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.