2762 - Dosimetric Benefits of Online Adaptive Accelerated Partial Breast Irradiation In Early-Stage Breast Cancer
Presenter(s)
O. Iskandar1, J. Cui2, A. T. Clark3, T. J. Galloway4, H. N. Yankey5, J. E. Meyer5, E. M. Horwitz4, and R. M. Shulman4; 1Fox Chase Cancer Center/ Temple University Hospitals, Philadelphia, PA, 2Biostatistics and Bioinformatics Facility, Fox Chase Cancer Center, Philadelphia, PA, 3Fox Chase Cancer Center, philadelphia, PA, 4Fox Chase Cancer Center, Philadelphia, PA, 5Department of Radiation Oncology, Fox Chase Cancer Center, Philadelphia, PA
Purpose/Objective(s): Online adaptive radiotherapy (OART) offers potential to enhance accelerated partial breast irradiation (APBI) by enabling daily plan adaptation to interfractional anatomical changes—such as lumpectomy cavity deformation. This can improve target coverage and organ-at-risk (OAR) sparing compared to non-adaptive approaches. In this study, we evaluated the dosimetric differences between adaptive and scheduled plans for APBI, stratified by key patient and tumor characteristics.
Materials/Methods: This retrospective, single-institution study included 32 patients with early-stage breast cancer treated with APBI to a dose of 30 Gy in 5 fractions between 10/2024- 10/2025 using OART on a CBCT-guided platform. Target volumes were defined as follows: lumpectomy cavity (including surgical clips) + 1.5 cm CTV (cropped to anatomical boundaries) + 3mm PTV. Dosimetric differences between the scheduled plan (initial plan recalculated on daily anatomy) and the adaptive plan (reoptimized on updated daily contours) were compared for each fraction using the Wilcoxon signed-rank test. An adaptive treatment was classified as beneficial if adaptation improved =8 of the 17 dosimetric parameters. Univariate logistic regression screened clinical and dosimetric variables for association with adaptive benefit (p < 0.10 threshold for inclusion); significant variables were then entered into a multivariate logistic regression model to determine independent predictors of benefit.
Results: Adaptive plans were selected for delivery in 119 of 160 fractions (74.4%). Although more patients had right-sided disease (53.1%), adaptive plans were selected more frequently for left-sided tumors (51.3% of fractions) and those in the upper outer quadrant (43.8%). Compared to scheduled plans, adaptive plans provided superior target coverage, with median lumpectomy cavity V28.5 Gy of 100% (vs. 99.7%; p < 0.001) and PTV V28.5 Gy of 99.2% (vs. 97.0%; p < 0.001). Adaptive plans significantly improved OAR sparing, including reduced spinal cord D0.1 cm³ (median 42 cGy vs. 47 cGy; p = 0.0016), ribs D0.03 cm³ (median 552 cGy vs. 557 cGy; p = 0.003), and heart mean dose (median 11 cGy vs. 13 cGy; p = 0.004). On multivariate logistic regression, tumors in the upper inner quadrant (OR 8.8, 95% CI 1.76–46.0; p = 0.009) and upper outer quadrant (OR 8.9, 95% CI 1.9–41.4; p = 0.006) demonstrated greater adaptive benefit compared to central tumors. Additionally, larger lumpectomy cavity volume (mean 25.8cc; SD 27.4) was independently associated with increased benefit, with each 1 cc increase conferring a 5% higher odds of adaptive benefit (OR 1.05, 95% CI 1.00–1.10; p = 0.03).
Conclusion: This study demonstrates that OART facilitates high-quality APBI for early-stage breast cancer patients, providing superior OAR sparing and improved target volume coverage particularly in patients with upper quadrant tumors and larger lumpectomy cavities.