Diagnostic Dilemma and Management of a Rapidly Enlarging Thigh Mass: A Case Report of Fibromatosis Mimicking Soft-tissue Sarcoma.

Due to its rarity and locally aggressive nature, fibromatosis often presents diagnostic challenges. We present a case of a 27-year-old female who presented with a painless, progressively enlarging mass in the left thigh that increased in size from 2 cm × 2 cm to 10 cm × 10 cm over 9 months. On physical examination, the mass was firm, immobile, and had well-defined margins. Magnetic resonance imaging (MRI) of the thigh and pelvis suggested soft tissue sarcoma, while histopathological examination of the trucut biopsy specimen demonstrated characteristic features of fibromatosis, including lack of nuclear atypia, proliferation of hypocellular spindle cells, and thick deposition of collagen. Immunohistochemistry confirmed the diagnosis with positive β-catenin staining and negative staining for desmin, S-100, and CD-34. The patient underwent wide local excision with R0 margins, and postoperative histopathology confirmed fibromatosis. Considering the risk of recurrence, the patient was advised regular follow-up with MRI. At 6-month follow-up, there was no evidence of recurrence.