Radiation-induced rib fractures after hypofractionated stereotactic body radiation therapy of non-small cell lung cancer: A dose– and volume–response analysis

Background and purposeThe aim of this study is to analyse the dose–response and the volume–response of radiation-induced rib fractures after hypofractionated stereotactic body radiation therapy (SBRT).Materials and methodsDuring the period 1998–2005, 68 patients with medically inoperable stage I non-small cell lung cancer (NSCLC) were treated with hypofractionated SBRT to 45 Gy in 3 fractions. Among the 33 patients with complete treatment records and radiographic follow-up exceeding 15 months (median: 29 months), 13 fractures were found in seven patients. Identifying all ribs receiving at least 21 Gy, 81 ribs (13 with and 68 without fracture) in 26 patients were separately contoured and their dose–volume histograms (DVHs) were obtained. The DVHs were assessed with the mean dose and cut-off models. Maximum likelihood estimation was used to fit dose–response and volume–response curves to each model.ResultsIt was possible to quantify the risk of radiation-induced rib fracture using response curves and information contained in the DVHs. Absolute volumes provided better fits than relative volumes and dose–response curves were more suitable than volume–response curves. For the dose given by the 2 cm3 cut-off volume, D2cm3, the logistic dose–response curve for three fractions was parameterised by D50 = 49.8 Gy and γ50 = 2.05. Consequently, for a median follow-up of 29 months, if D2cm3 < 3 × 7.0 Gy the risk is close to 0, and the 5% and 50% risks are given by D2cm3 = 3 × 9.1 Gy and 3 × 16.6 Gy, respectively.ConclusionsIn this group of patients, the risk for radiation-induced rib fracture following hypofractionated SBRT was related to the dose to 2 cm3 of the rib.