High-Grade Sarcomatous Transformation of Hybrid Hemosiderotic Fibrolipomatous Tumor-Myxoinflammatory Fibroblastic Sarcoma with Novel ADAMTS12::TERT Gene Fusion.

Hemosiderotic fibrolipomatous tumor (HFLT) and myxoinflammatory fibroblastic sarcoma (MIFS) are rare, locally aggressive soft tissue tumors with a predilection for distal extremities of middle-aged adults. Hybrid tumors (HFLT-MIFS) demonstrate overlapping features of both and share recurrent translocation (1;10) (p22; q24). We describe a tumor with high-grade sarcomatous transformation of a hybrid HFLT-MIFS, with a novel ADAMTS12::TERT gene fusion, presenting as a right foot soft tissue mass in an 85-year-old woman. Histologically, the tumor showed a high-grade sarcoma with areas of hybrid HFLT-MIFS in subsequent resection. The patient was treated surgically with a below-knee amputation and interval radiation therapy. On RNA-based next-generation sequencing, the tumor harbored a novel ADAMTS12::TERT gene fusion. In addition to reporting a novel gene fusion in a rare example of hybrid HFLT-MIFS with high-grade sarcomatous transformation, we also discuss relevant morphological features of sarcomatous HFLT and provide a brief review of reported neoplasms with TERT gene fusions.