Cutaneous malignant peripheral nerve sheath tumors

Cutaneous malignant peripheral nerve sheath tumors (MPNSTs) are rare entities compared with their deep soft tissue counterparts. We describe two cases of cutaneous MPNSTs. The first case, occurring in a 27‐year‐old woman with neurofibromatosis I, presented with recent growth of a pre‐existing nodule on her back. A biopsy showed a densely cellular area within a conventional neurofibroma composed of atypical, hyperchromatic epithelioid and spindled cells with frequent mitotic figures (MFs). The second case presented in an 88‐year‐old man with no stigmata of neurofibromatosis as a rapidly growing subcutaneous tumor of the right calf. A biopsy showed a diffuse neurofibroma that abruptly transitioned to a densely cellular proliferation of hyperchromatic atypical spindled cells arranged in short fascicles with frequent MFs. The diagnosis of MPNST was rendered in both cases. MPNSTs of the dermis and subcutis are rare sarcomas. They can occur as sporadic tumors or in the setting of neurofibromatosis. They are often associated with pre‐existing neurofibromas. Increase in size of pre‐existing neurofibromas is an indication for biopsy. Recognition of the cellular atypia, increased cellularity and mitotic activity is key to the diagnosis.