The patient underwent surgery. A fibrous lesion was found in the left distal femur which was curetted and a diagnosis of non-ossifying fibroma was made on histopathology.

A non-ossifying fibroma is the commonest non-neoplastic fibrous osseous lesion, usually seen in the metaphyses of long bones of young patients with immature skeleton ^1,2. Most of the patients are asymptomatic and it is commonly, discovered as an incidental finding on plain radiographs done for some other purpose; however, a few patients (with larger lesion and/or pathological fracture), can present with pain and swelling ^1,2.  Most of these lesions are seen in the long bones of the lower extremities, especially around the knee (distal femur and proximal tibia) ². Herget et al reported 79% lesions around the knee ². Because of increased risk of fracture, all non-fibular lower limb lesions > 33 mm in the vertical length and affecting > 50 % of the bone diameter in both planes (lateral & anterior-posterior), need close monitoring ². Because of the high incidence of spontaneous healing in most of the cases, most of the large NOFs can be conservatively managed ².

NOF has a classical imaging feature on plain radiography (well-marginated, eccentric, intramedullary radiolucent lesion with dense sclerotic margins towards medulla, aligned along the long axis of the bone, etc.) ^1,2. However, its MR imaging features are fluctuating and depend on the relative combinations of the fibrous tissue, collagen, foamy histiocytes, haemorrhage, and bone trabeculae (depending on the development or healing phase of the lesion) ³.

Fibrous cortical defect, aneurysmal bone cyst, chondromyxoid fibroma, intraosseous ganglion, and fibrous dysplasia are the major imaging differential diagnoses ¹.