<p>A 50-year-old female presented with a large palpable mass in the right buttock and proximal thigh, accompanied by sciatic paresthesia. She reported intermittent thigh and knee pain for 5 years, worsening over the previous 6 months. Plain radiography images revealed a pedunculated osteochondroma on the posterolateral aspect of the femur with extensive soft tissue involvement and soft tissue calcifications. A computed tomography (CT) scan showed an osteochondroma with a large adjacent bursa containing multiple foci of calcifications with a “snowstorm” appearance. A diagnostic dilemma arose regarding whether the condition represented reactive bursitis secondary to osteochondroma or a malignant transformation to chondrosarcoma with intra-bursal invasion. Magnetic resonance imaging (MRI) indicated a thick cartilage cap of osteochondroma measuring 53 mm and multiple cartilage nodules in the bursa along with heterogeneous enhancement in cartilage islands. Moreover, evidence of adjacent muscle invasion and sciatic nerve encasement was noted. These findings suggested an unusual diagnosis of malignant transformation of the osteochondroma to chondrosarcoma with intra-bursal invasion. The patient underwent wide-margin resection of the proximal femur and tumoral soft tissue, reconstructed with a proximal femoral mega prosthesis. Histologic evaluation confirmed a well-differentiated chondrosarcoma with intra-bursal invasion. This case highlights the critical role of diagnostic imaging in differentiating between benign and malignant complications associated with osteochondroma.</p>