OP18LONG TERM OUTCOMES OF SKULL-BASE LOW-GRADE CHONDROSARCOMA PATIENTS TREATED WITH PENCIL BEAM SCANNING PROTON THERAPY AT THE PAUL SCHERRER INSTITUTE

INTRODUCTION: Skull-base chondrosarcoma (ChSa) are rare malignant bone tumors. The current study was performed to evaluation the safety and effectiveness of pencil beam scanning proton therapy (PT) as well as prognostic factors in patients with skull base low-grade ChSa. METHOD: Between October 1998 and September 2014, 77 (male, 46%) patients with histologically confirmed low-grade ChSa were treated at the PSI. Median age was 38.9 years (range, 10.2-70.0). PT was delivered at initial diagnosis post-operatively to 62 (80.5%) patients. Median administered dose was 70.0 GyRBE (range, 64.0-76.0). Local control (LC), overall survival (OS) and toxicity-free survival (TFS) rates were calculated using the Kaplan Meier method. Toxicity was assessed according to the CTCAE v.4.0 scale. RESULTS: After a median follow-up of 58.2 (range, 4.6-190.8) months, 6 local (7.8%) failures were observed. The median time to local failure was 28.4 (range, 11.7-140.8) months. Five (6.5%) patients died. The actuarial 8-year LC and OS were 89.7% and 93.5%, respectively. High-grade (≥3) radiation induced toxicity was observed in 6 (7.8%) patients. Three (3.9%) patients presented with grade 3 hearing loss, 2 (2.6%) patients with grade 4 cerebellum or spinal cord necrosis and one (1.3%) patient with grade 4 optic neuropathy. The 8-year high-grade TFS was 90.8%. The univariate analysis (LC) results will be presented during the congress. CONCLUSION: This is the largest PBS PT series reporting the outcome of patients with low-grade ChSa of the skull of base. Our data indicates that protons are both safe and effective. Despite the high-dose administered to these patients, only a minority of patients of patients developed clinically significant toxicity.