Basal cell adenocarcinoma ex pleomorphic adenoma, tubular, trabecular and cribriform patterns, with oncocytic differentiation
Basal cell adenocarcinoma (BCAC) is a rare salivary gland neoplasm, and those occurring as carcinoma ex pleomorphic adenoma (CEPA) are even less common, with only one other case reported in the literature . Virtually any malignant salivary gland can manifest as CEPA, but certain neoplasms such as salivary duct carcinoma are more common in this setting [2,3]. The diagnosis of CEPA requires evidence of concurrent or prior pleomorphic adenoma at the same site. In this case, the area of cartilaginous matrix with embedded bilayered ductal structures is indicative of a pre-existing PA. A small subset of cases of BCAC may arise from a preexisting BCA, but the majority are thought to arise de novo .
This case of BCAC ex pleomorphic adenoma displayed extensive oncocytic differentiation, posing a diagnostic challenge. In the algorithmic approach to cytologic diagnosis of salivary gland neoplasms, a key component is the tinctorial characteristics of the tumor cells, which can be oncocytic, basaloid or clear . Due to unavoidable sampling bias on FNA, the oncocytic areas of this tumor were exclusively sampled, and thus the differential included entities such as Warthin tumor, oncocytoma, oncocytic cystadenoma and cellular pleomorphic adenoma. A basaloid neoplasm such as BCA or BCAC would naturally not be included in such a differential diagnosis.
Upon examination of the surgical resection specimen, it became clear that this tumor was indeed a predominantly basaloid neoplasm containing abrupt geographic islands of oncocytic cells throughout. The tumor was focally infiltrative with areas of perineural invasion, thus representing a salivary gland carcinoma. The histologic differential diagnosis included adenoid cystic carcinoma (ACC); however, this tumor is exceedingly rare in the context of CEPA, and the tumor cells did not show the hyperchromatic angulated nuclei of ACC. Furthermore, FISH study was negative for MYB translocation. By immunohistochemistry, the tumor cells showed nuclear positivity for B-catenin and LEF-1, which is a recently identified finding in many salivary BCA and BCAC that aids in the separation of these basaloid neoplasms from other entities in the differential diagnosis .
Although BCAC is known to rarely contain other heterotopic elements such as squamous and sebaceous elements, oncocytic differentiation in a BCAC has not been reported . On the other hand, oncocytic change in other salivary gland neoplasms such as pleomorphic adenoma, and even in normal salivary gland tissue as a consequence of aging, is a known occurrence . Because this is the first of its kind reported, the clinical significance of extensive oncocytic differentiation in BCAC of the salivary gland is unknown. While BCAC is typically a low grade lesion, this case showed atypical cytomorphology and increased mitotic index, which may indicate a higher grade lesion . Due to the positive margins, perineural invasion, and higher grade histology, the patient received postoperative radiation therapy, and has no evidence of recurrence at five months of follow-up.
Contributed by Stacey Mardekian, MD and Aaron Berg, MD