A Case of Telangiectatic Osteosarcoma in the Frontal Bone.

Telangiectatic osteosarcoma (TOS) is a subtype of osteosarcoma. TOS in the elderly and TOS in the skull are very rare. Here, we report a case of TOS in the frontal bone of an elderly patient. The patient was a 79-year-old woman who was identified as having a right frontal bone lesion. The patient was initially diagnosed with an intradiploic epidermoid cyst (IEC). A 60mm cystic lesion with bone destruction appeared 5 years later, which enlarged over 2 months. The fluid in the cyst seemed to be blood that had lost its clotting ability. Tumor and the surrounding tissue were resected. The postoperative course was favorable. Postoperative magnetic resonance imaging (MRI) showed no evidence of residual lesions. There was no metastases. Histopathologically, the cyst wall was composed of fibrous connective tissue and did not contain epithelial components. There were no skin appendages and keratinized tissues in the lesion. Based on these findings, the diagnosis changed to aneurysmal bone cyst (ABC). Subsequent immunohistochemical examinations confirmed that the MIB-1 index was 50% in some sections and there were atypical cells showing osteogenic properties in other sections. Based on these results, the patient was finally diagnosed with TOS. The differential diagnoses for cystic lesion presenting bone destruction include TOS, ABC, and IEC. In case of cystic lesion with bone destruction, early diagnosis based on histopathological study is important and complete resection with surrounding tissues is required, given the possibility of TOS.