A Rare Case of Extra-abdominal Desmoid-type Fibromatosis Arising from the Popliteal Fossa

Desmoids are rare soft-tissue tumors of the abdominal wall that may sporadically occur extra-abdominally. It manifests as clonal fibroblastic proliferation with an infiltrative tendency and capacity to recur without metastasizing. An adolescent male presented with a gradually increasing globular, non-tender, firm, non-pulsatile swelling (8 × 5 × 3 cm(3)) in the left popliteal fossa that had been present for five months. Following thorough investigation with imaging and Tru-cut biopsy, finally, an excisional biopsy was done. Histopathological examination confirmed a desmoid tumor, and the patient received adjuvant radiotherapy. At the one-year postoperative follow-up, there was no recurrence; the patient had been explained the prognosis. This case highlights a rare site of an extra-abdominal desmoid but with classical clinical presentation, imaging, intraoperative, and histopathological findings. Awareness and knowledge of this entity are of paramount importance for clinical practitioners.