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In the FAST-Forward trial, acute toxicity outcomes for patients receiving 26 Gy/5-fractions (F) were mild [1]. However, a slightly higher dose of 27 Gy/5F resulted in worse cosmetic and toxicity outcomes up to 5 years after treatment compared to the 26 Gy/5F group [1,2]. As dose distributions in the Netherlands may differ slightly from dose distributions of the FAST-Forward trial due to different planning schemes, the aim of this study was to assess clinician-reported acute toxicity in patients with early stage breast cancer treated with 1-week or 3-week adjuvant whole-breast (WBI) or partial-breast (PBI) irradiation without indication for regional nodal irradiation after breast-conserving surgery in the Netherlands.

This study was conducted within the prospective observational UMBRELLA cohort, including patients treated with 26 Gy/5F/1-week or 40 Gy/15F/3-weeks. From October 2021 onwards, the 26 Gy/5F/1-week scheme was clinically implemented as standard of care for selected adjuvant WBI or PBI without indication for regional nodal irradiation after breast-conserving surgery. A subgroup of patients aged ≥50 years who were treated with 40 Gy/15F between February 2020 and October 2021, was selected for comparison based on the inclusion criteria for the 26 Gy/5F scheme. Acute toxicity was graded using CTCAE criteria v5.0 before, during and up to four weeks after radiotherapy. Primary endpoint was defined as the proportion of patients with grade ≥ 1 toxicity for breast pain, dermatitis, lymphedema of the breast or fatigue at any time from the start of radiotherapy up to 4 weeks after completion.

In total, 150 patients were treated with 26 Gy/5F (80% WBI, 20% PBI) and 120 patients with 40 Gy/15F (82.5% WBI, 17.5% PBI). Prior to radiotherapy, breast pain grade 1 was observed in 13% of the 26 Gy/5F patients, dermatitis grade ≥ 1 in 0%, breast edema grade 1 in 5% and fatigue grade ≥ 1 in 15% (Table 1). In the 40 Gy/15F group, 21% experienced breast pain grade ≥ 1, 0% dermatitis grade ≥ 1, 3% breast edema grade 1, and 23% fatigue grade 1. Taking into account the worst CTCAE grade in the 26 Gy/5F group during and up to four weeks after completion of radiotherapy, breast pain grade 1 was experienced by 31% of all patients, dermatitis grade ≥ 1 by 62%, breast edema grade 1 by 17%, and fatigue grade ≥ 1 by 46%. In the 40 Gy/15F group, breast pain grade ≥ 1 was observed in 33% of the patients, dermatitis grade ≥ 1 in 71%, breast edema grade ≥ 1 in 19%, and fatigue grade ≥ 1 in 50%.

Mild to moderate acute toxicity in the 26 Gy/5F group during and up to four weeks after completion of radiotherapy was comparable to the 40 Gy/15F group. The current results will be complemented with clinician-reported outcomes of 200 patients treated with 26 Gy/5F in Maastro and patient-reported acute toxicity and quality of life up to three months after radiotherapy using the EORTC QLQ-C30/BR23 questionnaires of both UMC Utrecht and Maastro patients.