. The remarkable progress in pediatric oncology is measured by the dramatic *Director of Anatomic Pathology, Division of Surgical Pathology, Department of Laboratory Medicine and Pathology, University of Minnesota Medical School and Hospital, Minneapolis, Minnesota

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improvements in the prognosis of most childhood malignancies in the past 30 years. 68 The contemporary surgical-pediatric pathologist is not only confronted with the imperative of establishing "the" diagnosis but of recognizing certain favorable or unfavorable morphologic features in that neoplasm. With the beneficial response to therapy by many forms of childhood malignancy, morphologic subgroups of treatment-resistant and, thus, prognostically unfavorable tumors have emerged from neoplastic entities that were previously regarded as pathologically homogeneous. The prototypic example of this phenomenon was Wilms' tumor, which was universally considered a highly aggressive and invariably lethal malignancy requiring emergency nephrectomy if there was to be any hope for survival. In spite of surgical heroics, the overall survival was only 10 to 15 per cent, which is difficult to comprehend by those whose only perspective on Wilms' tumor is based on the experience of the past decade. As the majority of Wilms' tumors responded to combined treatment modalities, a minority of tumors proved to be unresponsive, and careful histologic studies distinguished between prognostically favorable and unfavorable pathologic subgroups. 6 The concept of favorable and unfavorable morphology in Wilms' tumor has been applied with variable success to rhabdomyosarcoma, hepatoblastoma, neuroblastoma, non-Hodgkin's malignant lymphoma, and acute leukemia. The prognostic parameters are no longer restricted to the presence or absence of certain histologic or cytologic features but now include chromosomal deletions and/or translocations and oncogene expression. 40, 60, 63 Molecular diagnostic techniques have yielded insights into the biology of childhood tumors at a rate that makes it difficult to remain current with even the most important observations. The transition of findings with clinical applicability from the basic sciences to the diagnostic laboratory are occurring with remarkable rapidity. A unique problem in pediatric tumor pathology is the pathogenetic question of whether a particular proliferation is a neoplasm or a developmental aberration and, if it is a neoplasm, whether it is benign or malignant. Another unusual quality of certain childhood tumors is the potential for spontaneous regression (hemangioendothelioma, myofibromatosis, retinoblastoma, and neuroblastoma).74 There are also a number of well-defined, reproducible constitutional chromosomal anomalies, somatic malformations, and syndromes whose presence may herald the development of certain neoplasms (Table 1).7 Knudson's "two-hit" hypothesis has served to explain the dysontogenetic-hamartomatous lesions resulting from a prezygotic mutational event and a post-zygotic hit producing a neoplasm in some instances. 56 Heredity as an etiologic factor in oncogenesis was known for retinoblastoma for some time and was recognized later in a subset of patients with Wilms' tumors. It is estimated that 40 per cent of all cases of Wilms' tumor are hereditary; this percentage is 100 per cent when the tumors are bilateral. 3. 34 The deletion of the p13 band of chromosome 11 is a marker of the aniridia-bilateral Wilms' tumor syndrome (Miller's syndrome) (Fig. 1).72 There are other examples of neoplasms, not exclusively presenting in the pediatric age population, with accompanying chromosomal breakages, in-

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. Table 1. Pediatric Tumors, Chromosomal Anomalies, and Malformations Wilms' tumor

Chromosome 11 deletion (11pI3 band) Aniridia Hemihypertrophy Beckwith-Wiedemann syndrome Male pseudohermaphroditism (Drash's syndrome)

Hepatoblastoma

Hemihypertrophy Beckwith-Wiedemann syndrome Familial colonic polyposis

Neuroblastoma

Chromosome 1 deletion (short arm) Von Recklinghausen's disease

Retinoblastoma

Chromosome 13 deletion (13q14 band) Autosomal dominant (bilateral)

Sacrococcygeal teratoma

Sacral agenesis Imperforate anus

Gonadal germ cell neoplasm

XY male pseudohermaphroditism XY/XO mixed gonadal dysgenesis Klinefelter's syndrome

Neurofibroma

Von Recklinghausen's disease

Myofibromatosis

Autosomal dominant Gardner's syndrome

Dysplastic nevus

Autosomal dominant Malignant melanoma

Medullary carcinoma of thyroid

Multiple endocrine adenomatosis type 2b

Malignant lymphoma

Primary immunodeficiency syndrome (severe combined immunodeficiency, Wiskott-Aldrich syndrome)

Hemophagocytic syndrome

X-linked lymphoproliferative syndrome Chediak-Higashi syndrome Familial lymphohistiocytosis

Squamous cell carcinoma (skin)

Xeroderma pigmentosa Epidermodysplasia verruciformis Epidermolysis bullosa dystrophica

Basal cell carcinoma

Basal cell nevus syndrome Xeroderma pigmentosa

Adenocarcinoma of stomach, colon

Ataxia telangiectasia Familial adenomatous polyposis Familial juvenile polyposis Peutz-Jeghers syndrome

Acute nonlymphoid leukemia

Trisomy 21

herited primary immunodeficiency states, or an autosomal transmission (see Table 1), Fatalities from malignant disorders in childhood are only exceeded by accidental deaths in the United States and other developed countries. 8, 57, 68 Approximately 6000 to 7000 children under 16 years of age are newly diagnosed each year in the United States with a malignancy. Several

Figure 1. A G-band karyotype of one metaphase cell from the peripheral blood of a child with aniridia and bilateral Wilms' tumor, showing the constitutional deletion of band 13 (arrow head) from the short arm of one chromosome 11. The inset demonstrates another number 11 chromosome from a different metaphase cell. (Figure courtesy of D. C. Arthur, MD, Minneapolis, Minnesota.)

