3161 - Electron Boosts for Breast Tumor Bed: How Treatment Parameters Change When CT-Based Dose Calculations are Introduced
Presenter(s)
J. N. Wancura, A. Sacher, A. C. Hartford, I. L. Kuo, C. D. Bradford, Y. Ding, and T. J. FitzGerald; Department of Radiation Oncology, University of Massachusetts Chan Medical School, Worcester, MA
Purpose/Objective(s): Electron beam radiotherapy is a well-established modality for boosting breast tumor bed volumes. In a typical clinical workflow, boost volumes and skin landmarks are drawn on CT images, resulting in a beam’s-eye-view skin rendering that is used to inform a clinical setup based on light-field visualization and the heuristic application of electron principles. The goal of this retrospective study was to evaluate whether the introduction of a CT-based dose calculation would have changed energy, block shape, gantry/table angle, isocenter position, or monitor units (MU).
Materials/Methods: A cohort of 20 breast boost patients previously treated using a clinical workflow at our institution were selected, with clinical target volumes (CTV) and 5-mm margin planning target volumes (PTV) having already been drawn on CT images by the prescribing physician. Each clinical plan was reconstructed in a CT-based treatment planning system. Dose was calculated using a commercial electron Monte Carlo algorithm preset to the clinical MU determined from TG-71 formalism for a prescribed isodose line and a measured output factor. Replanning was performed to achieve dose volume histogram (DVH) target coverage objectives (CTV D98% > 100%, PTV D95% > 95%, PTV D99%> 90%) while constraining hotspots (PTV D2%) and lung dose (V50%).
Results: Energy was lowered in 5/20 cases, and bolus was added in an additional 5/20 cases, yielding an average reduction in lung V50% of 34cc, while maintaining or improving target coverage. Block enlargements in 6/20 cases, gantry/table angles changes in 5/20 cases, and isocenter shifts >1cm in 2/20 cases were applied to improve target coverage and robustness. Replanned cases normalized based on DVH metrics demonstrated enhanced uniformity, with the interquartile range for PTV D50% narrowing from 11.5% to 1.9%. Replanned cases also showed reduced hotspots, with PTV D2% > 120% dropping from 9/20 to 0/20. The average MU change across all cases was -5.4%.
Conclusion: Compared with clinical plans, replans using a CT-based dose calculations improved target coverage and uniformity, while reducing low-dose lung spill. These findings support clinical practice guidelines that recommend incorporating CT-based dose calculation into routine treatment planning.