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Basal cell adenomas of the parotid gland: CT scan features
European Journal of Radiology 58 (2006) 260–265
Basal cell adenomas of the parotid gland: CT scan features Ashish J. Chawla, Tiong Yong Tan, Gerald J.S. Tan ∗ Department of Radiology, Changi General Hospital, 2 Simei Street 3, Singapore 529889, Singapore Received 18 September 2005; received in revised form 30 November 2005; accepted 2 December 2005
Abstract Basal cell adenoma (BCA) is a rare tumor of the parotid gland, and except for a few case reports, the imaging features of this pathological entity are not well described. We describe the computed tomography (CT) features of 14 cases of histologically proven BCA, the largest series to date. In all cases, the tumor appeared as a round or oval, sharply marginated mass. Three cases showed homogenous enhancement. In the other 11 cases, the enhancement was inhomogeneous, due to either cystic areas, linear bands or stellate-shaped areas of non-enhancement. The latter two patterns have not been previously described in BCA or other parotid tumors, and may therefore aid in distinguishing BCA from other benign parotid neoplasms on CT. © 2005 Elsevier Ireland Ltd. All rights reserved. Keywords: Basal cell adenoma; Basaloid monomorphic adenoma; Pleomorphic adenoma; Warthin’s tumor; Parotid gland; Computed tomography
1. Introduction Basal cell adenomas (BCAs) are uncommon benign tumors of the salivary gland. Histologically, these tumors are distinguished from pleomorphic adenoma by the presence of a uniform epithelial pattern and the absence of chondromyxoid stroma [1–5]. BCAs account for about 2% of salivary gland tumors and most of these are found in the parotid gland. The use of computed tomography (CT) for the assessment of parotid tumors is well established. Whereas much has been written on the CT appearance of the pleomorphic adenomas and Warthin’s tumors [6–12], the detailed imaging features of BCAs of parotid gland have not previously been described in the English language literature. In this study, we describe the CT appearance of 14 histologically proven cases of basal cell adenomas of the parotid gland.
2. Materials and methods A search through the histopathology records in our institution, revealed 20 histopathologically confirmed cases of ∗
Corresponding author. Tel.: +65 685 048 46; fax: +65 626 017 03. E-mail address: [email protected] (G.J.S. Tan).
0720-048X/$ – see front matter © 2005 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.ejrad.2005.12.001
BCAs of the parotid gland between January 1996 and October 2004, out of a total of 632 parotid tumors. Fourteen of these cases, which had a pre-operative CT scan, formed our study population. All of these patients underwent surgery; either superficial or total parotidectomy, depending upon the location of the lesion. The patients were scanned in GE Hispeed spiral CT/i scanner or TOSHIBA Aquilion multislice CT SCAN SYSTEM. All the cases were imaged with 3 mm contiguous slice thickness; scan level from the external auditory canal to the hyoid bone, with the plane of scan parallel to the hard palate. On the GE scanner, images were acquired at 120 kV and 180 mA with a pitch of 1 mm and 1 s scan time, followed by reconstruction using the soft tissue algorithm. On the four-slice Toshiba system, images were obtained using 135 kV and 250 mA with a pitch of 4.5, scan time of 1 s, and distance of 7 mm per rotation. The FC 10 soft tissue algorithm was used. Bolus intravenous dose of 75 ml of non-ionic contrast (350 mg I/ml) was given to all the patients at the rate of 1.5 ml per second. Scanning was initiated 45 s after the onset of contrast injection. Non-enhanced and delayed scans were not performed. A senior qualified head and neck radiologist retrospectively reviewed the CT images. The tumors were categorized in terms of location, size, margins, contour, enhancement and enhancement pattern.
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Tumor location was divided into superficial and deep lobes of the parotid. Size was expressed in terms of maximal diameter measured to the nearest millimeter. A tumor was considered to have ‘sharp margins’ if it was well demarcated from the rest of the parotid gland throughout; otherwise it would be considered to have ‘indistinct margins’. A ‘lobulated’ tumor was one showing an undulated margin with indentations or sulci; as opposed to a ‘smooth’ tumor where such indentations were lacking. Although pre-contrast images were not acquired, the tumors were presumed to be enhancing if they appeared significantly brighter than adjacent parotid parenchyma and ipsilateral muscles on visual comparison, and showed CT density of more than 100 HU. The enhancement patterns were divided into homogenous (uniform) and inhomogeneous (non-uniform). The non-homogenously enhancing tumors could be further subdivided into three more categories. These were the linear band, stellate-shaped and cystic area patterns of non-enhancement. A lesion having a central hypodense area with three or more thin, outwardradiating spokes was termed ‘stellate-shaped’, while a ‘cystic area’ was defined as having a CT scan density of 20 HU or less, and having a round or ovoid shape. The contralateral parotid gland was also evaluated for multifocal lesions. Fig. 1. Small BCA that was found incidentally during parotidectomy for sialometaplasia (black arrow). Note the stellate-shaped non-enhancing area (small white arrow).
