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Case Reports

Abstract

We present a case of a congenital melanocytic nevus with SESN3::RAF1 demonstrating both nevoid and lipofibromatosis-like morphology, with the latter component lacking expression of melanocytic markers. A 20-month-old female presented with a 2.5 centimeter back mass that was noted at birth and grew in proportion with her. Histologic sections of an incisional biopsy specimen demonstrated a lesion with two distinct components. In the deep dermis, there was a proliferation of nevoid melanocytes with epithelioid morphology and expression of Sox10, S100, HMB45, and Melan-A, compatible with a congenital melanocytic nevus.  In the deeper dermis, there were islands of bland spindled cells arranged around adnexa and along the neurovascular bundle with encroachment on adipose tissue imparting a morphologic appearance similar to lipofibromatosis. This spindle cell component expressed patchy CD34 and lacked expression of the aforementioned melanocytic markers. Although NRAS and BRAF mutations are the most common genetic drivers of congenital nevi, BRAF fusions and even less frequently RAF1 fusions that also result in MAP kinase pathway activation are detected in a minor subset. Interestingly, kinase-rearranged spindle cell tumors are a relatively new diagnostic category with a wide morphologic spectrum that includes lipofibromatosis-like histology and variable expression of CD34 and S100 without Sox10 expression. The most commonly reported genetic abnormalities are fusions and rearrangements involving NTRK1, but BRAF and RAF1 have also been implicated. This is an illustrative case of a congenital melanocytic nevus containing a primitive lipofibromatosis-like component with both morphologies, linked by the presence of SESN3::RAF1.