Metastatic malignant melanoma with 'rhabdoid' features

The authors propose the addition of malignant melanoma to the list of extrarenal neoplasms that may be predominantly composed of polygonal cells with the cytologic features of 'rhabdoid' tumor. A review of 313 metastatic melanomas disclosed 49 examples with rhabdoid features, from which 31 had sufficient material for further pathologic and immunohistologic characterization. A control group of 46 nonrhabdoid metastatic melanomas was examined in parallel fashion. In 39% of cases, rhabdoid melanomas manifested relative deletion of S100 protein compared with the control tumors. However, there were no differences in staining with HMB-45. Vimentin immunoreactivity was concentrated in the paranuclear cytoplasm of rhabdoid melanoma cells. However, ultrastructural studies of these cases failed to show corresponding whorls of intermediate filaments and instead demonstrated paracrystalline paranuclear inclusions in profiles of rough endoplasmic reticulum. It is concluded that metastatic rhabdoid melanoma exhibits significant morphologic similarity to other rhabdoid tumors at a light-microscopic level. However, it usually retains enough melanocytic attributes to allow for accurate diagnostic recognition. Probably because patients with metastatic melanoma have an extremely poor prognosis overall, no worsening of biologic behavior was associated with rhabdoid cytomorphologic findings in this tumor type when compared with the control cases.