Background: This study evaluates the impact of body contouring changes on delivered dose and proposes a method to identify target dose deviations exceeding 5%. Materials and Methods: Five CT datasets were created by simulating body contouring reductions of 3mm, 6mm, 9mm, 12mm, and 15mm from the original planning CT. Using the same iso-center and Volumetric Modulated Arc Therapy (VMAT) plan, new plans (P3, P6, P9, P12, P15) were generated for new five CT datasets (body contouring reductions of 3mm, 6mm, 9mm, 12mm, and 15mm) respectively. Dose distributions and changes in dosimetric parameters for the Planning Target Volume (PTV) and Organs at Risk (OARs), including the small intestine, rectum, bone marrow, femoral head, and bladder, were analyzed. Results: Progressive weight loss increased doses to PTVs and OARs. PTV D50 increased by 1.32%, 2.35%, 3.62%, 5.18%, and 6.24% for 3mm, 6mm, 9mm, 12mm, and 15mm reductions, respectively. The small intestine V45 exceeded 195cc and the rectum V50 surpassed 50% at 12mm and 15mm. Bone marrow doses remained below the V40 threshold of 37%. When reductions reached 12mm and 15mm, regions with dose deviations >5% (250cGy) covered 31.81% and 178.08% of the PTV. Conclusion: Weight loss-induced body contouring reductions significantly affect PTV and OAR doses. Re-scanning and re-planning are recommended when contour reductions exceed 12mm or waist circumference decreases by >75mm.
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