"Benign" Metastasizing Fibrous Histiocytoma Harboring PRKCD Gene Fusions With Malignant Clinical Course.

Dermatofibroma/fibrous histiocytoma is a common tumor of the dermis, composed of myofibroblasts, dendrocytes, and macrophages with characteristic entrapment of dermal collagen fibers at the periphery of the tumor. Various subtypes of dermatofibroma, including cellular, aneurysmal, and atypical dermatofibroma, are associated with an increased risk of recurrence, especially with incomplete excision. Two cases of benign fibrous histiocytoma with recurrence and metastasis are described. Patient 1 had a cellular fibrous histiocytoma with KIRREL1::PRKCD fusion presenting as a superficial and deep soft tissue mass involving the scapula and lateral clavicle. Patient 1 subsequently developed metastasis to the chest wall and thigh. Patient 2 had a recurrent aneurysmal fibrous histiocytoma with LAMTOR1::PRKCD fusion presenting as a large shoulder mass with invasion of the clavicle and lymph node metastasis. Patient 2 developed rapid local recurrence and PET-avid pulmonary nodules, radiologically consistent with metastasis. Fibrous histiocytoma is typically a morphologically benign neoplasm that only rarely exhibits clinically aggressive behavior with multiple recurrences and metastases. Although uncommon, recognition of this entity is important so that pathologists and sarcoma oncologists can ensure accurate diagnosis, multidisciplinary management, and appropriate clinical surveillance. In addition, complex chromosomal abnormalities may serve as useful indicators of fibrous histiocytoma with the risk of aggressive clinical behavior.