PurposeTo investigate the safety and efficacy of intensity modulated carbon ion radiation therapy (IM-CIRT) using pencil beam scanning technology for patients with unresectable sacrococcygeal chordoma (SC).Methods and MaterialsA total of 35 patients with unresectable SC were retrospectively analyzed, including 54.3% (19/35) recurrent cases. In 68.6% (24/35) cases, tumor was located in S2 or above, and all cases were treated with hypofractionated IM-CIRT. The median dose was 70.4 Gy (range, 69-80 Gy) (relative biologic effectiveness) in 16 fractions (range, 16-23 fractions), typically delivered over 5 fractions per week.ResultsThe 3-year overall survival, cause-specific survival, progression-free survival, locoregional progression-free survival, and distant metastasis-free survival rates with a median follow-up time of 42 months (range, 12-91 months) for the entire cohort were 93.2%, 96.3%, 61.8%, 80%, and 77.3%, respectively. Multivariate analysis revealed that gross tumor volume (hazard ratio, 3.807; 95% CI, 1.044-13.887; P = .043) was the only significant prognostic factor for progression-free survival and the dose for the gross tumor volume ≥70.4 Gy (relative biologic effectiveness) was relevant with significantly better locoregional progression-free survival (hazard ratio, 0.190; 95% CI, 0.038-0.940; P = .042). No significant prognostic factor for overall survival, cause-specific survival, and distant metastasis-free survival and no severe (ie, 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.ConclusionsIM-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 radiation therapy may improve outcomes for local control and seems to be feasible even for postoperative recurrent SC.