Osteoblastoma Recurring as High-grade Osteosarcoma

Abstract Introduction/Objective Osteoblastoma is a benign, bone-forming neoplasm that presents as a well-circumscribed lesion amenable to excision. The prognosis is good despite a 20% recurrence risk. Malignant transformation is very rare. Osteoblastoma-like osteosarcoma is a rare form of osteosarcoma that shares similar clinical, radiographic, and pathological characteristics Methods/Case Report A 24-year-old female presented with a large palpable mass in the posterior occiput. The lesion progressively enlarged over 2 years, finally becoming symptomatic. Magnetic resonance imaging demonstrated an extradural posterior fossa expansile skull mass measuring 6.0 x 5.7 x 4.7 cm that showed multiple fluid/fluid levels. The mass was resected. Pathology demonstrated a well-circumscribed neoplasm that was cellular and composed of large polyhedral osteoblasts with eccentric oval nuclei and abundant amphophilic cytoplasm. Tumor cells were intimately associated with numerous small interconnecting, irregular bony trabeculae. Scattered mitotic figures of normal configuration were noted. The findings were compatible with an epithelioid osteoblastoma with prominent secondary aneurysmal bone cyst-like changes. The patient was lost to follow-up but presented 3 years later with severe migraines. Imaging demonstrated a large heterogeneous enhancing mass centered about the occipital cranioplasty with both intracranial and extracranial components. Reexcision was performed. Sections showed a well- circumscribed tumor, delineated by a rim of subperiosteal reactive bone, with an epithelioid morphology. However, it also demonstrated areas of necrosis, extensive sheet-like growth, conspicuous mitoses, and cytological atypia not seen in the previous specimen. Therefore, diagnosis of osteoblastoma-like osteosarcoma was rendered after expert consultation. Results (if a Case Study enter NA) NA Conclusion This case highlights the exceedingly rare evolving nature of osteoblastomas and their high-grade transformation potential.