3. Results Table 1 shows the gender distribution, age and the radiological features of the BCAs. Of the 14 cases, 10 patients were female and 4 were males with age between 33 and 68 years (mean age of 58.2 years). Eleven patients presented with a parotid swelling, two with pain and one as an incidental surgical finding. This last patient presented with a 3 cm cystic parotid lesion, which was diagnosed as sialometaplasia on histology. However, CT scan demonstrated another small (8 mm) lesion in the same gland, initially thought to represent an intraparotid lymph node, but was later found on histological to be a co-existing BCA (Fig. 1).
The BCA was located in superficial lobe of parotid gland in 12 patients and in the deep lobe in 2 patients (Figs. 2 and 3). It was solitary and unilateral in all our patients. The size of the tumor ranged from 0.8 to 3 cm with average size of 1.9 cm. All the tumors showed sharp margins, with lobulations in three patients. The tumor showed homogenous enhancement in three cases (21%) (Fig. 4). The remaining 11 of the 14 tumors (79%) showed inhomogeneous enhancement, with stellateshaped non-enhancing area in three tumors (Figs. 1, 3 and 5), linear non-enhancing bands in three (Fig. 6) and cystic areas
Table 1 Case no.
Gender/age (year)
Location
Size (mm)
Margins
Contour
Enhancement
Comments
1 2 3 4 5 6 7 8 9 10 11 12 13 14
M/50 F/67 M/62 F/39 M/45 F/64 F/61 F/37 M/64 F/51 F/33 F/61 F/50 F/68
Superficial lobe Deep lobe Superficial lobe Superficial lobe Deep lobe Superficial lobe Superficial lobe Superficial lobe Superficial lobe Superficial lobe Superficial lobe Superficial lobe Superficial lobe Superficial lobe
30 25 20 18 8 22 15 11 20 15 8 17 13 26
Sharp Sharp Sharp Sharp Sharp Sharp Sharp Sharp Sharp Sharp Sharp Sharp Sharp Sharp
Smooth Smooth Smooth Lobulated Lobulated Smooth Lobulated Smooth Smooth Smooth Smooth Smooth Smooth Smooth
Inhomogeneous Inhomogeneous Inhomogeneous Inhomogeneous Inhomogeneous Inhomogeneous Inhomogeneous Homogeneous Inhomogeneous Homogeneous Homogeneous Inhomogeneous Inhomogeneous Inhomogeneous
Linear non-enhancing band Cystic areas Stellate-shaped low density area Stellate-shaped low density area Stellate-shaped low density area Cystic areas Cystic area Cystic area
Linear non-enhancing band Linear non-enhancing band Cystic area with enhancing nodules
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Fig. 2. White arrow points to a deep lobe BCA in the left parotid gland. Note the cystic component (white asterisk) and the enhancing portion (black cross).
in five (Figs. 2 and 7). One patient had an enhancing nodule within the cystic area (Fig. 8). None of the tumors showed presence of calcification. All patients underwent surgery. The histopathologic diagnosis of basal cell adenoma was made in all 14 cases. Since this was a retrospective study, the gross pathological specimens were no longer available at the time of this study. We made an attempt to correlate the appearance of the tumors on CT scan with the microscopic specimens that were available to us, although we agree that this was not the best way of comparison. In those tumors showing stellate non-enhancing areas, the microscopic specimens showed collagen bands extending from the tumor capsule into the substance of the
Fig. 3. White arrow points to a deep lobe BCA of the left parotid gland. Note the stellate-shaped non-enhancing area (small black arrow).
Fig. 4. White arrow points to a homogenously enhancing BCA with smooth margins in the superficial lobe of the right parotid gland.
mass. We concluded that these collagen bands would most likely be responsible for the non-enhancing stellate areas on the CT scan (Fig. 5). For those tumors showing band-like non-enhancing areas, the microscopic specimen showed large
Fig. 5. Enhancing BCA with a lobulated margin in the superficial lobe of the left parotid gland (white arrow). Small black arrow points to the stellateshaped non-enhancing area.
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Fig. 6. Enhancing BCA with lobulated margin in the superficial lobe of the left parotid gland (black asterisk). Arrow points to the non-enhancing linear band.
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Fig. 8. Arrow points to a BCA with smooth margin in the superficial lobe of the left parotid gland. It contains a cystic component with a mural nodule (black asterisk).
hemorrhagic areas within the tumors. This led us to conclude that these hemorrhagic areas were likely to be responsible for the band-like non-enhancing areas on the CT scan (Fig. 6). The tumors with cystic areas in the CT scan also had corresponding cystic areas in the microscopic specimen.
