Calcifying Aponeurotic Fibroma: A Review and Update

Calcifying aponeurotic fibroma (CAF) is a rare benign but locally aggressive mesenchymal tumor that primarily occurs in the distal extremities of children and adolescents. It typically presents as a slow-growing, painless, poorly circumscribed mass, often of prolonged duration. Radiographs may reveal a soft-tissue mass with a variable extent of fine stippled calcifications. On magnetic resonance imaging, CAF usually appears as an ill-defined subcutaneous mass with low to intermediate signal intensity on T1-weighted sequences and heterogenous high signal intensity on T2-weighted sequences. Areas of calcification exhibit low signal intensity on all pulse sequences. Intense heterogeneous enhancement is seen after intravenous contrast administration. Histologically, CAF is characterized by a fibromatosis-like component and a nodular calcified component. By immunohistochemistry, the tumor cells are variably positive for smooth muscle actin, muscle-specific actin and CD99 but negative for desmin and β-catenin. Moreover, frequent expression of ETS transcription factor ERG (ERG) and epidermal growth factor (EGF) has been demonstrated. Recent molecular studies have identified the presence of a recurrent fibronectin 1 (FN1)-EGF gene fusion. Surgical excision is the treatment of choice for CAF, but local recurrence is common due to its infiltrative nature. This review provides an updated overview of the clinical, radiological, morphological, immunohistochemical and molecular genetic features of CAF and discusses the differential diagnosis of this uncommon condition.