2798 - Simultaneous Integrated Proton Boost vs. Sequential Proton Boost in Node-Positive Breast Cancer: A Dosimetric Analysis of Normal Tissue Exposure and Biologically Effective Dose
Presenter(s)
M. Linnik1, N. Martynova1, N. Vorobyov1,2, E. Spiridenko1, A. Kalesnik1, E. Andronova1, G. Andreev1, K. Suprun1,3, I. Plugar'1, and I. Avryasov1; 1Dr. Berezin Medical Institute, Saint-Petersburg, Russian Federation, 2Saint-Petersburg State University, St.Petersburg, Russian Federation, 3Saint-Petersburg State University, Saint-Petersburg, Russian Federation
Purpose/Objective(s): Sequential proton boost (SEQ) prolongs treatment and may expose uninvolved breast tissue to high radiation doses due to field overlap. Simultaneous integrated proton boost (SIB-P) offers a hypofractionated approach with potential dosimetric advantages. This study compares SIB-P versus SEQ regarding physical and biologically effective doses to uninvolved breast tissue (NBT), skin, and other organs at risk (OAR).
Materials/Methods: A retrospective paired-plan analysis was performed for 71 consecutive patients (2025) receiving whole breast and regional nodal irradiation. SEQ delivered 15×2.67 Gy(RBE) to breast + 5×2.67 Gy(RBE) boost. SIB-P delivered 15×2.67 Gy(RBE) to breast/nodes with SIB of 15×3.2 Gy(RBE) to tumor bed. Both adhered to isotoxic NCCN constraints. NBT (CTV breast minus CTV tumor bed) physical Dmean and EQD2 (a/ß=3) were assessed. Skin EQD2 (a/ß=3) was evaluated within a 1 cm boost expansion ROI. OAR parameters included heart (V8Gy, V16Gy, Dmean), ipsilateral lung (V4Gy, V8Gy, V16Gy), contralateral breast Dmax, and LAD (Dmean, Dmax). Boost location (medial/lateral/central) was analyzed for left-sided patients (n=44). Statistics: Wilcoxon, paired t-test, ANOVA, Spearman.
Results: SIB-P reduced physical NBT Dmean by 2.05 Gy (p<0.001), but NBT EQD2 increased by 2.67 Gy (p<0.001). Skin sparing improved: median Dmax EQD2 decreased from 61.0 Gy to 59.8 Gy (p<0.001); mean dose within skin ROI decreased by 1.92 Gy (55.47 vs. 57.72 Gy, p<0.001). Plans exceeding 65 Gy (telangiectasia threshold) dropped from 14% to 4% (p=0.02). Ipsilateral lung V16Gy was lower with SIB-P (18.70% vs. 19.49%, p=0.018). Heart, LAD, and contralateral breast doses showed no significant differences. Skin dose exhibited pronounced individual variability (?~0.55); greatest reduction occurred in superficial tumors (skin-to-boost distance <5 mm, mean EQD2 reduction >3 Gy). Boost location did not significantly influence NBT EQD2 change (p=0.51). Baseline NBT dose correlated with physical dose reduction (?=0.44, p<0.001).
Conclusion: SIB-P reduces physical dose to NBT, lung, and skin while maintaining cardiac safety. However, increased NBT EQD2 may impact long-term fibrosis and cosmesis. These advantages, combined with shorter treatment, support SIB-P as a preferred technique, particularly in patients with high baseline NBT exposure, warranting prospective evaluation of cosmetic outcomes.