4. Discussion
Fig. 7. Arrow points to a smooth margin BCA with a cystic component (white asterisk) in the superficial lobe of the right parotid gland.
Benign adenomas account for 65.5% of salivary gland tumors [13]. Basal cell adenomas were recognized as a histologically distinct entity in 1991 by the World Health Organization (WHO) [14]. An incidence of 1–7% of all primary salivary gland tumors is often cited [1]. The incidence of 3.2% in our hospital falls within this range. BCA is a benign tumor composed of basaloid cells sharply delineated from the stroma by basement membrane. A mesenchymal component or chondromyxoid stroma should, by definition, be absent [2–5]. Other terms that have been associated with this tumor include tubular adenoma, trabecular adenoma, dermal “analogue” tumor, cancalicular adenoma, basaloid adenoma, clear cell adenoma and monomorphic adenoma [1]. “Basal cell” adenoma and basaloid monomorphic adenoma are the most widely used terms for this subset of adenomas. The pattern of growth may be tubular, trabecular or solid. Although it is most often found in females over 60 years of age, there is a wide age distribution from 32 to 87 years, and BCA should be considered as a differential, even in the persons in the 8th and 9th decades of life [3]. It typically presents as a solitary, slowly growing, otherwise asymptomatic mass, usually in the parotid or the minor salivary gland of the
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upper lip [1,3,5,15]. They have also been described in the submandibular gland and within periparotid or intraparotid lymph nodes [1–3]. Basal cell adenoma may rarely undergo malignant cell transformation (4%), giving rise to basaloid cell carcinomas (basal cell adenocarcinoma, adenoid cystic carcinoma) and, less commonly, non-basaloid carcinoma (salivary duct carcinoma, adenocarcinoma) [2,15]. Surgical excision is the treatment of choice. Clinically, BCAs behave similarly to other benign parotid tumors, such as pleomorphic adenomas and Warthin’s tumors. However, unlike pleomorphic adenomas, which have a recurrence rate of up to 32.5% after surgical resection due to pseudopodial extensions, recurrence in BCAs is rare, presumably due to the absence of such pseudopodia [16]. The exception to the low recurrence rate is the rare membranous (also known as the dermal analogue) subtype, where there is a post-resection recurrence rate of up to 25% [3]. The reason is that this subtype, which forms 10% of BCAs, is often multicentric, multilobular and non-encapsulated, resulting in a higher probability of incomplete surgical resection. The malignant transformation rate of the membranous subtype is also higher, at 28%, compared with the overall malignant transformation rate of BCAs, which is 4% [3]. Imaging features of BCA are rarely reported in the English literature because of their low prevalence. Previously reported cases include Suzuki et al., who reported a case of bilateral parotid BCAs [17]. The adenoma in the right parotid gland was well-circumscribed and showed homogenous enhancement, while that on the left was predominantly cystic with calcification in the wall. In our series, cystic areas were seen in 5 out of 14 cases, although none showed calcification. Kiyosue et al. first reported the MRI findings in BCA of the parotid gland [18]. He described four cases, all of which were well-circumscribed with a rounded contour. Two of these tumors were predominantly cystic and the rest entirely solid. The solid part of the tumor exhibited lower intensity than that of the surrounding parotid tissue on both T1- and T2weighted images, and demonstrated moderate enhancement on contrast scan. Takeshita et al. also described CT and MRI features in a case of BCA of the parotid gland [19]. The BCA showed homogenous moderate enhancement on contrast-enhanced CT. Capsule-like enhancement was also seen. On MRI, the adenoma was homogenously isointense to muscle on T1-weighted images. On T2-weighted images, it showed homogenous moderate intensity, higher than that of muscle, but lower than that of surrounding parotid tissue. The adenoma demonstrated homogenous moderate enhancement on contrast-enhanced T1-weighted images, with capsule-like enhancement similar to that seen on the CT. Jang et al. reported CT and MRI features in three cases of BCA [13]. The first was a well defined and homogenously enhancing mass within the parotid gland on CT scan. The second showed a cystic mass in the deep lobe of the parotid gland
