Failed Photon Beam Therapy Following Endoscopic Endonasal Resection of a Clival Chordoma with Oculomotor Nerve (CN III) Compression

Abstract Introduction Clival chordoma is a rare, locally aggressive tumour whose proximity to the brainstem, optic apparatus and internal carotid arteries limits both surgery and radiotherapy. Case Description A 38-year-old woman presented with a left oculomotor nerve palsy from a clival chordoma encasing the internal carotid arteries. She underwent subtotal endoscopic endonasal resection followed by adjuvant photon radiotherapy to 54 Gy in 27 fractions, as particle therapy was unavailable. The dose could not be escalated because the brainstem and optic apparatus were near tolerance, and no clinical target volume was used. After initial regression, the tumour progressed about 20 months later, causing a new right oculomotor nerve palsy, invading the right cavernous sinus and spreading perineurally along the mandibular nerve (V3) through the foramen ovale into the infratemporal fossa. A second endoscopic decompression of the oculomotor nerve achieved only subtotal resection, and MRI two months later showed further progression. Discussion This case shows where and why the tumour recurred. Adjacent critical structures limited the dose to 54 Gy, well below the 74–76 Gy of contemporary series, and progression arose outside the treated volume, compressing the oculomotor nerve and spreading perineurally along V3, a rarely reported route that target volumes should cover.