To evaluate effectiveness and safety of spot-scanning-based proton-radiotherapy (PT) for extracranial chordomas (ECC).

Between 1999-2006, 40 patients with chordoma of C-, T-, and L-spine and sacrum were treated at Paul Scherrer Institute (PSI) with PT using spot-scanning. Median patient age was 58 years (range, 10-81 years); 63% were male, and 36% were female. Nineteen patients (47%) had gross residual disease (mean 69 cc; range, 13-495 cc) before PT, and 21 patients (53%) had undergone prior titanium-based surgical stabilization (SS) and reconstruction of the axial skeleton. Proton doses were expressed as Gy(RBE). A conversion factor of 1.1 was used to account for higher relative biological effectiveness (RBE) of protons compared with photons. Mean total dose was 72.5 Gy(RBE) [range, 59.4-75.2 Gy(RBE)] delivered at 1.8-2.0 Gy(RBE) dose per fraction. Median follow-up time was 43 months.

In 19 patients without surgical stabilization, actuarial local control (LC) rate at 5 years was 100%. LC for patients with gross residual disease but without surgical stabilization was also 100% at 5 years. In contrast, 12 failures occurred in 21 patients with SS, yielding a significantly decreased 5-year LC rate of 30% (p = 0.0003). For the entire cohort, 5-year LC rates were 62%, disease-free survival rates were 57%, and overall survival rates were 80%. Rates were 100% for patients without SS. No other factor, including dosimetric parameters (V95, V80) were predictive for tumor control on univariate analysis.

Spot-scanning-based PT at PSI delivered subsequently to function-preserving surgery for tumor debulking, decompression of spinal cord, or biopsy only is safe and highly effective in patients with ECC without major surgical instrumentation even in view of large, unresectable disease.