TH‐E‐BRC‐11: Practical Methods for Improving Dose Non‐Uniformity in Monte Carlo‐ Based IMRT Planning of Lung Tumors Treated with Stereotactic Body Radiotherapy (SBRT)

Purpose: Current commercially available planning systems which utilize MC algorithm‐based final dose calculation in IMRT planning employ pencil‐beam algorithms in the optimization process. Consequently, dose coverage for SBRT lung plans can be quite non‐uniform, featuring cold‐ spots in the tumor periphery for “island” lesions within the lung, and, for other locations, hot‐spots within nearby normal organs (example: rib‐cage). This study evaluated practical approaches to reducing dose non‐uniformity within the target and surrounding normal organs in MC‐based IMRT planning. Methods: We evaluated two different IMRT‐based approaches. (A) Iterative planning where the MC calculation (with pencil‐beam‐based optimization) is initially performed. The resultant cold spot is then contoured and used as a simulatneous boost volume. The MC‐based dose is re‐computed and the prescription dose re‐normalized to 95% of the PTV. Ten SBRT lung cases with tumors seated near the lung‐wall/rib‐cage interface were planned. (B) Planning in which coplanar and non‐coplanar beam angles with limited path through lung tissue were selected. Both techniques were evaluated against the conventional coplanar‐beam approach: a single MC calculation and prescription dose normalization to 95% of the PTV. Results: Technique A: conformity index (CI) and PTV dose uniformity (U_PTV) improved in seven of ten plans. Average improvement (+/− standard error) was 10.8%+/−2.7%, and 22.4%+/−5.4%, respectively. Non‐significantly improved plans had PTVs near the skin, trachea and/or very small lung involvement. The maximum dose to 1cc volume (D1cc) of surrounding OARs decreased in nine often plans (average 10.6%+/−4.3%), with only the skin‐adjacent PTV plan showing no improvement. Technique B: we demonstrated an improvement of 11.2% and 2.6% in CI and U_PTV, respectively, and a D1cc reduction of 7.8% to surrounding OARs. Conclusions: The proposed practical approaches improve dose conformity in MC‐based IMRT planning of lung tumors treated with SBRT, improving target dose coverage and potentially reducing toxicities to surrounding normal organs. Supported in part by NIH/NCI Grant No. 106770.