excellent reviews on the epidemiologic aspects of childhood cancer are available and should be consulted for details not addressed in this discussion,57, 68, 78 but, in general, acute leukemias account for 25 to 30 per cent of cases followed by central nervous system tumors (20 per cent), malignant lymphomas of non-Hodgkin's and Hodgkin's types (12 per cent), neuroblastoma and related neuroectodermal tumors (7 per cent), Wilms' tumors and other nephrogenic neoplasms (6 to 8 per cent), soft tissue sarcomas, principally rhabdomyosarcoma (7 to 8 per cent), and bone tumors (4 per cent).8. 44 Germ cell neoplasms, retinoblastoma, and hepatic tumors each represent 3 per cent or less of the total cases. With the exception of the malignant lymphomas, bone sarcomas, and gonadal neoplasms, the other major categories present principally in the first 5 years of life, and males are affected slightly more commonly than females. 57 Geographic, ethnic, and environmental factors influence the incidence and frequency of some tumors in children; the mechanisms are either unknown or incompletely defined. 41 One of the several particularly intriguing topics in pediatric oncology and pathology is the occurrence of neoplasms in the fetus. To qualifY as a congenital neoplasm, the clinical manifestations must be evident at or very shortly after birth, otherwise it is defined as a connatal or neonatal tumor when diagnosed in the first few days or first month of life. 27 Most studies in the literature have eliminated from consideration some of the more common congenital tumefactions or proliferations, like the melahocytic nevus, hemangioma, or lymphangioma. A recent review of congenital and

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Figure 2. Transient myeloproliferative syndrome in a neonate with trisomy 21, shOWing circulating blasts in a peripheral blood smear. These cells have the phenotype of megakaryocytes. (Magnification x 400.) (Figure courtesy ofR. Brunning, MD, Minneapolis, Minnesota.)

neonatal tumors from the Children's Hospital of Los Angeles revealed that slightly in excess of 80 per cent were either teratomas (38 per cent), various soft tissue neoplasms (20 per cent), neuroblastomas (14 per cent), or leukemias (10 per cent). 47-49 Most teratomas presented in the sacrococcygeal region, and the soft tissue tumors were hemangiomas, fibromatosis, fibrosarcomas, and cystic hygromas. The neuroblastomas originated in the adrenal or retroperitoneum, and the leukemias were mainly nonlymphocytic. 95 Some putative congenital neonatal leukemias may be a transient myeloproliferative syndrome and undergo spontaneous resolution, especially in infants with trisomy 21 (Down's syndrome) (Fig. 2).12.22.35,89

METHODS AND APPROACHES TO PATHOLOGIC DIAGNOSIS Immunohistochemistry The "state of the art" techniques presently available to the pathologist in the diagnosis of undifferentiated or poorly differentiated neoplasms in children have progressed well beyond those that were discussed by Favara in 1976,32 At that time, five basic methods were considered in detail: standard histologic preparations, cytochemistry, enzyme cytochemistry, ultrastructural methods, and cell marker studies, Today, the peroxidaseantiperoxidase (PAP) technique and the avidin-biotin-peroxidase modification of immunohistochemistry have had a profound impact upon diagnostic histopathology, because it is possible with a constantly expanding menu of antibodies to detect structural constituents like intermediate filaments (vimentin, desmin, cytokeratin, glial fibrillary acidic protein, and neurofil-

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ament) and actin, cellular products (enzymes, oncofetal antigens), and membrane antigens (epithelial membrane antigen, leukocyte common antigen, synaptophysin) in paraffin-embedded tissues. 50. 64 Most of the antibodies in use are monoclonal, unlike the earlier generation of polyclonal antibodies with problems in specificity. In many cases, it is no longer an imperative for the clinician who takes the biopsy nor the pathologist who receives the specimen to be immediately concerned about special fixatives other than standard buffered 10 per cent formalin or absolute ethanol. Most PAP immunostains can be performed quite satisfactorily on formalin-fixed tissue, but the period of fixation should not exceed 6 hours, because formalin progressively binds more epitopes with time. If the desired epitope is cross-linked, then the antibody will not recognize it. Pre-digestion with one of the proteolytic enzymes is generally performed before the PAP reaction in order to expose epitopes. To avoid the latter step, ethanol fixation is recommended. Cytogenetics In addition to the PAP technique, the tumor can be analyzed by How cytometry for its DNA content (ploidy analysis) and by molecular probes. Chromosomal translocation and amplified segments of DNA representing, in many instances, proto-oncogene amplification are two major abnormalities detectable by molecular analysis. 3 , 9, 16, 50, 63. An important observation from these studies is that most breakpoints uniting two separate chromosomal segments are located in the region of a proto-oncogene, suggesting that this is the mechanism of oncogene activation and replication. Certain trans locations are found consistently in the following childhood malignancies: small, noncleaved cell lymphoma (Burkitt's lymphoma), t (8;14); Ewing's sarcoma and peripheral neuroepithelioma (Askin's tumor), t (11;22), and alveolar rhabdomyosarcoma, t (2;13).3, 13, 40, 43, 81 On the basis of the identical translocation in Ewing's sarcoma and peripheral neuroepithelioma, it has been postulated that Ewing's sarcoma is a neural crest-derived neoplasm. 15, 61, 92 Similar reasoning about histogenesis has been applied when overlapping immunophenotypes are discovered, but caution should be exercised, because neither chromosomal translocations nor immunophenotypes are irrefutable proof on the point of a common histogenesis. Clinician-Pathologist Consultation

If it has not become implicitly apparent in the discussion, then a brief digression about clinician-pathologist interaction may be appropriate. The

clinician caring for the patient and the pathologist should discuss the problem case before the biopsy or resection. We have found it extremely edifying in selected cases to review the clinical presentation when it is unusual or perplexing or when the biopsy reveals a poorly differentiated tumor. The latter situation is not uncommon in our experience, because this is the rype of case that is most likely to be referred to a tertiary care facility. A dialogue between the clinician and the pathologist before a biopsy or definitive resection can facilitate formulation of a plan for special studies to be performed, which is far more productive than hasty, last-minute preparations. All of the potential studies may not be necessary, but

