Periarticular Spindle Cell Sarcoma of the Knee in a Patient With Rheumatoid Arthritis: A Rare Diagnostic Dilemma in an Atypical Location and Age Group.

Spindle cell neoplasms are a heterogeneous group of mesenchymal tumors with variable biological behavior, ranging from benign lesions to high-grade sarcomas, and often mimic benign inflammatory conditions, posing diagnostic challenges. Myofibroblastic sarcomas are rare, particularly in periarticular locations such as the knee. Chronic inflammatory disorders like rheumatoid arthritis may further increase malignancy risk through persistent cytokine-mediated pathways. We report the case of a 63-year-old female with rheumatoid arthritis who presented with a progressively enlarging, painful swelling over the anteromedial aspect of the left knee for six months. Clinical examination showed a superficial erythematous swelling with vesicular changes. Initial imaging suggested a subcutaneous hematoma with Baker's cysts, while MRI revealed a heterogeneously enhancing soft tissue lesion suspicious for malignancy. The patient underwent a wide local excision. Histopathology demonstrated a high-grade spindle cell neoplasm with features suggestive of myofibroblastic differentiation (French Federation of Cancer Centers Sarcoma Group (FNCLCC) Grade 3), including high mitotic activity and focal necrosis. Adjuvant radiotherapy was administered. A new lesion was detected during follow-up and successfully treated with repeat excision. This case underscores the diagnostic difficulty of spindle cell tumors presenting as benign-appearing periarticular lesions. In patients with chronic inflammatory conditions, atypical or persistent swellings should raise suspicion for malignancy. Early diagnosis, histopathological confirmation, and multidisciplinary management are essential for optimal outcomes.