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Incidence of rib fractures after stereotactic body radiotherapy for peripheral lung lesions clinical experience and dose response estimation
Wolfgang A. Tomé, C. Wesley Hodge, Minesh P. Mehta, Søren M. Bentzen

Purpose/Objective(s): 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.

Materials/Methods: 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 (PTV). Both planned maximum dose and maximum Fraction-size equivalent dose (FED) 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.

Results: 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 D50 (123FED50) estimate of 66.71 Gy (73.52 Gy). The steepness of the dose-response curve was quantified by the m and γ50 value, estimated at m = 0.1663 and γ50 = 2.39 for the maximum dose probit dose response model and at m = 0.2747 and γ50 = 1.45 for the maximum 123FED probit dose response model.

Conclusions: 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 123FED 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.

Keywords: Lung; Cancer; Rib fractures,Stereotactic body radiotherapy; Radiotherapy; NTCP

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