Purpose/Objective(s)The purpose of this study is to clarify the incidence, the risk factors, and the dose-volume relationship of radiation induced rib fracture after hypofractionated stereotactic body radiation therapy (SBRT).Materials/MethodsBetween April 2003 and March 2007, 174 consecutive patients treated with hypofractionated SBRT for primary or metastatic lung cancer were reviewed. The inclusion criteria were at least 3 months of follow-up by CT scan and no previous overlapped radiation exposure. Radiation induced rib fractures were defined as rib fractures located in the radiation field, detected by CT scan after treatment. The risk factors considered; age, gender, GTV diameter, chest wall - tumor distance were reviewed and each parameter was divided into two groups. Dose-volume histogram analysis was conducted on ribs received over 20 Gy at maximal point dose. The max dose and absolute volume received; ≥10 Gy, ≥20 Gy, ≥30 Gy and ≥40 Gy were determined for each ribs as the dosimetric parameters. The 3- and 5- year Kaplan-Meier (KM) estimates of rib fracture were calculated. Each risk factor was assessed by a log-rank test. The optimal cut off value for each dosimetric parameter was analyzed through the use of receiver-operating characteristic (ROC) curves. The area under the curve (AUC) values were also calculated. To estimate the cumulative risk of fracture, the ribs were divided into two groups according to the cutoff value and compared by log-rank test.ResultsFrom 129 patients, 409 ribs met the inclusion criteria. Median follow-up period was 19 months. Among the 129 patients, 26 patients (44 ribs) experienced radiation induced rib fractures. The KM estimates of rib fracture at 3 years and 5 years were 35.3%, 53.7%, respectively. As a risk factor, chest wall - tumor distance (≥2cm vs. <2cm) was significantly correlated with radiation induced rib fracture (p = 0.0001). Among the dosimetric analyses, AUC values for the max dose, V40, V30, V20 and V10 were 0.84, 0.82, 0.79, 0.71 and 0.56, respectively. The 5- year estimated risk of rib fracture is 53.5% vs. 3.0% (max dose: ≥44.9 Gy vs. < 44.9 Gy), 61.8% vs. 2.0% (V40: ≥0.43cc vs. <0.43cc), 54.0% vs. 2.1% (V30: ≥1.35cc vs. <1.35cc), 51.5% vs. 8.4% (V20: ≥3.64cc vs. <3.64cc), 30.2% vs. 14.6% (V10: ≥6.01cc vs. <6.01cc), respectively.ConclusionsThe incidence of radiation induced rib fractures after hypofractionated SBRT is relatively high. High dose volume is more strongly correlated with the risk of rib fractures. Purpose/Objective(s)The purpose of this study is to clarify the incidence, the risk factors, and the dose-volume relationship of radiation induced rib fracture after hypofractionated stereotactic body radiation therapy (SBRT). The purpose of this study is to clarify the incidence, the risk factors, and the dose-volume relationship of radiation induced rib fracture after hypofractionated stereotactic body radiation therapy (SBRT). Materials/MethodsBetween April 2003 and March 2007, 174 consecutive patients treated with hypofractionated SBRT for primary or metastatic lung cancer were reviewed. The inclusion criteria were at least 3 months of follow-up by CT scan and no previous overlapped radiation exposure. Radiation induced rib fractures were defined as rib fractures located in the radiation field, detected by CT scan after treatment. The risk factors considered; age, gender, GTV diameter, chest wall - tumor distance were reviewed and each parameter was divided into two groups. Dose-volume histogram analysis was conducted on ribs received over 20 Gy at maximal point dose. The max dose and absolute volume received; ≥10 Gy, ≥20 Gy, ≥30 Gy and ≥40 Gy were determined for each ribs as the dosimetric parameters. The 