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contingency plans should be made. When lapses occur in these preparations, one can invariably predict the consequences. We found that the pathologist is guilty as often as the clinician in these cases. Diagnostic Approaches We will consider several specific problems in differential diagnosis to illustrate some principles of contemporary histopathologic evaluation and interpretation. One of the familiar diagnostic dilemmas is posed by the histologically similar but phenotypically distinct neoplasms composed of a more or less homogeneous population of small cells with prominent hyperchromatic nuclei and relatively inconspicuous cytoplasm. 85, 92, 93, 96 The character of the cytoplasm varies among the individual pathologic types of small blue cell tumors and even between neoplasms in the same nosologic group. For instance, one embryonal rhabdomyosarcoma is readily diagnosed by light microscopy because of the intensely acidophilic cytoplasm and the presence of cross striations, whereas another embryonal rhabdomyosarcoma requires electron microscopy and/or immunohistochemistry to establish its identity. When the biopsy reveals a monotonous population of obviously malignant small blue cells, the differential diagnosis is neuroblastoma, rhabdomyosarcoma, Ewing's sarcoma, peripheral neuroepithelioma (primitive neuroectodermal tumor), or a hematolymphoid malignancy of myelocytic, lymphocytic, or monocytic-histiocytic type. The age at presentation, the primary site, and the anatomic distribution of disease will often favor one or another of these choices despite the microscopic similarities. If the opportunity to evaluate the tumor by electron microscopy and immunohistochemistry is made available, a phenotypic diagnosis is possible in approximately 95 per cent of cases. 28 , 85, 96 Poor tissue preservation, excessive formalin fixation, extensive necrosis, or an inadequate sampling are the common reasons to explain a generic diagnosis of "undifferentiated small cell tumor" or "no diagnosis." However, there are occasional primitive small cell neoplasms with undifferentiated features and an unexpressed, seemingly anonymous phenotype. The widespread use of immunohistochemistry has engendered the impression among some pathologists and even clinicians that electron microscopy is no longer necessary; this is a mistaken idea in our opinion, especially when the differential diagnosis is between a peripheral neuroepithelioma or a Ewing's sarcoma and an embryonal rhabdomyosarcoma or a primitive sarcoma (Fig. 3). The principal ultrastructural and immunohistochemical features of the commonly encountered small cell tumors of childhood are summarized in Table 2. Leukocyte common antigen, neuronspecific enolase or synaptophysin, muscle-specific actin or des min, and vim en tin are the preferred markers to initiate the process of immunophenotyping (Fig. 4).29,54,65,67,76.90,94 We have found muscle-specific actin more sensitive than desmin in the recognition of rhabdomyosarcomas, especially in those tumors with primitive histologic features. Both desmin and musclespecific actin are expressed equally well in the differentiated embryonal and alveolar rhabdomyosarcomas. Unfortunately, neuron-specific enolase is only available commercially as a polyclonal antibody whose specificity for

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Figure 3. Ewing's sarcoma in this electron micrograph, showing a uniform round cell without processes, relatively scanty cytoplasmic organelles, and rare primitive intercellular junctions. The undifferentiated nature of the cell is a distinguishing feature from the other "small blue cell tumors" of childhood.

Table 2. Ultrastructural and Immunohistochemistry Features of Undifferentiated Small Cell Tumors IMMUNOCYTOCHEMICAL PROFILE TUMOR TYPE

Neuroblastoma

FINE STRUCTURAL FEATURES

Interdigitating elongated cytoplasmic processes Microtubules (neurotubules) Electron-dense granules in cell processes

NSE SYN VIM CK NF MSA DES LCA

+

+

±

Rhabdomyosarcoma Variable content of intermediate filaments Delicate filaments (actin) Variable Z-band material Glycogen Basal lamina-like material

±

+

Ewing's sarcoma

Round, monotonous cells Variable intermediate filaments Primitive cell attachments

±

+

Malignant lymphoma

Round to convoluted nucleus Minimal filaments

±

-

+

+

±

+

Abbreviations: NSE = neuron-specific enolase, SYN = synaptophysin, VIM = vimentin, CK = cytokeratin, NF = neurofilament, MSA = muscle-specific actin, DES = desmin, LCA = leukocyte common antigen.

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Lymphoma, Leukemia Small Cell Tumor LCA, NSE, VIM, MSA

Ewing's Sarcoma Rhabdomyosarcoma

Rhabdomyosarcoma Neuroblastoma

Neuroblastoma Ewing's Sarcoma Rhabdomyosarcoma

Ewing's Sarcoma Rhabdomyosarcoma

Figure 4. The differential diagnosis of the small blue cell tumor and the application of immunohistochemistry to arrive at a final pathologic diagnosis. Key: VIM = vimentin, NSE = neuron-specific enolase, LeA = leukocyte-common antigen, DES = desmin, MSA = muscle-specific actin, SYN = synaptophysin.

neuroectodermal tumors is less than satisfactory so that we come to rely more upon synaptophysin, a membrane-associated glycoprotein, which has been isolated from presynaptic vesicles. 67. 76. 90 Vimentin, like desmin, is an intermediate filament that is expressed in a number of mesenchymal tissues (smooth and striated muscle, blood vessels, fat, fibrous tissue) and also in melanocytes, mesothelial cells, meningothelial cells, and lymphoid cells. Vim en tin is the only consistent reactant in Ewing's sarcoma; cytokeratin is infrequently expressed in Ewing's sarcoma. 70 Neuron-specific enolase reactivity in Ewing's sarcomas has served as the basis to support the neuralcrest origin for this enigmatic neoplasm of bone and soft tissues. 43. 92. 93 The cell membrane-associated glycoprotein, leukocyte common antigen, is present on the surface of all leukocytes (B and T lymphocytes, granulocytes, and monocytes-histiocytes), but it is not uniformly expressed by ReedSternberg cells. 65 When the differential diagnosis has been correctly formulated, the results of these immunostains should provide the desired answers in the majority of cases (see Fig. 4). To illustrate the value of immunohistochemistry in the diagnosis of several perplexing small cell tumors and to demonstrate the importance of devising the appropriate differential diagnosis on the basis of light microscopic findings, the follOwing cases are presented. Case 1. A 5-year-old female, previously in good health, presented with anorexia, lethargy, and increasing abdominal girth of several weeks' duration. Three days before admission, she became icteric. A chest roentgenogram revealed multiple nodules in both lung fields. The CT scan showed pleural, pericardial, and peritoneal effusions and a 4-cm extrahepatic mass obstructing the common bile duct. An open lung biopsy was performed. Compact neoplastic nodules with a bronchilocentric orientation partially replaced the pulmonary parenchyma. The tumor cells were small, oval to spindle-shaped, and had hyperchromatic nuclei; a distinctly myxoid background loosely separated the small spindle cells in some foci (Fig. 5). These areas contained cells with provocative rhabdomyoblastic features; this impression was confirmed with immunoreactivity for muscle-specific actin and des min (Fig. 6). Tumor cells with abundant intermediate filaments and focal condensations suggestive of Z-band material were detected by electron microscopy. It was concluded that this child had metastatic embryonal rhabdomyosarcoma, the probable primary site being the common bile duct.