with enhancing anterior and posterior mural nodules. MRI imaging of this case revealed a well-marginated mass with peripheral solid and central cystic component. The solid component was slightly hypointense/hyperintense compared with the muscle on T1- and T2-weighted images, respectively. In the third case, CT scan showed a well-demarcated mass in the deep lobe of parotid gland that protruded to the superficial lobe. The mass showed homogenous enhancement. The common feature in cases reported by all authors is the presence of contrast enhancement in a well-marginated mass. Our study showed inhomogeneous enhancement in 11 out of 14 cases. Five out of 11 cases demonstrated cystic areas within the tumor, with one case showing an enhancing mural nodule within the cystic area. In addition, we described the presence of non-enhancing linear band in three cases and stellate areas in three cases. These findings have not been previously described in any BCA, nor are they characteristic of any other parotid tumor. By correlating the CT appearance and the microscopic specimens, we postulate that the non-enhancing linear bands on CT were most likely due to hemorrhagic areas within the tumors. This is in agreement with the case report by Yerli et al., which showed that the non-enhancing portion of the tumor on CT scan corresponded to an area of hemorrhage on MRI [20]. We also concluded that the stellate areas in some of our cases were most likely due to collagen bands extending from the capsule into the substances of the tumor. From the standpoint of the CT scan findings, the most important differential diagnoses for BCA of the parotid gland include benign pleomorphic adenoma and Warthin’s tumor. The CT features of these two more common tumors have been described in detail in the literature. An intraparotid mass with a lobulated contour favors a pleomorphic adenoma [10]. Pleomorphic adenomas also show little or no enhancement in the immediate post-contrast scan but manifest increased enhancement in the delayed scan [11,12]. Warthin’s tumors show enhancement in the immediate post-contrast scan but decreased enhancement in the delayed scan [12]. Radiological demonstration of cyst formation is more common in Warthin’s tumor [12]. Moreover, Warthin’s tumor is more common in elderly men than women and can be multifocal and bilateral in 10–15% of patients [21,22]. There is some overlap between the CT features of BCA and these two tumors, limiting the ability to predict the histopathologic typing on imaging [10]. However, there are certain characteristics CT features that may help in differentiation. As opposed to pleomorphic adenomas, BCAs enhance well in the immediate post-contrast scan, and this feature can be used to differentiate BCA from pleomorphic adenoma. Yerli also reported a case of BCA showing early enhancement with gradual washout of contrast. Delayed scanning could therefore be another useful feature in differentiating BCAs from pleomorphic adenomas. Delayed images can also help to distinguish BCAs from malignant tumors, 95% of which do not show decreased attenuation in the delayed phase
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[12]. Radiologically, BCAs share some CT scan features with Warthin’s tumor, such as similar enhancement characteristics, well-circumscribed margins and the presence of cystic areas. However, a solitary mass, with above CT scan features, within the parotid gland in an elderly woman, will favor the diagnosis of BCA over a Warthin’s tumor. Furthermore, the presence of non-enhancing stellate shape area or linear band could be a distinguishing feature of BCA.
5. Conclusion On CT scan, BCAs present as well-circumscribed, enhancing mass lesions with smooth or lobulated margins. They may contain non-enhancing areas of linear bands, stellate or cystic configuration. These imaging features allow BCAs to be differentiated from pleomorphic adenomas on most occasions. While BCAs may appear identical to Warthin’s tumors on CT scan, they can still be distinguished based on patient’s demographic characteristics in some cases. The presence of linear band or stellate-shaped non-enhancing areas might be a useful specific imaging feature of BCAs.
References [1] Ellis GL, Auclair PL. Tumors of the salivary glands, third series, Fascicle 17. In: Atlas of tumor pathology. Washington, DC: Armed Forces Institute of Pathology; 1996. p. 80–94. [2] Rosai J. Major and minor salivary glands. In: Rosai J, editor. Ackerman’s surgical pathology, vol. 1. Mosby-year Book, Inc.; 1996. p. 830. [3] Luna MA. Salivary glands. In: Pilch BZ, editor. Head and neck surgical pathology. Lippincott Williams & Wilkins; 2001. p. 312–5. [4] Batasakis JG, Luna MA, Ei-Naggar AK. Pathology consultation: basaloid monomorphic adenomas. Ann Otol Rhinol Laryngol 1991;100:687–90. [5] Ehab YH, James YS. Neoplasms of the salivary glands. In: Charles WC, John MF, Lee AH, Charles JK, Mark AR, David ES, editors. Otolaryngology Head & Neck Surgery, vol. 2. Mosby-year Book, Inc.;1998. p. 1267–68.
