Dose-effect analysis of radiation induced rib fractures after thoracic SBRT

To determine a dose-effect relation for radiation induced rib fractures after stereotactic body radiation therapy (SBRT) in early stage non-small cell lung cancer (NSCLC). Automatic rib delineation has enabled the analysis of a large patient group.

Four-hundred and sixty-six patients with stage I/II NSCLC received SBRT with a median of 54Gy in 3 fractions. The optimal EQD2-corrected dose parameter to predict (a)symptomatic fractures was found using Cox regression. Three normal tissue complication probability (NTCP) models based on this optimal parameter were constructed: (1) at a median follow up (FU) of 26months, (2) for all data, with time to toxicity taken into account and (3) at a FU of 26months, excluding low dose ribs.

The median time to fracture was 22 (range 5-51) months. Maximum rib dose best predicted fractures. The TD₅₀ (dose with 50% complication) of the second NTCP model was 375Gy. The TD₅₀ was significantly higher for the other models indicating an under-estimation of the dose effect at the median follow-up time and/or when excluding low dose ribs.

The risk of symptomatic rib fractures after SBRT was significantly correlated to dose, and was <5% at 26months when D_max<225Gy.