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Figure 5. Embryonal rhabdomyosarcoma presenting with pulmonary metastases, showing loosely and compactly arranged small hyperchromatic cells around a bronchiole in the lung periphery. (Magnification x 160.)

Figure 6. Embryonal rhabdomyosarcoma, showing cytoplasmic immunoreactivity for muscle-specific actin. (Magnification x 400.)

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Table 3. Immunohistochemical Reactivity of Uniform Round Cell and Epithelioid Cell Neoplasms of Childhood IMMUNOHISTOCHEMICAL REACTION TUMOR TYPE

Ewing's sarcoma Epithelioid sarcoma Epithelioid hemangioma Melanoma of soft parts* Chordoma Undifferentiated carcinoma Large cell lymphoma

CK

VIM

±

+ + + + +

+ + +

±

+

EMA

+ + +

F-VIII

HMB-45

+

UEA

LCA

5-100

+ +

+ ±

+

*Formerly clear cell sarcoma of tendon sheath Abbreviations: CK = cytokeratin, VIM = vimentin, EMA epithelial membrane antigen, F-VIII = factor VIII-related antigen, HMB-45 = melanocytic antibody, UEA = Ulex europaeus, LCA = leukocyte common antigen, S-100 = S-100 protein.

Case 2. A 3-week-old male presented with failure to thrive and ascites. There were no major laboratory abnormalities. A CT scan failed to disclose any thoracic or abdominal mass, and an exploratory laparotomy disclosed diffuse peritoneal thickening and multiple nodules on the peritoneum. A primary site was not apparent after a thorough exploration. Multiple biopsies were obtained and showed an undifferentiated small cell neoplasm with solid blastematous-appearing nests that alternated with dispersed spindle cells in a myxoid background. Some similarities to the histologic appearance of the neoplasm in Case 1 were noted, but the tumor cells only reacted for vimentin and the fine structural examination disclosed primitive cells with collections of intermediate filaments in the cytoplasm. Even though an embryonal rhabdomyosarcoma was strongly suspected, there were no objective phenotypic markers. A diagnosis of embryonal or primitive sarcoma was made. The next case is an example of a uniform round cell neoplasm in a child with the differential diagnosis of Ewing's sarcoma or a primitive neuroectodermal tumor on the basis of the light microscopy. An unexpected diagnosis emerged with the assistance of electron microscopy and immunohistochemistry . Case 3. A 15-year-old female presented with a firm mass in the region of the right hip measuring approximately 5 cm in greatest dimension. There was no history of a pigmented cutaneous lesion or lymphadenopathy. ACT scan demonstrated a homogeneous mass in the soft tissues without bony involvement. An incisional biopsy showed formless sheets and nests of uniform round cells with clear cytoplasm and discrete cell borders. The nuclei had vesicular chromatin and a single prominent eosinophilic nucleolus. Areas of coagulative necrosis were present throughout the biopsy. Soft tissue Ewing's sarcoma was initially suspected on biopsy, but the conspicuous nucleolus was an atypical finding; for that reason, the antibody to HMB-45, a melanocytic marker, was applied in addition to the other immunostains (Table 3).42 There was reactivity for vimentin, S-100 protein, and HMB-45, and it was concluded that the diagnosis was melanoma of soft parts (clear cell sarcoma of tendon sheath) (Fig. 7).18 Electron micros-

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Figure 7. Melanoma of soft parts, showing strong cytoplasmic immunoreactivity for vimentin (left) and HMB-45 (right). (Magnification x 400.)

copy confirmed the presence of premelanosomes and melanosomes in the cytoplasm. Chordoma, an uncommon neoplasm in childhood, may also mimic the histologic features of Ewing's sarcoma, malignant melanoma, epithelioid sarcoma, and even malignant fibrous histiocytoma. 88 Electron Microscopy Electron microscopy should not be forgotten by any means in the diagnostic evaluation of undifferentiated tumors in children. 30 Because of the limits on the specificity of some immunostains, especially neuron specific enolase, the fine structural features of most small cell tumors are sufficiently characteristic that it is possible to differentiate the principal types by eleCtron microscopy (see Table 2). Subcellular Studies Molecular genetics and cytogenetics of pediatric tumors have almost attained the status of standard diagnostic techniques. 3, 9, 11, 16,45,63 Oncogene amplification and its clinical application are well documented for childhood neuroblastoma with the correlation between N -myc amplification and the clinical stage of the tumor. 10, 11. 20 Ploidy analysis by flow cytometry has added another prognostic parameter in acute leukemia, neuroblastoma, and Wilms tumor. 38, 45, 86 Some of these studies are not available in every clinical laboratory, Diagnostic Problems Other Than Tumor Differentiation Most pathologists encounter questions on a regular basis about the benign or malignant nature of a particular proliferation. An atypical follicular and parafollicular reaction in a lymph node is one example and whether there is fibroblastic and myofibroblastic proliferation in the soft tissues is another. In the former case, the issue is one of malignant lymphoma; in the latter, a sarcoma must be differentiated from nodular fasciitis, fibro-

