Merkel cell carcinoma with divergent differentiation: histopathological and immunohistochemical study of 15 cases with PCR analysis for Merkel cell polyomavirus

Aims To report on 15 cases of Merkel cell carcinoma (MCC) with divergent differentiation, to characterize its clinicopathological spectrum and its relationship with Merkel cell polyomavirus (MCV). Methods and results Fifteen patients with a mean age of 81years were included. Follow-up was available for 13 cases (range 12days to 6years; median 6months). Recurrence, metastasis and mortality rates were 15.4%, 53.8% and 61.5%, respectively. All tumours showed the typical histological and immunohistochemical features of MCC, with at least one additional divergent component. Eight cases had a single aberrant component (squamous in six cases, follicular in one case, and porocarcinoma in one case), six cases had two aberrant components (squamous and sarcomatous in three cases, glandular and squamous in two cases, and sarcomatous and neuroblastic in one case), and one case had three aberrant components (glandular, squamous, and sarcomatous). All cases had dysplastic changes in the overlying epithelium, and four of 15 showed epidermotropism. PCR analysis for Merkel cell polyomavirus (MCV) gave negative results in all 12 cases tested. Conclusions Merkel cell carcinoma with divergent differentiation is a highly aggressive tumour that might be difficult to recognize, owing to its wide histological variability. Negativity for MCV suggests that the virus is not implicated in the development of this subtype of MCC.