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[6] Bryan RN, Miller RH, Ferreyro RI, Sessions RB. Computed tomography of the major salivary glands. Am J Roentgenol 1982;139:547–54. [7] Casselman JW, Mancuso AA. Major salivary gland masses: comparison of MR imaging and CT. Radiology 1987;165:183–9. [8] Sone S, Higashihara T, Morimoto S, et al. CT of parotid tumors. Am J Neuroradiol 1982;3:143–7. [9] Golding S. Computed tomography in the diagnosis of parotid gland tumors. Br J Radiol 1982;55:182–8. [10] Silvers AR, Som PS. Salivary glands. Radiol Clin North Am 1998;36:941–66. [11] Lev MH, Khanduja K, Morris PP, Curtin HD. Parotid pleomorphic adenomas: delayed CT enhancement. Am J Neuroradiol 1998;19:1835–9. [12] Choi DS, Na DG, Byun HS, et al. Salivary gland tumors: evaluation with two-phase helical CT. Radiology 2000;214:231–6. [13] Jang M, Park D, Lee SR, et al. Basal cell adenoma in the parotid gland: CT and MR findings. Am J Neuroradiol 2004;25:631–5. [14] Seifert G, Sobin LH. Histological typing of salivary gland tumors. In: World Health Organization International Histological Classification of Tumors. 2nd ed. Berlin: Springer-Verlag; 1991. p. 20–21. [15] Cheuk W, Chan JKC. Salivary gland tumors. In: Fletcher CDM, editor. Diagnostic histopathology of tumors, vol. 1. Churchill Livingstone; 2000. p. 245–7. [16] Phillips PP, Olsen KD. Recurrent pleomorphic adenoma of the parotid gland: report of 126 cases and a review of the literature. Ann Otol Rhinol Laryngol 1995;104(2):100–4. [17] Suzuki S, Okamura H, Ohtani I. Bilateral parotid gland basal cell adenomas case report. ORL J Otorhinolaryngol 2000;62:278–81; Otolaryngology Head and Neck Surgery, vol. 2. Mosby-year Book, Inc.; 1998. p. 1267–1268. [18] Kiyosue H, Mori H, Okahara M, Hori Y, Miyake H. Diagnostic imaging of parotid gland tumors. Jpn J Diagn Imaging 2000;20:38– 54. [19] Takeshita T, Tanaka H, Harasawa A, Kaminaga T, Imamura T, Furui S. CT and MRI findings of basal cell adenoma of parotid gland. Rad Med 2004;22:260–4. [20] Yerli H, Teksam M, Aydin E, Coskun M, Ozdemir H, Agildere AM. Basal cell adenoma of the parotid gland: dynamic CT and MRI findings. Br J Radiol 2005;78(931):642–5. [21] Howlett DC, Kesse KW, Hughes DV, Sallomi DF. The role of imaging in the evaluation of parotid disease. Clin Radiol 2002;57:692–701. [22] Eveson JW, Cawson RA. Salivary gland tumors: a review of 2410 cases with particular reference to histological types, site, age and sex distribution. J Pathol 1985;146:51–8.
Basal cell adenomas of the parotid gland: CT scan features Ashish J. Chawla, Tiong Yong Tan, Gerald J.S. Tan ∗ Department of Radiology, Changi General Hospital, 2 Simei Street 3, Singapore 529889, Singapore Received 18 September 2005; received in revised form 30 November 2005; accepted 2 December 2005
Abstract Basal cell adenoma (BCA) is a rare tumor of the parotid gland, and except for a few case reports, the imaging features of this pathological entity are not well described. We describe the computed tomography (CT) features of 14 cases of histologically proven BCA, the largest series to date. In all cases, the tumor appeared as a round or oval, sharply marginated mass. Three cases showed homogenous enhancement. In the other 11 cases, the enhancement was inhomogeneous, due to either cystic areas, linear bands or stellate-shaped areas of non-enhancement. The latter two patterns have not been previously described in BCA or other parotid tumors, and may therefore aid in distinguishing BCA from other benign parotid neoplasms on CT. © 2005 Elsevier Ireland Ltd. All rights reserved. Keywords: Basal cell adenoma; Basaloid monomorphic adenoma; Pleomorphic adenoma; Warthin’s tumor; Parotid gland; Computed tomography
1. Introduction Basal cell adenomas (BCAs) are uncommon benign tumors of the salivary gland. Histologically, these tumors are distinguished from pleomorphic adenoma by the presence of a uniform epithelial pattern and the absence of chondromyxoid stroma [1–5]. BCAs account for about 2% of salivary gland tumors and most of these are found in the parotid gland. The use of computed tomography (CT) for the assessment of parotid tumors is well established. Whereas much has been written on the CT appearance of the pleomorphic adenomas and Warthin’s tumors [6–12], the detailed imaging features of BCAs of parotid gland have not previously been described in the English language literature. In this study, we describe the CT appearance of 14 histologically proven cases of basal cell adenomas of the parotid gland.
2. Materials and methods A search through the histopathology records in our institution, revealed 20 histopathologically confirmed cases of ∗
Corresponding author. Tel.: +65 685 048 46; fax: +65 626 017 03. E-mail address: [email protected] (G.J.S. Tan).
