Deep Angiomyxoma of the Knee: a Rare Case Report.

Background: Deep angiomyxoma (DAM) is a slow-growing benign tumor with high risk of local recurrence after surgical resection. Although DAM in a lower extremity is extremely rare, clinicians must be aware of its possible occurrence. Suspicion can be made based on clinical examination and radiological imaging but final diagnosis is confirmed on histopathological examination and immunohistochemistry. We intend to present an extremely rare case of DAM in the knee, managed successfully with marginal excision. Case Presentation: A 4-year-old male child presented with the complains of a painless, progressively increasing, soft, non-tender, and fluctuant swelling in his right knee. The plain radiograph showed a non-calcified soft-tissue swelling and MRI revealed a multi-loculated cystic lesion with multiple septations. A pre-operative diagnosis of a benign cystic lesion was made. It was managed by marginal excision of the tumor and a histological diagnosis of DAM was made. IHC staining showed positivity for SMA, CD34 and vimentin were focally positive, while desmin and calponin were negative. At 12 months of follow-up, the patient had a normal painless gait and full knee ROM, without any local recurrence. Conclusion: DAM is a rare tumor which is often misdiagnosed. In this report, we present a rare case of benign cystic lesion which turned out to be DAM on HPE of resected specimen. Marginal excision of this lesion revealed good outcomes with no recurrence until 12 months of final follow-up. With this, we conclude that surgical excision should be the gold standard in cases of DAM.