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Fig. 5.16. Membranous basal cell adenoma: jigsaw like pattern: multiple epithelial islands surrounded by large amounts of basal membrane-like material. The latter is also present within the cytoplasm of some of the small dark hyperchromatic basal cells. There is little cellular pleomorphism

Non-membranous BCAs have an equal sex incidence and arise mostly in the major glands. They probably represent part of the spectrum of myoepithelio-ma and pleomorphic adenoma [70, 237]. The tumours are ovoid, well-circumscribed masses in which islands, nests and trabeculae of basaloid cells are each surrounded by a distinct thin PAS-positive basement membrane. The component cells may take two forms -small with scanty cytoplasm and a round, dark nucleus, and larger with amphophilic or eosinophilic cytoplasm and an ovoid paler staining nucleus. These two types are intermixed, but the smaller cells tend to be arranged around the periphery of the nests and trabec-ulae, giving the appearance of palisading. Ductal differentiation may or may not be apparent, but can be highlighted by EMA. There is little pleomorphism and mitotic figures are rare. The stroma varies in amount and cellularity, but S-100 protein-positive spindle cells may be numerous. S-100 positive cells are also present within the islands of epithelial cells, which react strongly with cytokeratins [220].

Membranous BCA (dermal analogue tumour) occurs predominantly in men, and can be multicentric. Most arise in the major glands, including within in-traparotid lymph nodes [128]. Microscopically, they are not encapsulated and appear multinodular, often with a jigsaw-like pattern. The most characteristic feature is the deposition of large amounts of hyaline basement membrane material, which is brightly eosinophilic and PAS-positive. It surrounds the epithelial cell islands and blood vessels, and is present within the islands as small droplets. There is little pleomor-phism or mitotic activity. In about 40% of cases, the salivary adenoma is associated with synchronous and often multiple skin appendage tumours of sweat gland or hair follicle origin, usually cylindromas or eccrine spiradenomas.

The most important differential diagnosis of all types of BCA is adenoid cystic carcinoma. Useful pointers to adenoma include lack of invasiveness and cytological pleomorphism, low mitotic and proliferative activity, and whorled eddies of epithelial cells. S-100 protein pos-itivity of spindled stromal cells may help, as this does not occur in adenoid cystic carcinoma [70]. BCA closely resembles basal cell adenocarcinoma, which may lack cytological pleomorphism and mitotic figures, the diagnosis then depending principally on the presence of genuine invasion (see Sect. 5.9.7).

The recurrence rate for non-membranous BCA is extremely low (0 out of 102 patients in one series) [16], and local excision with clear margins is sufficient treatment. There is a low rate of malignant transformation (about 4%) into basal cell adenocarcinoma [127]. In contrast, up to 24% of membranous BCAs recur after surgery [16], probably reflecting multicentricity and, in addition, malignancy (also as basal cell adenocarcinoma) develops in 28% [127]. Surgery for this subtype needs to be more extensive [16, 119, 130].

5.8.4 Warthin's Tumour

ICD-O:8561/0

Warthin's tumour (WT; adenolymphoma) is the second commonest neoplasm of the parotid gland, and is the easiest salivary tumour to diagnose by microscopy [66]. It arises almost exclusively in the parotid gland (usually the tail) and occasionally in periparotid lymph nodes. The mean age at diagnosis is 62 years (range 29-88), and WT is uncommon in blacks. Previously, there was a marked male predominance (as much as 26:1), but now there is an almost equal sex distribution. It is eight times more frequent in heavy smokers. WT is multicentric in 12% of patients, and bilateral in 6%.

There are two theories of its histogenesis [62] - a true epithelial neoplasm that attracts a heavy lymphoid reaction, or alternatively a non-neoplastic lesion arising from ectopic salivary inclusions in intraparotid lymph nodes. The latter theory is supported by a molecular genetic study using HUMARA analysis, which has shown that WT is not a clonal process [100].

Pathological examination shows a well-circumscribed oval mass, composed of slits or cystic spaces with papillary infoldings lined with two layers of oncocytic epithelium; the inner cells are columnar with nuclear palisading, deep to which are flattened or cuboidal basal cells (Fig. 5.17). Occasional mucous and squamous cells may be seen. The stroma comprises usually plentiful lymphoid tissue with germinal centre formation. Special stains and immunohis-tochemistry have little to offer in practice; myoepi-

Fig. 5.17. Warthin's tumour. Cystic and slit-like spaces with pap- Fig. 5.20. Canalicular adenoma of the upper lip. It is composed illary infolding lined with oncocytic cells. Lymphoid tissue occu- of bi-layered strands of basal-like cells embedded in a loose oede-pies the cores of most papillae matous stroma

Fig. 5.17. Warthin's tumour. Cystic and slit-like spaces with pap- Fig. 5.20. Canalicular adenoma of the upper lip. It is composed illary infolding lined with oncocytic cells. Lymphoid tissue occu- of bi-layered strands of basal-like cells embedded in a loose oede-pies the cores of most papillae matous stroma

Fig. 5.18. Metaplastic (infarcted) Warthin's tumour. There is extensive necrosis. A surrounding thin rim of viable tissue shows squamous metaplasia
Warthin Tumour Fna Pap
Fig. 5.19. Oncocytoma. Light and dark oncocytic cells are arranged in a solid, trabecular and tubular configuration

thelial markers are negative. Histological variants include a stroma-poor form, and metaplastic WT - in the latter, much of the original oncocytic epithelium has been replaced by squamous cells, and there is extensive necrosis, fibrosis, inflammation, and granuloma formation (Fig. 5.18) [66]. This not uncommon lesion follows trauma, particularly FNA [55], and can be mistaken for squamous or mucoepidermoid carcinomas (see Sect. 5.5.5). WT generally has a good outcome, with recurrence rates of about 2%. Malignancy occurs in less than 1% of cases, involving either epithelial or lymphoid elements leading to carcinomas or lymphomas [62].

5.8.5 Oncocytoma

ICD-O:8290/0

Oncocytic change in salivary tumours is common (see Sect. 5.6) [65]. Oncocytoma is a true benign neoplasm composed of oncocytes. It is rare and is often associated with MNOH (see Sect. 5.6) [158]. It comprises a well-demarcated mass of oncocytic cells (both light and dark) with a solid, trabecular, or tubular configuration (Fig. 5.19). There is a surrounding, usually incomplete fibrous capsule, and only a little internal fibrous stroma. There is a rare clear cell variant [61].

5.8.6 Canalicular Adenoma

ICD-O:8149/0

Canalicular adenoma also has a basaloid appearance. Its location is almost exclusively intraoral, particularly affecting the upper lip [119] and less often the palate. As a result, most tumours present when small

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