0720-048X/$ – see front matter © 2005 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.ejrad.2005.12.001
BCAs of the parotid gland between January 1996 and October 2004, out of a total of 632 parotid tumors. Fourteen of these cases, which had a pre-operative CT scan, formed our study population. All of these patients underwent surgery; either superficial or total parotidectomy, depending upon the location of the lesion. The patients were scanned in GE Hispeed spiral CT/i scanner or TOSHIBA Aquilion multislice CT SCAN SYSTEM. All the cases were imaged with 3 mm contiguous slice thickness; scan level from the external auditory canal to the hyoid bone, with the plane of scan parallel to the hard palate. On the GE scanner, images were acquired at 120 kV and 180 mA with a pitch of 1 mm and 1 s scan time, followed by reconstruction using the soft tissue algorithm. On the four-slice Toshiba system, images were obtained using 135 kV and 250 mA with a pitch of 4.5, scan time of 1 s, and distance of 7 mm per rotation. The FC 10 soft tissue algorithm was used. Bolus intravenous dose of 75 ml of non-ionic contrast (350 mg I/ml) was given to all the patients at the rate of 1.5 ml per second. Scanning was initiated 45 s after the onset of contrast injection. Non-enhanced and delayed scans were not performed. A senior qualified head and neck radiologist retrospectively reviewed the CT images. The tumors were categorized in terms of location, size, margins, contour, enhancement and enhancement pattern.
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Tumor location was divided into superficial and deep lobes of the parotid. Size was expressed in terms of maximal diameter measured to the nearest millimeter. A tumor was considered to have ‘sharp margins’ if it was well demarcated from the rest of the parotid gland throughout; otherwise it would be considered to have ‘indistinct margins’. A ‘lobulated’ tumor was one showing an undulated margin with indentations or sulci; as opposed to a ‘smooth’ tumor where such indentations were lacking. Although pre-contrast images were not acquired, the tumors were presumed to be enhancing if they appeared significantly brighter than adjacent parotid parenchyma and ipsilateral muscles on visual comparison, and showed CT density of more than 100 HU. The enhancement patterns were divided into homogenous (uniform) and inhomogeneous (non-uniform). The non-homogenously enhancing tumors could be further subdivided into three more categories. These were the linear band, stellate-shaped and cystic area patterns of non-enhancement. A lesion having a central hypodense area with three or more thin, outwardradiating spokes was termed ‘stellate-shaped’, while a ‘cystic area’ was defined as having a CT scan density of 20 HU or less, and having a round or ovoid shape. The contralateral parotid gland was also evaluated for multifocal lesions. Fig. 1. Small BCA that was found incidentally during parotidectomy for sialometaplasia (black arrow). Note the stellate-shaped non-enhancing area (small white arrow).
3. Results Table 1 shows the gender distribution, age and the radiological features of the BCAs. Of the 14 cases, 10 patients were female and 4 were males with age between 33 and 68 years (mean age of 58.2 years). Eleven patients presented with a parotid swelling, two with pain and one as an incidental surgical finding. This last patient presented with a 3 cm cystic parotid lesion, which was diagnosed as sialometaplasia on histology. However, CT scan demonstrated another small (8 mm) lesion in the same gland, initially thought to represent an intraparotid lymph node, but was later found on histological to be a co-existing BCA (Fig. 1).
The BCA was located in superficial lobe of parotid gland in 12 patients and in the deep lobe in 2 patients (Figs. 2 and 3). It was solitary and unilateral in all our patients. The size of the tumor ranged from 0.8 to 3 cm with average size of 1.9 cm. All the tumors showed sharp margins, with lobulations in three patients. The tumor showed homogenous enhancement in three cases (21%) (Fig. 4). The remaining 11 of the 14 tumors (79%) showed inhomogeneous enhancement, with stellateshaped non-enhancing area in three tumors (Figs. 1, 3 and 5), linear non-enhancing bands in three (Fig. 6) and cystic areas
Table 1 Case no.
Gender/age (year)
Location
Size (mm)
Margins
Contour
Enhancement
Comments
1 2 3 4 5 6 7 8 9 10 11 12 13 14
M/50 F/67 M/62 F/39 M/45 F/64 F/61 F/37 M/64 F/51 F/33 F/61 F/50 F/68
Superficial lobe Deep lobe Superficial lobe Superficial lobe Deep lobe Superficial lobe Superficial lobe Superficial lobe Superficial lobe Superficial lobe Superficial lobe Superficial lobe Superficial lobe Superficial lobe
30 25 20 18 8 22 15 11 20 15 8 17 13 26
Sharp Sharp Sharp Sharp Sharp Sharp Sharp Sharp Sharp Sharp Sharp Sharp Sharp Sharp
Smooth Smooth Smooth Lobulated Lobulated Smooth Lobulated Smooth Smooth Smooth Smooth Smooth Smooth Smooth
Inhomogeneous Inhomogeneous Inhomogeneous Inhomogeneous Inhomogeneous Inhomogeneous Inhomogeneous Homogeneous Inhomogeneous Homogeneous Homogeneous Inhomogeneous Inhomogeneous Inhomogeneous
Linear non-enhancing band Cystic areas Stellate-shaped low density area Stellate-shaped low density area Stellate-shaped low density area Cystic areas Cystic area Cystic area
Linear non-enhancing band Linear non-enhancing band Cystic area with enhancing nodules
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Fig. 2. White arrow points to a deep lobe BCA in the left parotid gland. Note the cystic component (white asterisk) and the enhancing portion (black cross).
