Recurrent Dedifferentiated Clival Chondrosarcoma Presenting 32 Years after Initial Treatment: A Case Report.

Background Skull base chondrosarcomas are rare and locally aggressive tumors with heterogeneous biology accounting for 0.1-0.2% of all intracranial tumors. Dedifferentiated chondrosarcoma is an exceptionally uncommon and aggressive variant, seldom seen at the skull base. We report a recurrent clival chondrosarcoma with a small, dedifferentiated component, and an unusually long latency after initial treatment. Case Report A 66-year-old woman was referred with a recurrent progressive clival lesion, initially suspected to be a chordoma. She had undergone subtotal resection followed by single-fraction radiosurgery (25 Gy) 32 years earlier. Surveillance imaging demonstrated interval growth compared with imaging 5 years earlier, prompting repeat subtotal resection. Histopathology revealed a conventional chondrosarcoma with a dedifferentiated component comprising <5% of tumor area and an isocitrate dehydrogenase 1 (IDH1) R132 mutation. The patient declined adjuvant radiotherapy. Three years later, she developed worsening diplopia, new right oculomotor palsy, and headache with radiologic progression, prompting repeat subtotal resection. She currently awaits postoperative proton beam radiotherapy. Conclusion This case highlights a key diagnostic challenge of distinguishing chondrosarcoma from chordoma, and underscores the importance of meticulous histologic sampling to detect small dedifferentiated foci that may otherwise result in undergrading and undertreatment. The heterogeneity in clinical behavior observed across histopathological subtypes further underscores the necessity for grade/subtype-driven treatment regimens. Therefore, accurate subtype classification and molecular annotation may better inform treatment strategy and follow-up. Lastly, the rarity of dedifferentiated skull base chondrosarcoma reinforces the need for multicenter registries to further understand the behavior and vulnerabilities of this rare disease.