A distinct histopathological variant of a malignant melanoma with perivascular pseudorosettes: A case report

Although a rare condition, rosette formation in malignant melanoma has been previously documented. The present study describes the second documented case of malignant melanoma with perivascular pseudorosettes. A 38-year-old male presented with a black nodule on his back. Histopathological study revealed diffuse proliferation of neoplastic cells in the dermis and subcutis. A section of the tumor (~30%) was composed of a conventional malignant melanoma component. The remaining area was comprised of medium-sized polygonal cells with slightly eosinophilic cytoplasm and small-to-medium, round nuclei. Melanin pigment was rarely observed. A noteworthy observation was the presence of perivascular pseudorosette formations, which were characterized by their radial arrangement around the blood vessels, with a perivascular, anuclear zone. Immunohistochemically, the neoplastic cells were diffusely positive for S-100 protein and Melan-A and focally positive for HMB-45. Clinicopathological analyses of cases of malignant melanoma with rosette formations revealed that the types of rosette included the Homer-Wright type (two cases), perivascular pseudorosettes (two cases) and an unclassifiable type (one case). Immunohistochemical analysis is a useful method for forming a diagnosis as Melan-A or HMB-45 are generally expressed in all cases. Rosette formation in malignant melanoma is a distinct histopathological variant and may be an under-recognized phenomenon. Therefore, its recognition is significant for obtaining an accurate diagnosis of malignant melanoma.