2923 - Dosimetric Comparison of Spot-Scanning Proton Arc (SPArc) vs. Intensity Modulated Proton Therapy (IMPT) for Patients with Loco-Regionally Advanced Breast Cancer Receiving Synchronous Bilateral Breast or Chest Wall and Regional Nodal Irradiation
Presenter(s)
X. Cao1, X. Ding1, P. Liu1, L. Zhao2, K. Salari3, X. Cong1, X. Xu1, and J. T. Dilworth1; 1Department of Radiation Oncology, Corewell Health William Beaumont University Hospital, Royal Oak, MI, 2Department of Radiation Oncology,Stanford University, Palo Alto, CA, 3Winship Cancer Institute, Emory University, Atlanta, GA
Purpose/Objective(s): To investigate the feasibility and normal tissue sparing of SPArc therapy for patients receiving synchronous bilateral breast or chest wall and regional nodal irradiation.
Materials/Methods: Twenty patients with loco-regionally advanced breast cancer who underwent a CT-based simulation for radiation planning were selected. Ten patients had bilateral intact breasts, and five patients had undergone bilateral total mastectomies (five of whom had reconstruction with either permanent implants or tissue expanders, and five had no reconstruction). Clinical target volumes for the breast or chest wall and the axillary levels I-III, supraclavicular, posterior cervical, and internal mammary nodal regions were contoured per the RADCOMP Breast Cancer Atlas. Near-surface skin rind volumes were defined as the volume bound by the breast/chest wall from the skin surface to 3 mm depth. For each patient, IMPT and SPArc plans were generated with a prescription dose of 50 Gy in 25 fractions, with goal of 95% prescription dose coverage of 95% of target volumes. Near-surface dose optimization was applied for both plans. Dosimetric parameters of target volume and organ-at risks, including the skin rind, were evaluated. Subgroup analyses were conducted in patients with thick chest walls (defined as those with bilateral intact breasts or bilateral implants post-mastectomy) and patients with thin chest walls (defined as patients who underwent mastectomy without reconstruction). Paired t-tests were used for the statistical analysis of normally distributed parameters; otherwise, the Wilcoxon rank-sum tests were employed.
Results: With comparable target coverage, SPArc significantly reduced dose to the lungs, the heart and the left anterior descending artery, compared to IMPT. Notably, SPArc reduced the mean left lung dose by 45.5% (475.40 cGy ± 102.32 cGy vs. 872.35 cGy ± 213.24 cGy, p<0.001) and the mean right lung dose by 36.2% (512.65 cGy ± 105.50 cGy vs. 803.41 cGy ± 280.73 cGy, p < 0.001). SPArc also significantly decreased the average skin rind dose by an average of 4.1 Gy (40.8 Gy ± 3.1 Gy vs. 44.9 Gy ± 2.4 Gy, p < 0.001). In subgroup analyses, SPArc reduced the skin rind dose compared to IMPT by an average of 4.7 Gy (44.6 ± 2.7 Gy vs. 39.9 ± 3.0 Gy) in patients with thick chest walls (n = 15) and by 2.3 Gy (45.8 ± 0.4 Gy vs. 43.6 ± 1.6 Gy) in those with thin chest walls (n = 5).
Conclusion: SPArc demonstrates dosimetric superiority over IMPT in synchronous bilateral breast or chest wall and regional nodal irradiation, achieving significant sparing of organs at risk, notably the lung and skin near-surface region, irrespective of the presence or absence of natural breast anatomy.