Late complications of SBRT include radiation related rib fractures. We estimate the incidence of rib fracturesas a function of maximum absorbed rib dose after stereotactic body radiotherapy (SBRT) for early stage lung cancer.

Of 23 patients treated with image guided SBRT (60 Gy in 5 fractions) between 2003 and 2006, 4 developed pathological rib fractures near the SBRT planning target volume (). Both planned maximum dose and maximum Fraction-size equivalent dose () to the combined rib volume lying within the prescription isodose volume was determined and a probit dose response model was fitted to the observed rib fracture data for each.

17 patients were evaluated, all with a minimum of 15 months follow-up. Median followup was 43 months (range 15-60 months). The median time to rib fracture was 26.5 months (range 15-34 months). The maximum rib dose ranged from 23.8-74.7 Gy (median 57.8 Gy) in 5 fractions. Dose was a significant predictor of rib fracture (p=0.02), with a D () estimate of 66.71 Gy (73.52 Gy). The steepness of the dose-response curve was quantified by the m and 50 value, estimated at = 0.1663 and = 2.39 for the maximum dose probit dose response model and at = 0.2747 and = 1.45 for the maximum probit dose response model.

Maximum rib dose should be carefully considered in SBRT with appropriate risk counseling of patients whose maximum rib dose exceeds a dose of 50 Gy in 5 fractions or a maximum of 43.1 Gy, estimated to be associated with a 6.6 % risk of rib fractures. Hence, the inclusion of ribs as an "organ at risk" in intensity modulated radiotherapy (IMRT) planning should be considered as a way to reduce the likelihood of rib fractures.