WE‐C‐ValA‐02: The Impact of 4D Breathing Motion Effects Versus Tissue Heterogeneity in Lung Cancer Treatment Planning

Abstract

Purpose: To investigate the relative magnitudes and clinical importances of the dosimetric effects related to 4D breathing motion and tissue heterogeneity for thoracic tumors treatment planning. Methods: Scans were acquired at normal exhale/inhale breathing phases. The target was the union of the exhale and inhale GTVs, uniformly expanded by 5mm(ITV). Patients were planned with both AP/PA and 3‐D conformal plans using the exhale (“static”) dataset, assuming unit density, for 100±5% ITV dose coverage. Each of these plans was further used to calculate: (a) heterogeneous “static” dose; (b) homogeneous cumulative dose; (c) heterogeneous cumulative dose. The same number of MU were used for each of the calculations and was based on the homogeneous “static” plan. Cumulative dose distributions consisted of a time‐weighted sum of exhale and inhale doses. Doses were calculated using the DPM_MC code which includes secondary electron transport for the heterogeneous computations. Results: Relative to unit‐density plans, tumor EUD, and lung NTCP increased in the heterogeneity corrected plans; primarily due to the reduced beam attenuation through lungs and the larger than coin‐size tumors investigated. In comparing 4D cumulative dose plans with static plans, clinical EUD and NTCP estimates were relatively unchanged. The insignificant tumor EUD change was a consequence of good target design, while the small lung NTCP change was due to its large volume effect. Accounting for tissue heterogeneity resulted in average changes of 10% in MLD. Accounting for 4D breathing motion effects resulted in <1% changes in MLD from the static value. The magnitude of these effects was not correlated with the dose distribution conformality. Conclusions: In this study we found that tissue heterogeneity effects are likely to have a larger clinical significance on tumor (if ITV is properly designed) and normal lung clinical treatment evaluation metrics than occurs with 4‐D respiratory‐induced changes.Supported by P01‐CA59827, R01‐CA106770.