Sacral insufficiency fractures after high-dose carbon-ion based radiotherapy of sacral chordomas

Background: This study aimed to analyse the frequency and clinical relevance of sacral insufficiency fractures (SIFs) after high-dose carbon-ion based irradiation of sacral chordomas. Methods: A total of 56 patients were included in this retrospective study. Twenty one patients (37%) were treated with definitive radiotherapy (RT), and 35 patients (63%) received postoperative RT using carbon ions, either in combination with photons or as single-modality treatment (median radiation dose 66 Gy RBE, range 60-74 Gy). Follow-up examinations including MRI of the pelvis were performed at 3-monthly intervals in the first year and consecutively at 6-monthly intervals. Median follow-up was 35.5 months (range 2-83). Results: SIFs were diagnosed in 29 patients (52%) after a median follow-up of 11 months (range 1-62 months). Most sacral fractures (79%) occurred within 2 years after RT. For the overall study population, the fracture-free survival probability amounted to values of 0.68 (95% CI, 0.53-0.79) after 1 year, 0.46 (95% CI, 0.31-0.60) after 2 years, and 0.31 (95% CI, 0.16-0.47) after 5 years. Statistical analysis showed no significant difference regarding the fracture rates between patients who received an operation and postoperative RT and patients treated with definitive RT. About one third of the patients with SIFs (34%; 10 of 29 patients) had associated clinical symptoms, most notably pain. All patients with symptomatic fractures required strong analgesics and often intensive pain management. Conclusions: Sacral fractures after high-dose carbon ion-based RT of sacral chordomas were shown to be a considerable radiogenic late effect, affecting about half of the treated patients. However, only one third of these fractures were clinically symptomatic requiring regular medical care and pain therapy. Further hazard factor analysis in the future with larger patient numbers will possibly enable the identification of high-risk patients for developing SIFs with the ultimate goal to prevent symptomatic fractures.