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Tricholemmoma, a benign follicular neoplasm with outer root sheath differentiation, typically comprises clear or pale cells, and when multiple is pathognomic of Cowden's syndrome. The tumor is probably underrecognized and in basaloid examples can be difficult to distinguish from basal cell carcinoma (BCC). We studied 55 tricholemmomas (including 15 basaloid cases) and compared immunohistochemical profile with nodular BCC from our archives. Basaloid and non-basaloid tricholemmomas had similar staining characteristics. BerEP4 was focally positive (range 10%-20%) in only 3/39 (7.7%) tricholemmomas compared with widespread positivity in BCC (90.8%, 139 of 151 cases with ≥50% tumor area stained). CD34 was expressed, usually focally (median 20%, range 10%-90%), in 52/53 (98.1%) tricholemmomas and was negative in all 21 BCCs stained. EMA staining lacked sensitivity or specificity in differentiating tricholemmoma from BCC. Five or more Merkel cells were found in 7/17 (40.1%) tricholemmomas and 1/23 (4.3%) nodular BCCs studied. In summary, immunohistochemistry is helpful in distinction between tricholemmoma, including difficult basaloid examples (BerEP4 negative or focal, CD34 positive) compared with BCC (BerEP4 widespread in most cases, CD34 negative). The presence of 5 or more Merkel cells is a relatively specific but not a particularly sensitive discriminator.

We report the case of a 59-year-old Caucasian male with a primary in ltrative BCC with vascular invasion on the right temple. An excision biopsy was taken and histology con rmed BCC, 2.5mm thick, invading into the reticular dermis. The patient underwent a scar excision which demonstrated no further malignancy. The patient showed no clinical signs of recurrence 2 years a er diagnosis. Relevance:Basal cell carcinoma (BCC) is the most common human malignancy and its incidence is increasing. Less than 1% of BCCs metastasize and very few cases of intravascular invasion by a primary BCC have been reported. This report discusses the clinical features of a BCC, the population at risk and summarises all known cases of BCC with vascular invasion in the literature. Take Home Messages:Due to the rarity of basal cell carcinoma with vascular invasion, there is a poor understanding of the disease process and the metastatic potential, resulting in uncertainty for clinician and patient regarding appropriate management.The British Student Doctor, 2020;4(1):41-44