Merkel cell carcinoma frequently shows histologic features of basal cell carcinoma: a study of 30 cases.

Background: Merkel cell carcinoma (MCC) is a basaloid cutaneous neoplasm that may be mistaken for basal cell carcinoma (BCC).
Methods: Thirty MCCs were examined for areas that histologically resembled BCC.
Results: One of the histologic features of BCC (either a mucinous stroma or stromal artifactual retraction) was identified in all MCCs. A mucinous stroma was found in 28 MCCs (93%), stromal artifactual retraction in 27 (90%), mucin-containing gland-like spaces within tumor nests in 8 (27%), focal peripheral palisading in 8 (27%), epidermal involvement in 3 (10%) and dystrophic calcification in 1 MCC (3%). The cytologic features and absence of widespread peripheral palisading were the most reliable discriminators between MCC and BCC on routine sections. Squamous cell carcinoma was identified in four cases (13%). Two cases (7%) contained pagetoid intraepidermal spread (IES) of MCC. In one case, there was IES over the entire epidermal surface associated with intranuclear clearing, resembling the intranuclear cytoplasmic inclusions (INI) common in melanocytic tumors. INI were identified in six MCCs (20%).
Conclusions: MCCs frequently contain areas that histologically resemble BCC and other more common cutaneous malignancies. This can lead to diagnostic errors, particularly in small fragmented curettage specimens or frozen sections.