Fifty-eighth annual meeting of the american association of physicists in medicine
SU-F-T-455: Is Contouring the Whole Breast Necessary for Two-Field 3D Breast Planning?
To investigate the effect of contouring the whole breast on reducing the radiation dose to the heart and affected lung in tangential field-in-field 3D breast planning. We hypothesize that contouring the whole breast will simplify the plan normalization process, reduce dose to critical structures, while maintaining treatment plan quality and consistency.
Twenty previously treated breast cancer patients using tangential field-in-field 3D planning technique were randomly selected. The affected breast was contoured following the RTOG breast atlas guideline. Breast PTV was created by shrinking 5 mm from the breast contour. The same plan has been pasted to the new contour and normalized to 95% of the Breast PTV receiving the prescribed isodose line. Lung V20 Gy% and Heart V25 Gy% were the primary study endpoints. Homogeneity Index (HI) and Conformity Index (CI) were calculated based on the following equations. HI= Dmax/ D95 and Nakamura's Conformity Index= PIV/TVPIV × TV/TVPIV.
The average CI for previous plans was 1.68 vs. 1.66 for the new hybrid plan: both plans were conformal to the breast with similar quality. The HI for both the previous and the new hybrid plan was 1.24. Lung V 20% slightly increased from 4.27% to 4.82%. Heart V 25% for LT breast patients slightly decreased from 0.38% to 0.29%. Heart V 25% for RT breast patients was close to zero in both cases.
With similar conformal and homogeneity indices for the plan quality, by contouring the whole breast following RTOG breast atlas guideline will simplify the planning process. The study showed that contouring the whole breast for patients with left-sided breast cancer reduced the heart V 25%, although not significantly, while maintaining the CI and HI. There was no measurable gain seen with whole breast contour for right-sided breast cancer patients.