Clinicopathological analysis of basal cell carcinoma of the anal region and its distinction from basaloid squamous cell carcinoma

Basal cell carcinoma of the anal region is rare and morphologically difficult to distinguish from basaloid squamous cell carcinoma, particularly on biopsies. This distinction has therapeutic and prognostic implications. We reviewed morphological features of 9 basal cell carcinomas and 15 basaloid squamous cell carcinomas from the anal region diagnosed during 1993-2011 and determined the utility of Ber-EP4, BCL2, TP63, CK5/6, CDKN2A, and SOX2 as diagnostic tools. Immunostains were scored in a semi-quantitative manner (1+-1-10%, 2+-11-50%, 3+-450%). All basal cell carcinomas were located in the perianal region, while all basaloid squamous cell carcinomas originated in the anal canal/anorectum. Nodular subtype of basal cell carcinoma was the most common subtype. Retraction artifact was the only significant distinguishing histological feature of basal cell carcinoma compared with basaloid squamous cell carcinoma (88% vs 26%; P = 0.04). Atypical mitoses were more common in basaloid squamous cell carcinomas (71% vs 11%; P = 0.05). An in situ component was only present in basaloid squamous cell carcinomas, and was noted in 6/15 cases. Basal cell carcinomas had 2-3+ Ber-EP4 (basal cell carcinoma 100% vs basaloid squamous cell carcinoma 40%; P<0.001) and BCL2 immunoreactivity (basal cell carcinomas 100% vs basaloid squamous cell carcinoma 33%; P<0.001). Diffuse CDKN2A and SOX2 expression was seen only in basaloid squamous cell carcinomas (basal cell carcinoma 0% vs basaloid squamous cell carcinoma 93%; P<0.001). There was no difference in TP63 and CK5/6 expression. Perianal location, retraction artifact, and lack of atypical mitoses are histological features that help distinguish basal cell carcinoma from basaloid squamous cell carcinoma. An in situ component, when present, supports the diagnosis of basaloid squamous cell carcinoma. Immunostains are extremely helpful as diffuse Ber-EP4 and BCL2 expression is a feature of basal cell carcinoma and basaloid squamous cell carcinoma is typified by diffuse CDKN2A and SOX2 expression.