A case of telangiectatic osteosarcoma arising from the frontal bone.

Osteosarcoma of the calvarium is extremely rare, specifically of the telangiectatic subtype, which has few reported cases. Herein, we report a case of telangiectatic osteosarcoma occurring in the frontal bone of a 33-year-old man without notable family or medical history. The patient presented with a painless mass in the frontal region, and computed tomography (CT) imaging and magnetic resonance imaging (MRI) revealed a tumor destructively involving the inner and outer tables of the skull. On T1- and T2-weighted magnetic resonance imaging (MRI) images, the tumor exhibited an isointense signal similar to brain parenchyma with multiple hemorrhagic cavities internally. Furthermore, the solid component of the tumor enhanced intensely on enhanced T1-weighted image; moreover, biopsy revealed giant cells. Thus, based on imaging and biopsy findings, a preoperative diagnosis of aneurysmal bone cyst was initially suspected. However, pathological examination of the entire tumor after resection revealed atypical mitosis. Results of immunohistochemical tests revealed no H3G34W mutation, and molecular genetic analyses revealed no USP6 gene rearrangement, leading to a final diagnosis of telangiectatic osteosarcoma.