Abstract PO4-22-03: Intensity Modulated Proton Therapy (IMPT) for the Definitive Adjuvant Management of Women with Breast Cancer: A Single Institutional Experience of 5-Year Oncologic Outcomes
Authors: Nichols Elizabeth , Singh Gurbani , Lejano Desiree , McAvoy Sarah , Mishra Mark , Vyfhuis Melissa 
Published: May 2024

Abstract
Purpose: There is limited data reporting the oncologic outcomes of breast cancer patients treated definitively with adjuvant proton beam therapy. Here we report a single institutional experience of oncologic outcomes, treating a predominantly vulnerable breast cancer patient population and compare these to historical standards primarily utilizing photon therapy.
Materials and Methods: Unver IRB approval, we retrospectively reviewed 453 definitively treated breast cancer patients from 2016 – 2023 using adjuvant IMPT as part of their cancer care. Patients with recurrent disease, receiving re-irradiation, and those with less than 1-year of follow up were excluded from this analysis. Patients were treated with surgery and systemic therapies as per standards of care. Radiation therapy was administered using pencil beam scanning IMPT with daily image guidance. χ2 and Mann-Whitney U tests were performed to determine differences between select variables. Kaplan-Meier analysis and Cox proportional hazard models were used to analyze overall survival (OS).
Results: With a median follow up of 22 months for the entire cohort, 453 patients were treated at XXXX as part of definitive adjuvant management for their breast cancer. Nearly 30% of patients self-identified as black (n=135), average patient age was 54 and 56% (n=255) self-identified as being married with approximately 25% (n=109) of patients having Medicare/Medicaid insurance. 198 patients (43.8%) had an intact breast and the remainder mastectomy of which 37.4% had reconstruction at the time of their radiation. 229 patients (50.7%) received neoadjuvant chemotherapy while 55 patients (12%) received concurrent chemotherapy which was either her2+ directed or Xeloda. The number of patients (%) corresponding to the AJCC 8th edition anatomical staging is as follows: Stage 0: 12 (2.7%); Stage I: 91 (20.1%); Stage 2: 197 (43.6%); Stage 3: 152 (33.6%). Median RT dose delivered for patients receiving comprehensive treatment was 50.4 Gy with 81% of patients receiving a boost to either scar, lumpectomy cavity and/or LN regions. Median RT dose delivered for patients receiving whole breast radiation was 42.56 Gy with a 10 Gy lumpectomy cavity boost. 5-yr OS by stage was 0: 100%; stage I: 98.8%; stage 2: 93.5%; stage 3: 90.8%. Five-year OS for black, white and other race patients were 85.3%, 95.8% and 96%, respectively (White as Ref: HR 3.176, 95% CI: 1.313-7.681, p=0.010 for black patients). For the entire cohort, patients with triple negative disease had a 5-yr OS of 80.4% (HR: 4.33, 95% CI 1.87-10.04, p&lt; 0.001) and there was no difference in OS when insurance status was considered (p=0.378).
Conclusions: To the best of our knowledge this represents the largest series of definitively treated breast cancer patients utilizing adjuvant proton therapy and the 5-year OS rates in our analysis coincide with published standards1. Results from the on-going randomized RADCOMP study will provide further guidance as to which patients benefit the most from proton therapy and will further support equivalent oncological outcomes when compared to photon therapy.
1. Weiss A, Chavez-MacGregor M, Lichtensztajn DY, et al. Validation study of the American Joint Committee on Cancer eighth edition prognostic stage compared with the anatomic stage in breast cancer. JAMA Oncol. 4(2):203-209, 2018.
Citation Format: Elizabeth Nichols, Gurbani Singh, Desiree Lejano, Sarah McAvoy, Mark Mishra, Melissa Vyfhuis. Intensity Modulated Proton Therapy (IMPT) for the Definitive Adjuvant Management of Women with Breast Cancer: A Single Institutional Experience of 5-Year Oncologic Outcomes [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-22-03.