3- and 5- year Kaplan-Meier (KM) estimates of rib fracture were calculated. Each risk factor was assessed by a log-rank test. The optimal cut off value for each dosimetric parameter was analyzed through the use of receiver-operating characteristic (ROC) curves. The area under the curve (AUC) values were also calculated. To estimate the cumulative risk of fracture, the ribs were divided into two groups according to the cutoff value and compared by log-rank test. Between April 2003 and March 2007, 174 consecutive patients treated with hypofractionated SBRT for primary or metastatic lung cancer were reviewed. The inclusion criteria were at least 3 months of follow-up by CT scan and no previous overlapped radiation exposure. Radiation induced rib fractures were defined as rib fractures located in the radiation field, detected by CT scan after treatment. The risk factors considered; age, gender, GTV diameter, chest wall - tumor distance were reviewed and each parameter was divided into two groups. Dose-volume histogram analysis was conducted on ribs received over 20 Gy at maximal point dose. The max dose and absolute volume received; ≥10 Gy, ≥20 Gy, ≥30 Gy and ≥40 Gy were determined for each ribs as the dosimetric parameters. The 3- and 5- year Kaplan-Meier (KM) estimates of rib fracture were calculated. Each risk factor was assessed by a log-rank test. The optimal cut off value for each dosimetric parameter was analyzed through the use of receiver-operating characteristic (ROC) curves. The area under the curve (AUC) values were also calculated. To estimate the cumulative risk of fracture, the ribs were divided into two groups according to the cutoff value and compared by log-rank test. ResultsFrom 129 patients, 409 ribs met the inclusion criteria. Median follow-up period was 19 months. Among the 129 patients, 26 patients (44 ribs) experienced radiation induced rib fractures. The KM estimates of rib fracture at 3 years and 5 years were 35.3%, 53.7%, respectively. As a risk factor, chest wall - tumor distance (≥2cm vs. <2cm) was significantly correlated with radiation induced rib fracture (p = 0.0001). Among the dosimetric analyses, AUC values for the max dose, V40, V30, V20 and V10 were 0.84, 0.82, 0.79, 0.71 and 0.56, respectively. The 5- year estimated risk of rib fracture is 53.5% vs. 3.0% (max dose: ≥44.9 Gy vs. < 44.9 Gy), 61.8% vs. 2.0% (V40: ≥0.43cc vs. <0.43cc), 54.0% vs. 2.1% (V30: ≥1.35cc vs. <1.35cc), 51.5% vs. 8.4% (V20: ≥3.64cc vs. <3.64cc), 30.2% vs. 14.6% (V10: ≥6.01cc vs. <6.01cc), respectively. From 129 patients, 409 ribs met the inclusion criteria. Median follow-up period was 19 months. Among the 129 patients, 26 patients (44 ribs) experienced radiation induced rib fractures. The KM estimates of rib fracture at 3 years and 5 years were 35.3%, 53.7%, respectively. As a risk factor, chest wall - tumor distance (≥2cm vs. <2cm) was significantly correlated with radiation induced rib fracture (p = 0.0001). Among the dosimetric analyses, AUC values for the max dose, V40, V30, V20 and V10 were 0.84, 0.82, 0.79, 0.71 and 0.56, respectively. The 5- year estimated risk of rib fracture is 53.5% vs. 3.0% (max dose: ≥44.9 Gy vs. < 44.9 Gy), 61.8% vs. 2.0% (V40: ≥0.43cc vs. <0.43cc), 54.0% vs. 2.1% (V30: ≥1.35cc vs. <1.35cc), 51.5% vs. 8.4% (V20: ≥3.64cc vs. <3.64cc), 30.2% vs. 14.6% (V10: ≥6.01cc vs. <6.01cc), respectively. ConclusionsThe incidence of radiation induced rib fractures after hypofractionated SBRT is relatively high. High dose volume is more strongly correlated with the risk of rib fractures. The incidence of radiation induced rib fractures after hypofractionated SBRT is relatively high. High dose volume is more strongly correlated with the risk of rib fractures.