High-Dose Proton Beam-Based Radiation Therapy in the Management of Extracranial Chondrosarcomas

Radiation therapy (RT) improves local tumor control in axial chondrosarcomas (CS). It is, however, often difficult to safely deliver the high doses (range, 70.2-77.4 Gy) required for achieving a high likelihood of local control, especially in the spine, using photons. This, however, can be achieved with proton beam therapy (PBT) due to its unique physical characteristics. The main goal of our study is to evaluate the outcomes of CS patients treated with passive scattered PBT.

Forty-four patients (N = 44) were identified who received PBT as part of their treatment from 1990 to 2012. A retrospective review of their medical and RT treatment records was conducted. Multivariate analyses were performed to identify patient- and tumor-related factors predicting for improved local control and overall survival.

Median age was 45.5 years and 55% were female. Median tumor size was 13 cm. Most common anatomical location was the spine (80%). Median follow-up was 29.1 months. Median external beam RT dose was 70.2 Gy relative biological effectiveness (RBE) at 1.8 Gy (RBE) per fraction typically administered using a combination of photon RT + PBT (77%) or PBT alone (23%). Local control was 76% and 57%, and overall survival was 90% and 68% at 2 and 5 years, respectively. Toxicity was acceptable, with the most frequent being wound complications (16%). On multivariate analyses, grade III tumors were significantly associated with decreased local control (P = 0.019), while female sex (P = 0.037) and grade III tumors (P = 0.005) were associated with a poorer overall survival.

High-dose proton-based RT in combination with surgery resulted in local tumor control in most of these high-risk CS patients. Female sex was predictive for decreased survival, while higher tumor grade (grade III) was predictive of decreased local control and survival. Proton beam therapy is an attractive treatment modality for these challenging tumors.