RESUMO
Ancient schwannoma, a rare subtype of schwannoma, a benign tumor originating from nerve sheaths, can arise from various nerves, except for the optic, olfactory, spinal, and autonomic nervous systems. Schwannomas are typically characterized by the presence of neoplastic Schwann cells and tend to develop eccentrically. Malignant transformations of schwannomas are exceptionally uncommon. In this case report, a 42-year-old male presented with a painful lump on the front of his left knee. The lump was described as an extra-articular swelling below the kneecap, situated over the patellar tendon. Initially, ultrasonography (USG) indicated the presence of a slow-flow vascular malformation in the infrapatellar region of the left knee. However, subsequent magnetic resonance imaging (MRI) revealed a well-defined mass in the subcutaneous plane below the knee, with minimal septations, leading to an initial suspicion of a large sebaceous cyst. Further investigation through histopathological analysis confirmed the diagnosis of an extra-articular schwannoma. This finding highlights the importance of thorough examination and diagnostic techniques in differentiating between various types of soft tissue masses. Schwannomas, although uncommon in certain locations, should be considered in the differential diagnosis of painful lumps, even in atypical anatomical sites such as the knee.