Unusual calcifying fibrous tumor of the distal femur: a case report and review of literature

Calcifying fibrous tumor (CFT) is a rare benign fibrous lesion of uncertain pathogenesis which shows distinct histological features. CFT has a broad anatomy distribution, and it was recently described in the subcutaneous and deep soft tissue, intra-cavity and intrinsic viscera. However, so far, there is no report to describe a CFT occurring in bone. We presented an unusual intraosseous CFT in a 63-year-old female patient with right knee joint pain for 2 weeks. Radiological imaging revealed a well-circumscribed osteolytic lesion with multiple calcifications or ossifications located in the right distal femur. The lesion was curetted totally. Histological examinations demonstrated that the tumor consisted of hyalinized, hypocellular collagen, scattered with fibroblast-like spindle cells, variably prominent chronic inflammatory cells infiltrates, and psammomatous or dystrophic calcifications. The morphologic diagnosis of primary CFT was made by its typical histological features and negative immunostaining for anaplastic lymphoma kinase (ALK). To the best of our knowledge, this is the first reported case of CFT that primarily developed in the bone. Awareness of CFT and its distinctive characteristics are important to avoid a diagnostic pitfall caused by histologic similarities to other spindle cell or calcifying lesions in unusual locations. Despite local recurrences have been reported, CFT still has favorable prognosis.