Long-term outcomes and prognostic factors of skull-base chondrosarcoma patients treated with pencil-beam scanning proton therapy at the Paul Scherrer Institute

Background Skull-base chondrosarcoma (ChSa) is a rare disease, and the prognostication of this disease entity is ill defined. Methods We assessed the long-term local control (LC) results, overall survival (OS), and prognostic factors of skull-base ChSa patients treated with pencil beam scanning proton therapy (PBS PT). Seventy-seven (male, 35; 46%) patients with histologically confirmed ChSa were treated at the Paul Scherrer Institute. Median age was 38.9 years (range, 10.2-70.0y). Median delivered dose was 70.0 Gy_RBE (range, 64.0-76.0 Gy_RBE). LC, OS, and toxicity-free survival (TFS) rates were calculated using the Kaplan Meier method. Results After a mean follow-up of 69.2 months (range, 4.6-190.8 mo), 6 local (7.8%) failures were observed, 2 of which were late failures. Five (6.5%) patients died. The actuarial 8-year LC and OS were 89.7% and 93.5%, respectively. Tumor volume > 25 cm³ (P =. 02), brainstem/optic apparatus compression at the time of PT (P =. 04) and age >30 years (P =. 08) were associated with lower rates of LC. High-grade (≥3) radiation-induced toxicity was observed in 6 (7.8%) patients. The 8-year high-grade TFS was 90.8%. A higher rate of high-grade toxicity was observed for older patients (P =. 073), those with larger tumor volume (P =. 069), and those treated with 5 weekly fractions (P =. 069). Conclusions This is the largest PT series reporting the outcome of patients with low-grade ChSa of the skull base treated with PBS only. Our data indicate that protons are both safe and effective. Tumor volume, brainstem/optic apparatus compression, and age were prognosticators of local failures.