Results of the ACCEL trial: Dosimetry in accelerated partial breast irradiation.

PURPOSE: To report the achieved dosimetry in the ACCEL trial and compare the results to reported dosimetry from the major accelerated partial breast irradiation (APBI) phase III trials. METHODS: The ACCEL trial was a single arm, phase II, prospective cohort study. A five-field, inverse-planned, IMRT strategy was employed using a class solution technique to increase planning consistency including high dose conformity and low normal tissue dose to the ipsilateral breast. RESULTS: Between 2016 and 2019, 283 patients were treated with 27 Gy in five fractions in consecutive days. The average PTV conformity index was 1.1. For the ipsilateral breast, the median (range) volume receiving 95% and 50% of the prescription dose was 9.7% (3.2-22.4) and 30.3% (11.1-54.6), respectively. Compared to major APBI phase III trial constraints, this reduction in irradiated volume at the 95% and 50% isodose levels represents a reduction of 209 cm3 and 265 cm3, respectively. CONCLUSION: The IMRT planning strategy employed in the ACCEL trial demonstrated consistent and superior dosimetry by comparison to conventionally used 3D CRT techniques. Future APBI trials should update dosimetric constraints.

One-Year Cosmesis and Fibrosis From ACCEL: Accelerated Partial Breast Irradiation (APBI) Using 27 Gy in 5 Daily Fractions.

PURPOSE: To report 1-year cosmesis and toxicity outcomes of a prospective, phase II trial of accelerated partial breast irradiation using intensity-modulated radiation therapy (RT) to deliver 27 Gy in 5 daily fractions. METHODS AND MATERIALS: Node-negative breast cancer patients after breast conserving surgery with clear excision margins, with physician-assessed excellent or good baseline cosmesis were invited to participate in a prospective clinical trial to receive 27 Gy in 5 daily fractions to the expanded primary site. Clinical photographs and European Organisation for Research and Treatment of Cancer cosmetic score were collected at baseline prior to RT and 1-year after radiation therapy. A protocol-specified, interim analysis was scheduled when 50 patients had completed 1-year follow-up. A panel of 6 physicians provided a consensus global cosmetic score (termed panel-assessed score) based on clinical photographs at baseline and 1-year. Fibrosis and telangiectasia were prospectively assessed by clinical research staff at clinic visits. RESULTS: At the interim analysis, 55 patients had baseline and 1-year post-RT images available. Most patients had either an improvement (53%) or no change (40%) in cosmesis from baseline to 1-year. Among 49 patients with excellent or good panel-assessed score at baseline, only 2 (4%) patients had a fair score at 1-year post-RT, indicating cosmetic deterioration. No patients had evidence of telangiectasia or grade 2 or higher fibrosis. There were no recurrences. CONCLUSIONS: APBI using 27 Gy in 5 daily fractions achieved acceptable 1-year cosmesis and no grade 2 fibrosis. A preplanned stopping rule of 5% grade 2+ fibrosis was not observed. The trial will continue to the planned target accrual of 274 patients.