Proton therapy for base of skull chordoma: a report for the Royal College of Radiologists. The Proton Therapy Working Party.
Chordomas of the base of the skull are rare. They are locally infiltrative and frequently arise close to radiosensitive structures, which limits the ability to deliver a high dose of radiotherapy. Complete surgical excision is not usually possible. Conventional postoperative radiotherapy can result in approximately 50% 5-year survival and effective palliation, but long-term local control and cure are rare. The well-defined Bragg peak of protons allows planning with a sharp cut-off outside the target volume. This permits a higher dose of radiotherapy to be delivered to the tumour while avoiding excessive irradiation to radiosensitive structures. Outcome after proton irradiation is superior to that reported for conventional photon irradiation. Radiotherapy schedules involving a mixed schedule of protons and photons have achieved an approximately 60% local control rate at 5 years. Some of this improvement may have resulted from better surgical techniques. Proton irradiation is also effective for base of skull chondrosarcomas. Protons of sufficient energy to treat base of skull tumours are not available in the UK. Patients have been referred to the proton facilities at the Harvard cyclotron, and at Orsay, France. They will continue to require referral abroad for proton therapy for base of skull tumours. Proton therapy has become standard treatment for ocular melanoma and this is available at the Douglas cyclotron at Clatterbridge Hospital. Proton therapy has the potential for improved dose distribution compared with conformal photon radiotherapy. This may be exploited effectively to irradiate target volumes close to radiosensitive structures such as the spinal cord. There is a need for further clinical research to evaluate proton therapy for tumours such as spinal and paraspinal sarcomas, and paediatric brain tumours.