Large Institutional Experience of Early Outcomes and Dosimetric Findings with Postoperative Stereotactic Partial Breast Irradiation in Breast Cancer

Abstract

Purpose: To analyze the dosimetric and toxicity outcomes of patients treated with postoperative stereotactic partial breast irradiation (S-PBI).

Methods: We identified 799 women who underwent S-PBI at our institution between January 2016 and December 2022. The most commonly used dose-fraction and technique were 30 Gy in 5 fractions (91.7%) and a robotic stereotactic radiation system with real-time tracking (83.7%). The primary endpoints were dosimetric parameters and radiation-related toxicities. For comparison, a control group undergoing ultra-hypofractionated whole breast irradiation (UF-WBI, n=468) at the same institution was selected.

Results: A total of 815 breasts from 799 patients, with a median planning target volume (PTV) volume of 89.6 cm3, were treated with S-PBI. Treatment plans showed that the mean and maximum doses received by the PTV were 96.2% and 104.8% of the prescription dose, respectively. The volume of the ipsilateral breast that received 50% of the prescription dose was 32.3±8.9%. The mean doses for the ipsilateral lung and heart were 2.5±0.9 Gy and 0.65±0.39 Gy, respectively. Acute toxicity occurred in 175 patients (21.5%), predominantly of grade 1. Overall rate of late toxicity was 4% with a median follow-up of 31.6 months. Compared to the UF-WBI group, the S-PBI group had comparably low acute toxicity (21.5% vs. 25.2%, p=.12) but significantly lower dosimetric parameters for all organs-at-risks (all p<.05).

Conclusion: In this large cohort, S-PBI demonstrated favorable dosimetric and toxicity profiles. Considering the reduced radiation exposure to surrounding tissues, external beam PBI with advanced techniques should at least be considered over traditional WBI-based approaches for PBI candidates.