A Sclerosing Perineurioma With Collagen Rosette Formation: Benign Mimic of Low-Grade Fibromyxoid Sarcoma.

We report the case of a sclerosing perineurioma with conspicious collagen rosette formation in a 20-year-old male presenting with a firm, painless nodule on the palmar side of his right ring finger. The main differential diagnosis is a low-grade fibromyxoid sarcoma. The distinction between these entities is important because low-grade fibromyxoid sarcoma has a metastatic potential, while, as a rule, perineuriomas are benign. The presence of collagen rosettes in this current case makes this distinction even more difficult given that approximately 30% of low-grade fibromyxoid sarcoma cases show the focal presence of collagen rosettes. The demonstration of the characteristic t(7;16), t(11;16) or t(11;22) translocations (resulting in the FUS-CREB3L2, FUS-CREB3L1 or EWSR1-CREB3L1 fusion genes, respectively) or immunoreactivity for MUC4, a recently described sensitive and specific marker for low-grade fibromyxoid sarcoma, remain the gold standard in the diagnosis of low-grade fibromyxoid sarcoma, differentiating it from perineurioma. This case is, to our knowledge, the first report on collagen rosettes in sclerosing perineurioma, extremely well mimicking low-grade fibromyxoid sarcoma, and further expanding the morphological spectrum of this rare subtype of perineurioma.