Incidence, risk factors, and dose-volume relationship of radiation-induced rib fracture after carbon ion radiotherapy for lung cancer

Background: The purpose of this study was to assess the incidence, risk factors, and dose-volume relationship of radiation-induced rib fracture (RIRF) after carbon ion radiotherapy for lung cancer. Material and methods: Fifty-seven ribs of 18 patients with peripheral stage I non-small cell lung cancer treated with carbon ion radiotherapy were analyzed on rib fracture. The patients were treated at a total dose of 52.8 Gy [relative biologic effectiveness (RBE)] or 60.0 Gy (RBE) in 4 fractions and were followed at least six months. Patient characteristics and dosimetric parameters were analyzed for associations with RIRF. Results: Eighteen patients and 57 ribs were included in this study. The median length of follow-up was 36.5 months. RIRF was observed in seven (39%) of the 18 patients, and in 11 (19%) of 57 ribs. Only one patient developed symptomatic fracture. The distance from the ribs to the tumor site was significantly shorter in fractured ribs than in non-fractured ribs (1.4 ± 0.3 cm vs. 2.5 ± 0.3 cm). Receiver operating characteristic curve analysis showed that D³_1cm as a cut-off value for discriminating RIRF had the largest area under the curve (AUC =0.78). Comparison of the two-year cumulative incidence of RIRF among two groups as determined by cut-off values, yielded the following result: 53% vs. 4% [D³_1cm, ≥ 38.2 Gy (RBE) or less]. Results from the two groups were significantly different (p < 0.05). Conclusion: The crude incidence of RIRF after carbon ion radiotherapy was 39% but incidence of symptomatic fracture was low. The D³_1cm as cut-off values may be helpful for discriminating the risk of RIRF.