Mast cell sarcoma with KIT p.D816V mutation and concurrent systemic mastocytosis.

Mast cell sarcoma (MCS) is an extremely rare and aggressive form of mastocytosis characterized by highly atypical mast cells with local invasion, metastatic potential, and poor prognosis. MCS is predominantly a de novo process without recurrent molecular findings or predisposing lesions including various myeloid neoplasms. However, there have been rare case reports of MCS with preceding or concurrent systemic mastocytosis (SM) or cutaneous mastocytosis (CM), which is suggestive of an uncommon progression from SM/CM to MCS. We hereby report a case of MCS in an 84-year-old male with a KIT p.D816V mutation and concurrent SM. KIT p.D816V point mutation is well known as the canonical variant in SM. In our case, MCS with KIT p.D816V mutation is a unique finding in the setting of concurrent SM, highlighting the potential relatedness of these two entities and the progression from SM to MCS, a currently poorly understood phenomenon.