Main Session
Sep
28
PQA 04 - Breast Cancer, Patient Reported Outcomes/QoL/Survivorship, Functional Radiation Medicine, Hematologic Malignancies, Palliative Care, and International/Global Oncology
2808 - Impact of Daily Anatomical Variations on Delivered Dose in Hypofractionated Whole Breast Proton Beam Therapy
Presenter(s)
Olivia Moncrief, BS - LSU Health Sciences Center Shreveport, Shreveport, LA
O. G. Moncrief1,2, L. R. Rosen2, E. Warren1,2, C. J. Wang2, H. T. Wu2, and Y. A. Walter2,3; 1Louisiana State University Health Shreveport, Shreveport, LA, 2Willis Knighton Cancer Center, Shreveport, LA, 3University of Jamestown, Fargo, ND
Purpose/Objective(s):
Inflammation is a known side effect of whole breast radiation therapy. In whole breast proton beam therapy (WB-PBT), swelling-induced changes in the radiological depth may impact the delivered dose. Additionally, variability in the breast position may lead to further perturbations. The purpose of this work was to quantify the impact of daily anatomical variations on delivered dose in hypofractionated WB-PBT.Materials/Methods:
Patients receiving WB-PBT to a dose of 4256 cGy (RBE 1.1) in 16 fractions were included. Patients were treated on an IBA ProteusONE (Ion Beam Applications) with on-board cone beam CT (CBCT) capability. Assessment plans were generated, covering 95% of the treated breast (WB CTV) with a gradient-matched 2-field arrangement. To assess the impact of depth perturbations, range uncertainty was not used in optimization. Delivered doses were calculated on virtual CT (VCT) sets generated from daily CBCT images in RayStation 2023B (RaySearch Laboratories) using a deformation-based method. The dose to 90-100% of the WB CTV (D90-100), max and mean dose to the heart and left anterior descending artery (LAD), and volume of lung receiving at least 5, 10, and 16 Gy (RBE) were recorded (V5-V16). Two-tailed paired t-tests were used for analysis, with effect sizes reported as Cohen’s d. Pearson correlation was used to assess relationships between the fraction number and change in dosimetric parameters.Results:
Data for 7 patients (5 left, 2 right breast) treated in 112 total fractions were collected for analysis. On average, the accumulated WB CTV D95 dropped by 6.3% ± 8.1% (range –24.3% to –0.5%, p=0.08). Similarly, accumulated WB CTV D90 (p=0.03, d=1.11), D98 (p=0.04, d=0.96), and D100 (p=0.02, d=1.21) unanimously decreased compared to the nominal plan. Pearson correlation showed a statistically significant relationship between fraction number and decreases in target coverage metrics (D90: r=-0.37, D95: r=-0.32, D98: r=-0.29, D100: r=-0.39, all p<0.01). Changes in organ-at-risk metrics were not statistically significant, though for patients treated in the left breast, the accumulated LAD maximum dose varied by up to 65.4% compared to the nominal plan.Conclusion:
Changes in the radiological depth throughout treatment significantly perturbed the delivered dose. Target coverage losses grew as patients progressed through therapy, potentially impacting efficacy. Alternative optimization strategies, including adaptive radiotherapy, may be assessed for viability as mitigation strategies in this context.