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matosis, or fibrous histiocytoma. These cases generally are resolved on the basis of the pathologist's experience or by consultation. For some pathologic entities, such as the inflammatory myofibroblastic tumors or inflammatory pseudotumor of the lung, omentum, mesentery, and liver, there is no consensus at the present time about the reactive or neoplastic nature of the spindle cell proliferation. 28 . Another area of some difficulty in pathologic interpretation is the cellular, mitotically active, and/or immature-appearing neoplasm in a child. Because these histologic features in an adult neoplasm are generally regarded as evidence for a malignancy, the extrapolation is logically transferred to the pediatric tumor except for the fact that experience has taught us that the congenital-infantile fibrosarcoma, congenital leiomyosarcoma, immature teratoma, infantile hemangioendothelioma, and even endodermal sinus tumor do not behave as high-grade malignancies in the young patient. For instance, it is generally acknowledged that the infantile endodermal sinus tumor of the testis is far more innocuous than its counterpart later in childhood. 26 Vascular neoplasms are another example of extremely cellular lesions with complicated papillary and anastomosing patterns that rarely behave as an angiosarcoma with metastases to the lung and bones. 1 Infantile hemangioendothelioma of the liver has very disturbing microscopic features in some cases, yet this neoplasm often undergoes spontaneous regression. The gonadal and extra-gonadal teratomas in infants are partially or totally composed of immature somatic tissues of a neuroepithelial type forming primitive neural tubules and neuroblast-like cells. 26 In the ovary, immature neuroepithelium and its quantity have served as the basis for the pathologic grading of the immature or malignant teratoma. 58 A comparable system of grading has been applied to extragonadal teratomas in the sacrococcygeal region, retroperitoneum, and head and neck in children under 2 years of age with marginal prognostic significance. 26 If the immaturity or maturity of the teratoma is equivalent to that of the patient, clinical concern about the prognosis can be diminished accordingly. The spindle cell neoplasms of the soft tissues or the viscera (lung, intestinal tract) presenting at birth or within the first year of life are variably diagnosed as fibrosarcoma, hemangiopericytoma, or leiomyosarcoma. 19 A pathologic interpretation of a sarcoma is appropriate morphologically because these tumors are very cellular and mitotically active but anaplasia is absent. Despite these unsettling histologic features, the prognosis is excellent in the vast majority of cases. 19, 28 A comparison of the prognosis of fibrosarcomas and leiomyosarcomas in adults and children discloses a significant difference in the outcome with an 80 to 90 per cent survival in the pediatric age population and 50 per cent or less in adults. Our experience has shown that most spindle cell sarcomas in children behave as their adult counterparts after 5 years of age. A proliferative lesion noted at birth that may have dire consequences to the patient either early in life or later is the congenital melanocytic nevus. The risk of malignant transformation is correlated with the size of the lesion; the "giant" congenital nevus may undergo malignant change in 5 to 40 per cent of cases. 28 Melanocytic dysplasia with highly atypical cytologic features is one of several findings in a congenital nevus. More

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often than not, the malignant component has an undifferentiated or sarcomatous appearance, resembling a neurogenic sarcoma rather than a typical melanoma. UPDATE ON MAJOR TUMOR TYPES IN CHILDREN This section can only highlight some of the more important trends and observations that have appeared in the literature in the past few years~ Other sources should be consulted for more detailed discussions. 28. 97 Neuroblastoma and Other Neuroectodermal Tumors Classic neuroblastoma is one of the most familiar childhood neoplasms, but in the past several years a presumably related group of small cell tumors, the peripheral neuroepitheliomas or primitive neuroectodermal tumors, have been reported in the soft tissues and bone.23. 62. 84. 88, 92 The so-called Askin's tumor of the chest wall (malignant small cell tumor of the thoracopulmonary region) is the most widely recognized example of the latter tumor type. 4 These tumors rarely differentiate beyond the formation of primitive neurites that are only identifiable by electron microscopy, which is in contrast to the more advanced differentiation seen in most classic neuroblastomas. In the prognostic assessment of a classic neuroblastoma, the histologic grade of the tumor has often been considered in the past. However, earlier clinicopathologic studies have yielded equivocal and even contradictory results on the grading of neuroblastomas. 24 A recent study by Shimada et al87 is innovative in its approach in that it incorporates an established nonmorphologic parameter of prognosis (age at diagnosis) and several histologic features (presence or absence of stroma, differentiation, mitoses, and karyorrhexis) into an integrated scheme that has yielded some promising preliminary results (Fig. 8), Whether this system and any other will eventually become obsolete when oncogene amplification is routinely determined on neuroblastomas must await future studies. 53 Wilms' Tumor and Congeners The classification of Wilms' tumors has undergone a dramatic revision in the last decade based upon the clinicopathologic studies of the National Wilms' Tumor Study (NWTS).6 Even before the histopathologic results from the NWTS, there were already suggestions that not all primary intrarenal tumors of childhood were typical Wilms' tumors. This revelation began with the redefinition of the "monomorphic spindle cell Wilms' tumor of infancy" as the congenital mesoblastic nephroma. This observation was followed by reports of a variant of congenital mesoblastic nephroma, the cellular mesoblastic nephroma, and the cystic nephroma or partially differentiated cystic Wilms' tumor. The latter tumor is closely related to the socalled unilateral multilocular cyst. The cellular mesoblastic nephroma may manifest locally aggressive behaviQr if incompletely excised or even metastasize in rare instances. . Nodular renal blastema and nephroblastomatosis were not only defined

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Figure 8. Classic neuroblastoma, showing a stroma-poor, low mitotic-karyorrhetic index pattern (left) and a stroma-rich, low mitotic-karyorrhetic index pattern (right). Although Homer-Wright rosettes are numerous in the tumor on the left and ganglion cells are abundant in the tumor on the right, neither of these findings are considered in the Shimada classification. (Magnification x 160.)

