SU‐GG‐T‐546: Dosimetric Impact of Beam Margin on 3D Conformal Treatment Plan in the Lung Cancer Stereotactic Body Radiotherapy

Purpose: This work investigates dosimetrically how beam margin (BM) affects characteristics of 3D conformal lung cancer stereotactic body radiotherapy (SBRT) treatment planning in terms of the conformality index (CI), high‐dose spillage and intermediate‐dose spillage. Method and Materials: Ten patient plans were generated in the Eclipse treatment planning system using the Anisotropic Analytical Algorithm (AAA) for BM of 0, 2 and 4 mm, respectively. All plans were normalized so that at least 95% of PTV receives the prescription dose and 99% of PTV receives at least 90% of the prescription dose. The high‐dose spillage was defined by the ratio of the volume of all tissue outside the PTV receiving a dose larger than 105% of the prescription dose to the PTV, and the intermediate‐dose spillage was evaluated by the mean dose at the 3mm shell distanced at 2 cm away from the PTV surface. Results: The CIs are much lower for 0 or 2 mm BM than those of 4 mm BM for all patient plans while for two patients, slightly higher for 0 mm BM than those for 2 BM. The high‐dose spillages were on average about 7% lower for 0 or 2 mm BM than those of 4 mm BM for all patient plans while for three patients, higher for 0 mm BM than those of 2 mm BM. The intermediate‐dose spillages were lower for 0 or 2mm BM than those of 4 mm BM. Conclusion: With assumption that PTV fully covers the microscopic disease, tumor motion and setup uncertainty, and following recent ASTRO/ACR guideline that every effort should be made to minimize the volume of surrounding normal tissues exposed to high dose levels, 3D conformal lung cancer SBRT plans appear superior with 2 mm BM or less depending on the tumor location, size and patient geometry.