Carbon Ion Radiation Therapy of Sacrococcygeal Chordomas: A Comparison of Different Fractionation Schemes.

Abstract

Sacrococcygeal chordomas are rare, radioresistant malignant bone tumors that are challenging to treat with conventional photon therapy. Carbon ion radiation therapy (CIRT) offers a favorable dose distribution and higher biological effectiveness, which may improve local control. This retrospective study from the Heidelberg Ion-Beam Therapy Center compares 2 carbon ion fractionation schemes for sacrococcygeal chordomas. The study included 157 patients treated between 2011 and 2025 with CIRT with either 64 GyRBE in 16 fractions of 4 GyRBE each (96.0 Gy EQD2) or 66 GyRBE in 22 fractions of 3 GyRBE each (82.5 Gy EQD2). Outcomes analyzed were local progression-free survival (LPFS), distant metastasis-free survival, overall survival (OS), and toxicity. Survival curves were analyzed using the Kaplan-Meier method, and sacral insufficiency fractures were evaluated with a competing risk analysis. Multivariable Cox regression was used to identify independent risk factors associated with local recurrence and OS. At 2 and 4 years, LPFS was 83.4% and 71.8% for the 64 GyRBE in 16 fractions group with a median LPFS of 58 months (95% CI, 39.8-76.2 months) versus 71.8% and 52.3% for the 66 GyRBE in 22 fractions group with a median LPFS of 53 months (95% CI, 35.0-71.0 months); the difference was not statistically significant (P = .140). In multivariable analysis, female sex (HR, 2.44; 95% CI, 1.46-4.08; P < .001) and treatment for recurrent disease (HR, 3.01; 95% CI, 1.49-6.06; P = .002) were independent predictors of local progression. OS at 2 and 4 years was 96.5% and 85.4% for 16 × 4 GyRBE versus 98.6% and 91.2% for 22 × 3 GyRBE, with no significant difference (P = .816). A larger clinical target volume was associated with worse OS. Sacral insufficiency fractures occurred more frequently after 16 × 4 GyRBE (47% vs 24%; P = .003). For sacrococcygeal chordomas treated with CIRT, the 2 regimens showed no significant difference in LPFS or OS, despite a slight numerical advantage of the 64 GyRBE in 16 fractions in LPFS. However, the 64 GyRBE in 16 fractions regimen was associated with a significantly higher rate of insufficiency fractures.