Sci—Thurs PM: Planning—06: A Simple, Robust IMRT Optimization Method for Lung Cancer, Accounting for Tissue Heterogeneity and Intra-Fraction Lung Tumour Motion



Background For lung cancer radiotherapy, respiratory motion broadens dose penumbra, increasing the amount of normal tissues irradiated and reducing the target dose near the edge. Traditionally, large PTV margins are used to ensure coverage of the tumour in the presence of motion. Unfortunately, organs at risk intersecting with the PTV also receive high doses. The objective of this work was to evaluate a robust lung strategy to account for the effects of respiratory motion on tumour coverage and normal tissue sparing. Hypothesis Accumulating dose from 4DCT phases using a deformable registration tool combined with penumbral and motion compensation IMRT techniques can be used to develop robust lung plans that reduce the dose to normal tissues and maintain therapeutic coverage of the PTV. Methods A deformable image registration tool was used to plan and accumulate dose over 10 phases of the breathing cycle for clinical IMRT plans and robust IMRT plans of 5 NSCLC patients. Robust plans have reduced beam apertures, but added segments which prefentially boost the portion of the target that falls outside of the reference phase (e.g. the exhale phase). The dose to this boost volume was set to 110% of the prescription dose inside the peripheral edge of the PTV. Clinical and robust plans were normalized and compared for CTV coverage and lung dose. Results for the ipsilateral lung showed that on average, V20, V10 and V5 decreased by approximately 3.0% with the robust approach. For all cases, the accumulated dose to CTV was increased. Conclusions Robust lung IMRT allows for reduction of geometric margins sparing ipsilateral lung and enhancing tumour coverage.