as specific morphologic entities but also spawned a histogenetic concept to explain an entire range of developmental neoplastic lesions. 6, 66 Both are recognized in the kidney as isolated pathologic findings or in association with a classic Wilms tumor. Based upon anatomic localization in the kidney and various other histologic features, nephroblastomatosis is divided into a perilobar or interlobar type. The microscopic perilobar rest and nodular renal blastema are the same lesion with predominant blastematous features. Limited data are available on the risk of nodular renal blastema evolving into a clinical Wilms' tumor; the impression is that it is very uncommon, although approximately 15 per cent of Wilms' tumors have microscopic foci of nodular renal blastema. 6 Massive infantile nephroblastomatosis (MIN) is the rarest and most obvious expression of perilobar nephroblastomatosis. A potentially metastasizing Wilms tumor may evolve from MIN. Interlobar nephroblastomatosis is the likely source for those Wilms' tumors arising in the region of the renal pelvis and presenting as a polypoid or botryroid growth. Once adjuvant therapy of Wilms' tumors had achieved the overall favorable results, a pathologic subset representing approximately 11 per cent of cases in the first NWTS accounted for more than 50 per cent of the tumor-related deaths. 6 An analysis of the prognostically unfavorable cases yielded the following categories: classic Wilms tumor with focal or diffuse anaplasia, clear cell sarcoma, and rhabdoid tumor (Fig. 9). Subsequent results have shown that the clinical stage 1 Wilms' tumor with anaplasia has a prognosis similar to its favorable histology counterpart. Improvements in the clinical outcome have occurred for the clear cell sarcoma as well, a

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Figure 9. Wilms' tumor, showing immature tubules, islands of blastema, and fibromyomatous stroma (left). This pattern is regarded as "favorable" histology, but in the same tumor, a focus of anaplasia (right) was identified in the presence of enlarged cells and bizarre mitotic figures. (Magnification x 160 [left], and x 460 [right).)

tumor with the potential for skeletal metastasis; however, the other sarcomatous variant, the rhabdoid tumor, has a very poor prognosis despite vigorous multi-modality therapy. There is agreement that the clear cell sarcoma is a nephrogenically derived neoplasm, but the histogenesis of the rhabdoid tumor is unresolved. One of the perplexing points relates to the fact that extra-renal rhabdoid tumors are described in the heart, soft tissues, and liver. Rather than a histogenetic entity, the rhabdoid tumor is more appropriately considered a distinctive histopathologic entity but with diverse origins analogous to the oncocytoma of the thyroid, salivary gland, liver, and kidney. Another unexplained aspect of the rhabdoid tumor of the kidney is its common association with a second primary neoplasm of the central nervous system with primitive neuroectodermal features. Finally, N-myc oncogene amplification has been reported in Wilms' tumor.75 Malignant Lymphoma, Hodgkin's and non-Hodgkin's Types Many questions have been answered over the last several years about the malignant lymphomas; the B-Iymphoid nature of small noncleaved cell lymphoma (Burkitt's and non-Burkitt's types), and the T-Iymphoid character of small, convoluted cell lymphoma (lymphoblastic lymphoma) are now established. Large cell lymphoma in children, like its adult counterpart, has the cytologic and cell marker features oflarge noncleaved B lymphocytes or immunoblasts. An association between an Epstein-Barr virus infection and atypical B-cell proliferations and immunoblastic lymphoma has been documented in children with primary and secondary immunodeficiency states;36 nuclear antigen or genomic material of Epstein-Barr virus has been demonstrated in B-celllymphomas in these patients. Most T-celllympho-

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mas in the pediatric age population are high-grade lymphoblastic lymphomas; this form oflymphoma is equivalent to acute lymphoblastic leukemia. 33. 99,100 Among the 407 cases of non-Hodgkin's lymphoma submitted to the Children's Cancer Study Group, 136 (33 per cent) were classified as the lymphoblastic type. 55 Cell marker studies have established the phenotype of the various malignant lymphomas, but cytogenetics and molecular genetics have identified specific translocations and gene arrangements in B- and T-cell lymphomas. The cells of a lymphoblastic lymphoma have the phenotype of a cortical thymocyte, and a minority of T-cell lymphomas in children express the phenotype of medullary thymocytes or peripheral Tcells. Cutaneous involvement is one of the typical clinical features of peripheral T -cell lymphomas, but the most common expression of the latter, mycosis fungoides, is rarely diagnosed in childhood. One of the principal difficulties in differential diagnosis in some cases, even in children, is the distinction of peripheral T-cell lymphoma from Hodgkin's disease and malignant histiocytosis. Hodgkin's disease accounts for 25 to 30 per cent of all malignant lymphomas in children; most cases are diagnosed after 10 years of age with the peak age attained in the third decade. 28 Nodular sclerosis Hodgkin's disease is the most common histologic subtype (70 to 80 per cent of cases). There is still no consensus about the nature of the Reed-Sternberg cell; some studies have suggested that the Reed-Sternberg cell is histiocyte, B lymphocyte, or T lymphocyte. Rearrangement of the T-cell receptor gene and the expression of Ki-l, Leu Ml, TIl, and T4 antigens have supported the argument for the T-lymphocyte derivation of Reed-Sternberg cells. 52 If Hodgkin's disease is a T-cell lymphoma, at least one subtype, nodular lymphocyte-predominant Hodgkin's disease, may be a B-celllymphoma. Histiocytic Disorders The mononuclear phagocytes constitute a family of bone marrowderived cells whose morphologic and functional representatives are found in virtually all organs and in the soft tissues. These cells are functionally responsible for processing and presenting antigen to the rest of the immune system and for the digestion of foreign and native organic and inorganic materials. Several types of mononuclear phagocytes are recognized: dendritic reticulum cells, Langerhans' cells (interdigitating cell), histiocyte (macrophage), and the circulating monocyte. A proposed classification of childhood histiocytoses by the Histiocytoses Club includes the following categories: Langerhans' cell histiocytosis (histiocytosis X, eosinophilic granuloma, Hand-Schuller-Christian disease, Letterer-Siwe disease), non-Langerhans' cell histiocytosis (reactive, infectionassociated hemophagocytic syndromes, familial erythrophagocytic lymphohistiocytosis; neoplastic, histiocytic sarcoma, malignant histiocytosis, acute monocytic-monoblastic leukemia). 17, 25, 37, 39, 59 There are presently reservations about the neoplastic nature of Langerhans' cell histiocytosis. Acute, disseminated infantile histiocytosis X or Letterer-Siwe disease has a virulent clinical course, yet the proliferating cells do not have "malignant" cytologic features; nor do these cells have the biologic markers of transformed cells. 28, 37 One of the difficulties in the pathologic diagnosis of Langerhans' cell

