Treatment with high marginal dose is mandatory to achieve long-term control of skull base chordomas and chondrosarcomas by means of stereotactic radiosurgery

Chordomas and chondrosarcomas of the skull base are rare. Since total resection of these tumors is difficult, adjuvant radiotherapy is necessary. This study was performed to evaluate the effect of stereotactic radiosurgery (SRS) for skull base chordomas and chondrosarcomas and to determine the optimal marginal dose for these tumors. Fourteen patients with histologically proven chordomas or chondrosarcomas underwent 16 sessions of SRS using gamma knife. The marginal doses ranged from 10 to 20 Gy (mean, 15 Gy). Lower marginal doses of 12 Gy on average (range, 10–12.5 Gy) were applied to four patients since they underwent prior fractionated radiotherapy, and partial treatment for which parts of tumors were excluded from planned target volume because of their proximity to critical structures was also applied to four patients. The whole tumors were covered with higher marginal doses of 18 Gy on average (range, 16–20 Gy) for six patients. The mean follow-up period was 65 months. Progression-free survival (PFS) rates at 3 and 5 years after SRS was 53 and 43%, respectively. Five-year PFS rates for patients who underwent SRS with higher and lower marginal doses were 80 and 14%, respectively, which were significantly different (P = 0.005). Tumor progression after partial irradiation mainly occurred from sites where delivered doses were reduced. Sufficient marginal doses at least 16 Gy appeared crucial. Proper combination with surgical resection to detach tumors from critical structures and to reduce tumor volume is necessary to completely deliver sufficient marginal doses for SRS.