Epithelioid Osteoblastoma: Clinicopathologic Features of a Classic Case with Aggressive Behavior

Epithelioid osteoblastoma is a rare variant of osteoblastoma, characterized by large osteoblasts with epithelioid morphology. Although benign, it can be locally aggressive, frequently affecting the spine and long bones, with up to 26% of cases in the craniofacial region, especially the mandible.

We present a case of a 12-year-old male with a mandibular epithelioid osteoblastoma causing cortical expansion and tooth displacement. CT revealed a 4.7 cm mass with calcifications and periosteal reaction. Histology showed epithelioid osteoblast proliferation within a highly vascular stroma, scattered osteoclasts, and hemorrhagic areas, with a low Ki-67 index (< 5%) and no atypical mitoses or infiltrative growth.

Osteoid osteoma, cementoblastoma, fibro-osseous lesions, and osteosarcoma were considered. Osteosarcoma was excluded due to the absence of nuclear pleomorphism, atypical mitoses, and infiltrative growth. Clinical, radiographic, and histologic features, along with FOS/FOSB rearrangements and c-fos immunohistochemistry, aided accurate diagnosis.

The patient underwent marginal resection with clear margins. Follow-up for 10 months showed no recurrence.

Epithelioid osteoblastoma reflects a histologic phenotype rather than clinical aggressiveness. Complete excision is recommended to prevent recurrence, while preoperative biopsy and careful differential diagnosis are crucial for distinguishing it from mimics, particularly osteosarcoma.