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histiocytosis is the objective confirmation that the histiocytic infiltrate in a skin, bone, or lung biopsy is composed of Langerhans' cells; the confirmatory tests are predicated on the demonstration of T6 antigen on the cell surface or on the presence of Birbeck granules in the cytoplasm of the histiocytes by electron microscopy.l7, 51, 73 Langerhans' cells have nuclear and cytoplasmic immunoreactivity for S-100 protein in paraffin sections; this reaction is sensitive because most Langerhans' cells are positive, but the immunoreactivity is not specific for these cells. A major advance in our understanding of the histiocytoses occurred with the separation of the "benign" hemophagocytic syndromes from malignant histiocytosis, Histiocytic medullary reticulosis assumed that all hemophagocytic proliferations with an acute multisystem presentation were clinically and pathologically malignant, More recently, doubt has been cast on the very existence of malignant histiocytosis as a pathologic entity and, with the exception of acute monocytic and monoblastic leukemias, all putative histiocytic malignancies are peripheral T-cell lymphomas. 31 In the French-American-British classification, acute monoblastic and monocytic leukemias are M5a or M5b, depending upon the degree of differentiation. 33 Some children with acute monoblastic or monocytic leukemia present at birth or in early infancy. 21,95 The clinical presentation may be extramedullary in the skin, lymph nodes, soft tissues, and liver. Monoblasts in the paracortex, interfollicular zones of the lymph node, the hepatic sinusoids, portal tracts, and the dermis are the various patterns of involvement. Hepatic Neoplasms Hepatoblastoma is the most common primary malignant tumor of the liver in children. 28, 98 The predominant fetal pattern seems to connote a favorable prognosis, whereas the embryonal and small cell hepatoblastomas have a poor outcome. When nonhepatic divergent tissues are present in a hepatoblastoma, it may be problematic to distinguish it from a teratoma; the designation of teratoid hepatoblastoma has been proposed for these neoplasms. Other tumor types unique to the livers of children are the hemangioendothelioma, undifferentiated sarcoma, and mesenchymal hamartoma. The undifferentiated sarcoma is a highly aggressive small cell, anaplastic neoplasm whose histogenesis is essentially unknown. There is no evidence to support its relationship to the rhabdomyosarcoma despite the histologic similarities. Hepatobiliary rhabdomyosarcoma typically occurs in the common bile duct and presents with obstructive jaundice, fever, and weight loss. Hemangioendothelioma is recognized in the first year of life as single or multifocallesions. Asymptomatic hepatomegaly is the usual clinical presentation, but a minority of infants have a consumptive coagulopathy or high-output congestive heart failure. Most hemangioendotheliomas undergo spontaneous regression through the process of thrombosis and subsequent dystrophic calcification and fibrosis. The histologic pattern is a complicated one; in some instances, it may lead to concern that the hemangioendothelioma has progressed to angiosarcoma, but this is a rare event. A recurrence of a previously resected infantile hemangioendothelioma is worrisome, but it does not imply the development of an angiosarcoma. Hepatocellular carcinoma in children basically occurs as a complication of chronic paren-

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chymal injury with progression to macronodular cirrhosis. Chronic hepatitis B and tyrosinemia are two of the more frequent conditions in childhood associated with cirrhosis and hepatocellular carcinoma. A variant of hepatocellular carcinoma with a more favorable prognosis, fibrolamellar (oncocytic) carcinoma, may present in an adolescent as a solitary mass in an otherwise normal liver rather than the more typical diffusely infiltrating neoplasm in a background of cirrhosis. Hepatic adenoma, focal nodular hyperplasia, and primary malignant lymphoma have been reported on occasion in childhood. 28 Germ Cell Neoplasms and Teratomas Several comprehensive clinicopathologic studies and reviews have appeared on the topic of germ cell-derived neoplasms in children and are summarized elsewhere. 26 With the application of immunohistochemistry, it is currently possible to localize the source of oncofetal antigens in these neoplasms. For instance, alpha-fetoprotein is not only synthesized in foci of endodermal sinus tumor (yolk sac carcinoma) but in immature enteric mucosa and fetal hepatocytes, comparable to the normal-developing fetus. Placental alkaline phosphatase is a sensitive marker for neoplastic primordial germ cells in intratubular germ cell neoplasia and germinomas. One of the major unresolved pathologic problems is the clinical importance of somatic immaturity in this group of neoplasms. 26 Skeletal Neoplasms Osteosarcoma and Ewing's sarcoma together account for the majority of primary sarcomas of bone in children. 77, 78 With the exception of the infrequent well-differentiated and periosteal osteosarcomas, the other types of osteosarcoma are pathologically high-grade neoplasms. 14 Preoperative chemotherapy of osteosaroma is virtually standard practice; this is followed by resection and an assessment of tumor viability based upon the pathologic examination. Rather than extensive necrosis in the tumor, the stroma is hypocellular and hyalinized in those cases with a presumed response to therapy, but the "malignant" osteoid remains architecturally intact. A favorable response to chemotherapy is predicated on 90 per cent or more of the tumor showing the sterilized or inactive appearance. Ewing's sarcoma has been discussed in the context of the small cell tumors of childhood. A curious and unexplained epidemiologic fact is the rarity of Ewing's sarcoma in black children. 78 In contrast to several years ago, surgical resection of Ewing's sarcoma is more widely utilized, especially in those primary sites where function is not appreciably compromised. The results of immunohistochemical and cytogenetic studies have raised some interesting and provocative discussions about the histogenesis of this tumor. Soft Tissue Tumors An overview of soft tissue neoplasms in children by Coffin and Dehner can be consulted for a more detailed discussion. 19 Rhabdomyosarcoma is the most common soft tissue sarcoma of childhood with a discernible phenotype. 69, 91 There are primitive or embryonal sarcomas of indeterminate differentiation with clinical and histologic simi-

