chondroblastoma

Chondroblastoma is a rare, benign primary bone tumor that arises from chondroblasts within the epiphysis or apophysis of long bones. It most often affects adolescents and young adults, with a slight male predominance. The most common sites are the distal femur, proximal tibia, and proximal humerus; pelvis and other epiphyses can be involved.

Clinically, patients present with localized joint pain, swelling, and sometimes reduced range of motion. Symptoms usually develop gradually and may follow minor trauma. The lesion often remains intraosseous for some time before causing noticeable symptoms.

Imaging typically shows a well-defined, lytic epiphyseal/metaphyseal lesion with a thin sclerotic margin. Expansion of the bone and cortical thinning may occur. Calcifications within the lesion are common, and secondary aneurysmal bone cyst changes can be seen. CT can delineate calcifications, while MRI provides tissue characterization with variable T2 signal and strong post-contrast enhancement.

Histology reveals round to polygonal chondroblasts with grooved nuclei embedded in a chondroid matrix, often arranged with characteristic pericellular “chicken-wire” calcifications. Multinucleated giant cells may be present. A recurrent somatic mutation in H3F3B (K36M) is commonly detected and is considered a molecular hallmark.

Management is typically intralesional curettage with adjuvant measures such as phenol, ethanol, or cryotherapy, followed by bone grafting or polymethylmethacrylate cement. En bloc resection may be required for difficult lesions or where curettage would compromise the epiphysis. Recurrence can occur, especially with suboptimal curettage or in the spine or pelvis. The prognosis is generally excellent after complete excision; metastasis is rare, and malignant transformation is uncommon.