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Background: Breast conserving surgery (BCS) is typically followed by adjuvant radiotherapy (RT) based on several landmark trials demonstrating improvements in disease control and survival. Since completion of these historical trials, the advent of molecular subtyping has revealed that breast cancer is not a single disease entity, but rather a class of cancers with differential risk profiles. We evaluated whether RT could be safely omitted following BCS for patients with the most favorable subtype as defined by the Prosigna PAM50 assay. Methods: We conducted a multicenter prospective single-arm cohort study with IRB approval and an FDA investigational device exemption (IDE). Eligible patients were women 50 to 75 years of age (inclusive) who had undergone BCS revealing tumors ≤2cm in size, that were estrogen or progesterone receptor positive (HR+), HER2 negative, grade 1-2, node negative (N0), with negative excision margins (no ink on tumor). Intent to take endocrine therapy was required. Upon registration, tumors were submitted for central Prosigna testing and those with Risk of Recurrence (ROR) score ≤40 were deemed eligible for the investigational omission of RT. The primary endpoint was the 5-year locoregional recurrence rate (LRR). Anticipating a total of 345 RT-omitting patients to enroll over 3.5 years, the study was designed with 90% power to exclude a 5-year LRR of 5% using a one-sample exponential test with one-sided type I error of 0.025. Results: From 2016 to 2020, 671 patients were registered from 13 centers, inclusive of affiliated regional network sites. Of these, 382 patients had a ROR Score ≤40 and opted to forego RT, comprising the main intention-to-treat (ITT) study population. Median age was 65 years (range 50 to 75), and median tumor size was 0.9 cm (range 0.1 to 2.0 cm). At a median follow-up of 26.9 months, 12 events were observed: 4 patients had ipsilateral in-breast recurrences, 7 had contralateral breast cancers, and 1 developed an unrelated melanoma. There were no regional-nodal or distant recurrences. The 2-year cumulative rate of LRR was 0.3% (95% CI: 0 – 1.0%). Of the 4 ipsilateral breast recurrences, 2 were in the same quadrant as the original primary tumor. Conclusion: In this preliminary report of the PRECISION trial, patients 50-75 years of age undergoing BCS and endocrine therapy for pT1N0 HR+ HER2-negative breast cancer with ROR score ≤40 had exceedingly low rates of LRR in the absence of adjuvant RT at a median follow-up of 26.9 months. Additional follow-up is required to determine whether these favorable results are durable. Citation Format: Lior Z. Braunstein, Julia Wong, Deborah A. Dillon, Yu-Hui Chen, Paul Catalano, Oren Cahlon, Mahmoud B. El-Tamer, Rachel Jimenez, Atif Khan, Carmen Perez, Rinaa Punglia, Ron Shiloh, Laura Warren, David Wazer, Jean Wright, Elizabeth Buckley, Tari King, Simon Powell, Eric Winer, Jennifer Bellon. Preliminary report of the PRECISION Trial (Profiling Early Breast Cancer for Radiotherapy Omission): A Phase II Study of Breast-Conserving Surgery Without Adjuvant Radiotherapy for Favorable-Risk Breast Cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr OT1-12-02.

Recent data have been published supporting the application of ultrashort radiation therapy (RT) regimens for women with early stage breast cancer after breast conserving surgery. What has remained controversial is whether and how to apply accelerated whole breast irradiation (AWBI) or accelerated partial breast irradiation (APBI) approaches in these patients, as well as the consideration of intraoperative RT (IORT).We performed a systematic review of the literature searching for randomized and prospective data published evaluating ultrashort RT delivered in 5 days or less with APBI, AWBI, or IORT.We identified 2 randomized studies evaluating AWBI (n = 5,011 patients) with 5 to 10 year follow-up, which supported the use of ultrashort course AWBI compared with hypofractionated whole breast irradiation (WBI). We identified 7 randomized trials evaluating APBI (compared with WBI) in 5 days or less (n = 8528) with numerous (n = 55) prospective studies as well, with the data supporting short course APBI. Finally, we identified 2 randomized trials evaluating IORT; however, both trials demonstrated elevated rates of recurrence with IORT compared with WBI.The current body of data available for ultrashort adjuvant RT regimens delivered in 5 days or less after breast conserving surgery overwhelmingly support their utilization. Although data for both exist, APBI regimens have, by far, greater numbers of patients and longer follow-up compared with AWBI. Also, given increased rates of recurrence seen with IORT with long-term follow-up, this should not be considered a standard approach at this time.

In recent years, results with mature follow-up have been reported for several Phase III trials randomizing women to receive whole breast irradiation (WBI) versus varying modalities of partial breast irradiation (PBI). It is important to recognize that these methods vary in terms of volume of breast tissue treated, dose per fraction, and duration of therapy. As such, clinical and technical guidelines may vary among the various PBI techniques.Members of the American Brachytherapy Society with expertise in PBI performed an extensive literature review focusing on the highest quality data available for the numerous PBI options offered in the modern era. Data were evaluated for strength of evidence and published outcomes were assessed.The majority of women enrolled on randomized trials of WBI versus PBI have been age45 years with tumor size3 cm, negative margins, and negative lymph nodes. The panel also concluded that PBI can be offered to selected women with estrogen receptor negative and/or Her2 amplified breast cancer, as well as ductal carcinoma in situ, and should generally be avoided in women with extensive lymphovascular space invasion.This updated guideline summarizes published clinical trials of PBI methods. The panel also highlights the role of PBI for women facing special circumstances, such as history of cosmetic breast augmentation or prior breast irradiation, and discusses promising novel modalities that are currently under study, such as ultrashort and preoperative PBI. Updated consensus guidelines are also provided to inform patient selection for PBI and to characterize the strength of evidence to support varying PBI modalities.