Abstract PS15-12: Initial outcomes of adjuvant proton pencil beam scanning radiation for patients with breast cancer requiring comprehensive nodal irradiation within a single institution

Background:Following lumpectomy or mastectomy, locally advanced breast cancer (LABC)requires adjuvant radiation (RT) to the chest wall (CW) and comprehensiveregional nodal basins (CNI). Proton therapy (PBT) has demonstrated dosimetricadvantages in heart, lung, and esophageal exposures compared to photon RT, butlittle is published regarding oncologic outcomes of PBT for LABC. Methods:Consecutive patients treated from 2016-2019 wereretrospectively reviewed. Men and women over the age of 18 requiring adjuvantRT with CNI were included; all patients had at least 6 months of follow up fromRT completion. Initial treatment volumes, excluding boosts to scars or grossdisease, (CTV_init) included CW-CNI per the RADCOMP atlas. CTV_init coveragewas prescribed as 95% CTV_init at 100% Rx dose. All patients were treated with proton pencil-beam scanning RT (PBS-PBT),typically with a two-field anterior SFO technique. Patient, tumor, and dosimetric characteristics wereanalyzed. Tumoral control and survival rates were estimated by the Kaplan Meiermethod. Toxicities were recorded prospectively by treating physicians andreviewed retrospectively. Local recurrence was defined as in-breast, or skin/chestwall; regional recurrence was defined as a nodal failure. Results:One hundred patients were included with a median follow upof 15.4 months (6-42). Ninety-eight percent were female, and 61% were white.Median age was 52 years, and 94% of patients had an ECOG PS ≤1.AJCC 8th edition anatomic staging was predominantly stage II (49%)or III (48%).Sixty-five percent of patients were treated for left-sideddisease; 7% of patient received bilateral RT; 87% received cytotoxicchemotherapy (63% neoadjuvant, 37% adjuvant). Twenty six patients receivedconcurrent systemic therapy with trastuzumab (H)/pertuzumab (P)(38% ),capecitabine (29%), H-emtansine (21%), or H (12%). The median initial RT dosewas 50.4 (45-50.4) while median total RT dose was 50.4Gy (45-70.2). Forty-twopercent of patient underwent an RT boost to nodal areas and/or the scar. Allpatients were treated in 1.8 or 2.0Gy fractions. Eighty-seven percent oflesions were invasive ductal carcinomas; 52% were ER+/Her2-, 17% weretriple-negative (TNBC), and 31% were Her2+.Nine (9%) of patients experienced a ≥G3 acute toxicity, all in theform of radiation dermatitis. There was one acute G4 incidence of skinnecrosis. There were no ≥G3 late toxicities or documented major cardiac events atthe time of last follow up. Median doses to critical organs at risk (OARs) were asfollows: mean heart 0.9Gy ( Citation Format: Cristina DeCesaris, Ariel Pollock, Emily Kowalski, Kayla Paulosky, Sung Choi, Mark Mishra, Elizabeth Nichols. Initial outcomes of adjuvant proton pencil beam scanning radiation for patients with breast cancer requiring comprehensive nodal irradiation within a single institution [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS15-12.