in five (Figs. 2 and 7). One patient had an enhancing nodule within the cystic area (Fig. 8). None of the tumors showed presence of calcification. All patients underwent surgery. The histopathologic diagnosis of basal cell adenoma was made in all 14 cases. Since this was a retrospective study, the gross pathological specimens were no longer available at the time of this study. We made an attempt to correlate the appearance of the tumors on CT scan with the microscopic specimens that were available to us, although we agree that this was not the best way of comparison. In those tumors showing stellate non-enhancing areas, the microscopic specimens showed collagen bands extending from the tumor capsule into the substance of the
Fig. 3. White arrow points to a deep lobe BCA of the left parotid gland. Note the stellate-shaped non-enhancing area (small black arrow).
Fig. 4. White arrow points to a homogenously enhancing BCA with smooth margins in the superficial lobe of the right parotid gland.
mass. We concluded that these collagen bands would most likely be responsible for the non-enhancing stellate areas on the CT scan (Fig. 5). For those tumors showing band-like non-enhancing areas, the microscopic specimen showed large
Fig. 5. Enhancing BCA with a lobulated margin in the superficial lobe of the left parotid gland (white arrow). Small black arrow points to the stellateshaped non-enhancing area.
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Fig. 6. Enhancing BCA with lobulated margin in the superficial lobe of the left parotid gland (black asterisk). Arrow points to the non-enhancing linear band.
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Fig. 8. Arrow points to a BCA with smooth margin in the superficial lobe of the left parotid gland. It contains a cystic component with a mural nodule (black asterisk).
hemorrhagic areas within the tumors. This led us to conclude that these hemorrhagic areas were likely to be responsible for the band-like non-enhancing areas on the CT scan (Fig. 6). The tumors with cystic areas in the CT scan also had corresponding cystic areas in the microscopic specimen.
4. Discussion
Fig. 7. Arrow points to a smooth margin BCA with a cystic component (white asterisk) in the superficial lobe of the right parotid gland.
Benign adenomas account for 65.5% of salivary gland tumors [13]. Basal cell adenomas were recognized as a histologically distinct entity in 1991 by the World Health Organization (WHO) [14]. An incidence of 1–7% of all primary salivary gland tumors is often cited [1]. The incidence of 3.2% in our hospital falls within this range. BCA is a benign tumor composed of basaloid cells sharply delineated from the stroma by basement membrane. A mesenchymal component or chondromyxoid stroma should, by definition, be absent [2–5]. Other terms that have been associated with this tumor include tubular adenoma, trabecular adenoma, dermal “analogue” tumor, cancalicular adenoma, basaloid adenoma, clear cell adenoma and monomorphic adenoma [1]. “Basal cell” adenoma and basaloid monomorphic adenoma are the most widely used terms for this subset of adenomas. The pattern of growth may be tubular, trabecular or solid. Although it is most often found in females over 60 years of age, there is a wide age distribution from 32 to 87 years, and BCA should be considered as a differential, even in the persons in the 8th and 9th decades of life [3]. It typically presents as a solitary, slowly growing, otherwise asymptomatic mass, usually in the parotid or the minor salivary gland of the
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upper lip [1,3,5,15]. They have also been described in the submandibular gland and within periparotid or intraparotid lymph nodes [1–3]. Basal cell adenoma may rarely undergo malignant cell transformation (4%), giving rise to basaloid cell carcinomas (basal cell adenocarcinoma, adenoid cystic carcinoma) and, less commonly, non-basaloid carcinoma (salivary duct carcinoma, adenocarcinoma) [2,15]. Surgical excision is the treatment of choice. Clinically, BCAs behave similarly to other benign parotid tumors, such as pleomorphic adenomas and Warthin’s tumors. However, unlike pleomorphic adenomas, which have a recurrence rate of up to 32.5% after surgical resection due to pseudopodial extensions, recurrence in BCAs is rare, presumably due to the absence of such pseudopodia [16]. The exception to the low recurrence rate is the rare membranous (also known as the dermal analogue) subtype, where there is a post-resection recurrence rate of up to 25% [3]. The reason is that this subtype, which forms 10% of BCAs, is often multicentric, multilobular and non-encapsulated, resulting in a higher probability of incomplete surgical resection. The malignant transformation rate of the membranous subtype is also higher, at 28%, compared with the overall malignant transformation rate of BCAs, which is 4% [3]. Imaging features of BCA are rarely reported in the English literature because of their low prevalence. Previously reported cases include Suzuki et al., who reported a case of bilateral parotid BCAs [17]. The adenoma in the right parotid gland was well-circumscribed and showed homogenous enhancement, while that on the left was predominantly cystic with calcification in the wall. In our series, cystic areas were seen in 5 out of 14 cases, although none showed calcification. Kiyosue et al. first reported the MRI findings in BCA of the parotid