Prognosticating tools in primary neuroendocrine (Merkel-cell) carcinomas of the skin: histopathological subdivision, DNA cytometry, cell proliferation analyses (Ki-67-immunoreactivity) and NCAM immunohistochemistry. A clinicopathological study in 25 patients.

Histopathologically, 18 of our patients had classical Merkel-cell carcinomas (MCC); seven had neuroendocrine (NE) carcinomas with features different from MCC, here called "aberrant MCC". These patients showed a progressive neoplastic disease with a fatal outcome in four of them. The cytometric DNA distribution pattern of the tumor cell nuclei of all the aberrant MCCs was found to be of the aneuploid type. By contrast, the neoplastic disease of the majority of patients with classical MCC ran a milder course; a fatal outcome occurred in only one of them. Here, the DNA ploidy pattern was of the euploid (diploid or tetraploid) type in eight cases and of the aneuploid type in another eight. Our recently described "proliferation cell index" (PCI), based on nuclear immunoreactivity (IR) with the proliferation "marker" antigen Ki-67, was significantly lower in those five MCCs of the classical "DNA-diploid" type than in the seven "DNA-aneuploid" ones. These five patients presented a mild neoplastic disease; only one had a local recurrence and none had metastases. Otherwise, neither the PCI values nor the NCAM IR of the MCC cells were found to be of any prognostic significance.