Epithelioid osteoblastoma with secondary aneurysmal bone cyst and FOS gene rearrangement.

Epithelioid osteoblastoma, sometimes equated with aggressive osteoblastoma, is a variant of osteoblastoma that typically demonstrates more worrisome imaging and pathological features compared to conventional osteoblastoma. These more aggressive features can overlap with those seen in osteosarcoma, creating a diagnostic challenge for radiologists and pathologists. Recent identification of FOS and FOSB gene rearrangements in osteoid osteoma and osteoblastoma has allowed for greater diagnostic confidence following biopsy, but careful radiological-pathological correlation remains a key component for guiding appropriate management. Although the imaging features of conventional osteoblastoma have been previously described, there are limited examples in the literature of the imaging appearance of epithelioid osteoblastoma, and none with secondary aneurysmal bone cyst. In this case report, we detail the clinical, imaging, and histological characteristics of a proximal femoral epithelioid osteoblastoma which was pathologically confirmed by FOS and FOSB genetic testing. The initial imaging impression favored a malignancy, but when the biopsy results were correlated in a multidisciplinary fashion with the imaging, epithelioid osteoblastoma became the leading diagnosis which was subsequently genetically confirmed. This case emphasizes the value of multidisciplinary radiology-pathology correlation in routine practice.