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larities to the rhabdomyosarcoma but without reactivity for desmin and muscle-specific actin or fine structural evidence of myogenic differentiation. Despite that fact, the clinical behavior of these tumors and their response to therapy are much like those of embryonal rhabdomyosarcoma. Approximately 65 to 70 per cent of childhood rhabdomyosarcomas are embryonic in type and have the gross appearance of sarcoma botyroides or a solid, circumscribed mass in the soft tissues.28 Several patterns of microscopic growth and cytomorphology of embryonal rhabdomyosarcoma, ranging from uniform small round cells in formless sheets and without a stroma background to a predominant spindle cell proliferation with prominent eosinophilic cytoplasm, are appreciated. When the ovoid to spindleshaped cells have identifiable cytoplasm, cross striations may be present by light microscopy; predictably, the cells are immunoreactive for desmin and muscle-specific actin. Alveolar rhabdomyosarcoma is a more aggressive neoplasm than embryonal rhabdomyosarcoma and occurs in later childhood and adolescence. 28 In a minority of cases, the patient presents with disseminated tumor in bone marrow, lymph nodes, and bone, simulating a leukemia or malignant lymphoma. 46 A nonrandom chromosomal translocation (2;13) has been reported in alveolar rhabdomyosarcoma. 81 A typical biopsy shows a monomorphic population of polygonal cells and individual multinucleated cells in noncohesive aggregates separated by delicate strands of connective tissue. On the other hand, the alveolar pattern is not always apparent; rather, the monomorphic cells are compactly arranged in sheets or nodules. Mixed embryonal and alveolar rhabdomyosarcoma has received little attention in the literature, yet 10 to 15 per cent of childhood rhabdomyosarcomas have composite histologic features. The practice has been to assign the composite rhabdomyosarcoma to one or the other histologic category based upon the predominant pattern. There are no data to determine whether such tumors behave like the prognostically favorable embryonal rhabdomyosarcoma or the less favorable alveolar rhabdomyosarcoma. The other category of soft tissue sarcomas exclusive of the peripheral primitive neuroectodermal tumor is characterized by a spindle cell pattern; synovial sarcoma, fibrosarcoma, leiomyosarcoma, and neurogenic sarcoma are the specific tumor types. Malignant fibrous histiocytoma is also recognized in children and a history of prior irradiation therapy is encountered in some cases. Central Nervous System Tumors This category of primary neoplasms is an important one in the overall context of pediatric oncology, especially in terms of incidence. 78 One measure of the dynamics in this area is the recently proposed classification of primary brain tumors of childhood by Rorke et al. 80 Some controversy has been generated on the diagnostic application of the term primitive neuroectodermal tumor, to medulloblastoma, cerebral neuroblastoma, ependymoblastoma, and pineoblastoma. Needless to say, this proposal has been greeted with less than total enthusiasm by Rubinstein. 82 Immunohistochemistry has contributed more to the debate than its resolution by the

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phenotypic identification of diverse neuroepithelial elements in the less mature, more primitive tumors.23 One category of primary brain tumors whose diagnosis has been facilitated by immunoperoxidase studies is the germ cell neoplasms, notably the pineal and hypothalamic germinomas and endodermal sinus tumors.83 For a general discussion of brain tumors in children from the viewpoint of a large institution, the reader is directed to Becker and Halliday.s Retinoblastoma and Other Ocular-Orbital Neoplasms Despite the fact that retinoblastoma is a rare childhood neoplasm (1 in 18,000 live births), this has not diminished the progress in the delineation of its histogenesis and pathogenesis. 78 There are three basic expressions of the disease: hereditary, nonhereditary, and chromosomal deletion types. 79 It is estimated that 2 to 3 per cent of patients have the 13-deletion form (13qI4). The nonhereditary or sporadic (unilateral) form appears to account for 95 per cent of all cases, but among these children, 15 per cent may have hereditary retinoblastoma. Approximately 5 per cent of affected children have bilateral tumors and autosomal dominant inheritance. Murphree and Benedice r have discussed the role of recessive cancer genes to explain not only the retinoblastoma but also the increased risk for a second primary malignancy in particular osteosarcoma. There is also the association of bilateral retinoblastomas and pineo blastoma (so-called trilateral retinoblastoma). Pathologically, most retinoblastomas are poorly differentiated small cell tumors with an immunophenotype similar to neuroblastoma, because these tumors are derived from primitive neuroepithelium of the retina. The other intraocular neoplasm of childhood is the medulloepithelioma, which is far less common than the retinoblastoma. In contrast to the posterior localization of the retinoblastoma, the medulloepithelioma originates in the region of the ciliary body. Immature neurotubular structures in a hyaline matrix are the basic microscopic findings. Flexner-Wintersteiner rosettes of retinoblastoma are not present in this tumor. Hemangioma and embryonal rhabdomyosarcoma are respectively the principal orbital neoplasms of childhood. Secondary involvement and metastasis to the eye and orbit are reported with acute leukemia (myeloid) and neuroblastoma.

SUMMARY The diagnosis of childhood malignancy is a complex endeavor that requires the cooperation of the clinician and pathologist. On occasion, potentially problematic situations arise when a biopsy cannot answer all of the questions or is nondiagnostic. Delays may occur before a final diagnosis is possible in an especially complicated case that requires several timeconsuming studies. The pathologist should appreciate the pressure placed on the clinician to initiate treatment and, in turn, the clinician should realize that several steps are necessary before the pathologic examination can be completed.

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