Long term outcomes of patients with skull-base low-grade chondrosarcoma and chordoma patients treated with pencil beam scanning proton therapy

Abstract Purpose To evaluate the long term tumor control and toxicity of skull base tumors treated with pencil beam scanning proton therapy (PT). Materials and methods PT was delivered to 151 (68%) and 71 (32%) chordoma and chondrosarcoma (ChSa) patients, respectively. Mean age of patients was 40.8 ± 18.4 years and the male to female ratio was 0.53. The postoperative tumor was abutting the brainstem or optic apparatus in 71 (32.0%) patients. The postoperative mean gross tumor volume (GTV) was 35.7 ± 29.1 cm 3 . The delivered mean PT dose was 72.5 ± 2.2 Gy RBE. Results After a mean follow-up of 50 (range, 4–176) months, 35 local (15.8%) failures were observed between 10.9 and 85.4 months. The estimated 7-year LC rate for chordoma (70.9%; CI95% 61.5–81.8) was significantly lower compared to the LC rate for ChSa patients (93.6%; 95%CI 87.8–99.9; P  = 0.014). The estimated 7-year distant metastasis-free- and overall survival rate was 91.6% (95%CI 91.6–98.6) and 81.7% (95%CI 74.7–89.5), respectively. On multivariate analysis, optic apparatus and/or brainstem compression, histology and GTV were independent prognostic factors for LC and OS. The 7-year high grade toxicity-free survival was 87.2 (95%CI 82.4–92.3). Conclusions PBS PT is an effective treatment for skull base tumors with acceptable late toxicity. Optic apparatus and/or brainstem compression, histology and GTV allow independent prediction of the risk of local failure and death in skull base tumor patients.