ObjectivesTo investigate the safety and efficacy of intensity modulated carbon ion radiotherapy (IM-CIRT) using pencil beam scanning technology for patients with unresectable sacrococcygeal chordoma (SC). Methods and MaterialsA total of 35 unresectable SC patients were retrospectively analyzed, including 54.3% (19/35) recurrent cases. 68.6% (24/35) cases’ tumor located in S2 or above, and all cases were treated with hypofractionated IM-CIRT. The median dose was 70.4 (range 69 - 80) Gy (RBE) in 16 (range 16 - 23) fractions, typically delivered over 5 fractions per week. ResultsThe 3-year OS, CSS, PFS, LRFS and DMFS rates with a median follow-up time of 42 (12∼91) months for the entire cohort were 93.2%, 96.3%, 61.8%, 80% and 77.3%, respectively. Multivariate analysis revealed that GTV volume (HR, 3.807; 95% CI: 1.044 - 13.887, p = 0.043) was the only significant prognostic factor for PFS and the dose for the GTV ≥ 70.4 Gy (RBE) was relevant with significantly better LRFS (HR, 0.190; 95% CI: 0.038 – 0.940, p = 0.042). No significant prognostic factor for OS, CSS and DMFS and no severe (i.e., grade ≥ 3) acute toxicity were identified. Severe late toxicities occurred in 3 patients (8.57 %): pain (1 patient), motor neuropathy (1 patient) and skin ulcer (1 patient). Furthermore, no severe toxicity related to urinary function or defecation was observed following IM-CIRT. Pain grades improved or remained unchanged in 85.7% of patients. ConclusionIM-CIRT produced acceptable 3-year outcomes without substantial late adverse effects, especially urinary and anorectal complications for SC, and did not appear to increase pain. IM-CIRT at high doses using hypofractionated RT may improve outcomes for local control and seems to be feasible even for postoperative recurrent SC.