Posterior Knee Mass Presenting as Benign Spindle Cell Tumour: A Case of Deep-Seated Fibromatosis

Fibromatosis is a rare, histologically benign soft tissue tumor that tends to be locally invasive and has a high risk of recurrence. We report the case of a middle-aged female who presented with a progressively enlarging, painless swelling in the posterior aspect of the right knee for two years. MRI suggested a benign spindle cell lesion, and FNAC findings supported this. Surgical excision was performed, and histopathological examination confirmed the diagnosis of desmoid-type fibromatosis. Postoperatively, the patient developed right foot drop due to nerve involvement during tumor dissection and later developed a surgical site infection. This case highlights the diagnostic difficulty posed by such tumors due to their ability to mimic vascular lesions radiologically and underscores the importance of histological evaluation for accurate diagnosis. Early surgical intervention and long-term follow-up are essential due to the tumor's infiltrative nature and recurrence risk.