gland [18]. He described four cases, all of which were well-circumscribed with a rounded contour. Two of these tumors were predominantly cystic and the rest entirely solid. The solid part of the tumor exhibited lower intensity than that of the surrounding parotid tissue on both T1- and T2weighted images, and demonstrated moderate enhancement on contrast scan. Takeshita et al. also described CT and MRI features in a case of BCA of the parotid gland [19]. The BCA showed homogenous moderate enhancement on contrast-enhanced CT. Capsule-like enhancement was also seen. On MRI, the adenoma was homogenously isointense to muscle on T1-weighted images. On T2-weighted images, it showed homogenous moderate intensity, higher than that of muscle, but lower than that of surrounding parotid tissue. The adenoma demonstrated homogenous moderate enhancement on contrast-enhanced T1-weighted images, with capsule-like enhancement similar to that seen on the CT. Jang et al. reported CT and MRI features in three cases of BCA [13]. The first was a well defined and homogenously enhancing mass within the parotid gland on CT scan. The second showed a cystic mass in the deep lobe of the parotid gland
with enhancing anterior and posterior mural nodules. MRI imaging of this case revealed a well-marginated mass with peripheral solid and central cystic component. The solid component was slightly hypointense/hyperintense compared with the muscle on T1- and T2-weighted images, respectively. In the third case, CT scan showed a well-demarcated mass in the deep lobe of parotid gland that protruded to the superficial lobe. The mass showed homogenous enhancement. The common feature in cases reported by all authors is the presence of contrast enhancement in a well-marginated mass. Our study showed inhomogeneous enhancement in 11 out of 14 cases. Five out of 11 cases demonstrated cystic areas within the tumor, with one case showing an enhancing mural nodule within the cystic area. In addition, we described the presence of non-enhancing linear band in three cases and stellate areas in three cases. These findings have not been previously described in any BCA, nor are they characteristic of any other parotid tumor. By correlating the CT appearance and the microscopic specimens, we postulate that the non-enhancing linear bands on CT were most likely due to hemorrhagic areas within the tumors. This is in agreement with the case report by Yerli et al., which showed that the non-enhancing portion of the tumor on CT scan corresponded to an area of hemorrhage on MRI [20]. We also concluded that the stellate areas in some of our cases were most likely due to collagen bands extending from the capsule into the substances of the tumor. From the standpoint of the CT scan findings, the most important differential diagnoses for BCA of the parotid gland include benign pleomorphic adenoma and Warthin’s tumor. The CT features of these two more common tumors have been described in detail in the literature. An intraparotid mass with a lobulated contour favors a pleomorphic adenoma [10]. Pleomorphic adenomas also show little or no enhancement in the immediate post-contrast scan but manifest increased enhancement in the delayed scan [11,12]. Warthin’s tumors show enhancement in the immediate post-contrast scan but decreased enhancement in the delayed scan [12]. Radiological demonstration of cyst formation is more common in Warthin’s tumor [12]. Moreover, Warthin’s tumor is more common in elderly men than women and can be multifocal and bilateral in 10–15% of patients [21,22]. There is some overlap between the CT features of BCA and these two tumors, limiting the ability to predict the histopathologic typing on imaging [10]. However, there are certain characteristics CT features that may help in differentiation. As opposed to pleomorphic adenomas, BCAs enhance well in the immediate post-contrast scan, and this feature can be used to differentiate BCA from pleomorphic adenoma. Yerli also reported a case of BCA showing early enhancement with gradual washout of contrast. Delayed scanning could therefore be another useful feature in differentiating BCAs from pleomorphic adenomas. Delayed images can also help to distinguish BCAs from malignant tumors, 95% of which do not show decreased attenuation in the delayed phase
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[12]. Radiologically, BCAs share some CT scan features with Warthin’s tumor, such as similar enhancement characteristics, well-circumscribed margins and the presence of cystic areas. However, a solitary mass, with above CT scan features, within the parotid gland in an elderly woman, will favor the diagnosis of BCA over a Warthin’s tumor. Furthermore, the presence of non-enhancing stellate shape area or linear band could be a distinguishing feature of BCA.
5. Conclusion On CT scan, BCAs present as well-circumscribed, enhancing mass lesions with smooth or lobulated margins. They may contain non-enhancing areas of linear bands, stellate or cystic configuration. These imaging features allow BCAs to be differentiated from pleomorphic adenomas on most occasions. While BCAs may appear identical to Warthin’s tumors on CT scan, they can still be distinguished based on patient’s demographic characteristics in some cases. The presence of linear band or stellate-shaped non-enhancing areas might be a useful specific imaging feature of BCAs.
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