Saturday, June 18, 20223:45 PM - 4:45PM MSS01 Presentation Time: 3:45 PM: A Single-Institution Retrospective Study of Three-Fraction HDR Accelerated Partial Breast Irradiation

Abstract

<h3>Purpose</h3> Accelerated partial breast irradiation (APBI) delivered with high-dose-rate brachytherapy is a standard of care treatment which is typically delivered over 8-10 fractions. The TRIUMPH-T multi-institutional study recently demonstrated promising results using a shorter three fraction regimen. However, there are limited additional published series using this regimen. Here, we report our experience and outcomes for patients treated as per the TRIUMPH-T regimen. <h3>Materials and Methods</h3> Retrospective single-institution analysis of patients who underwent lumpectomy followed by APBI (22.5 Gy in 3 fractions delivered over 2-3 days) using a SAVI applicator between November 2016 and January 2021. Per the TRIMPH-T protocol guidelines, the planning treatment volume (PTV) was 1 cm beyond the surgical cavity, limited to 0.5 cm from the skin surface, and subtracted from chest wall and SAVI cavity. Dose-volume metrics were obtained from the clinically treated plan. Chart review was performed to determine locoregional recurrence and toxicities according to CTCAEv5.0. <h3>Results</h3> From 2016-2021, 31 patients were treated per the TRIUMPH-T protocol. The median age was 64 years (range 46-82). 77% of patients had invasive ductal carcinoma, median tumor size was 1.2 cm (range 0.4-2.0), 84% were Grade 1-2, 90% were N0 (3 DCIS patients were Nx), and 100% were ER+. Median follow-up was 31 months (range 6-62 months) from completion of brachytherapy. PTV median dose metrics were as follows: D90: 94.6% (range 90-110%), V150: 22.0cc (range 12.5-38.2cc), V200: 11.0cc (range 3.5-21.2cc), and DHI: 0.51 (range 0.36-0.68). Median D0.03cc for the skin (3 mm inner margin on body) and rib were 68.3% (range 29.9-91.6%) and 70.6% (range 15.7-124.2%), respectively. There were no acute/late grade 3 or higher toxicities. Cumulative late Grade 1 and 2 toxicities were seen in 58% and 6% of patients, respectively. Grade 1-2 fibrosis was seen in 32% of patients. Other toxicities included grade 1 breast pain (26%), grade 1-2 seroma formation (23%), grade 1 hyperpigmentation (6%), symptomatic fat necrosis (6%), and breast infection (3%). Of note, four patients experienced locoregional recurrence: three ipsilateral breast tumor recurrences (two in the same quadrant as the lumpectomy cavity), and one nodal recurrence (patient was cN0/pN0). All three ipsilateral breast tumor recurrences occurred in patients who would be classified as "cautionary" based on ASTRO consensus guidelines due to age ≤50, lobular histology, or high grade. There were no distant recurrences or deaths of any cause. <h3>Conclusions</h3> Three-fraction HDR brachytherapy APBI delivered per the TRIMUPH-T protocol was well-tolerated in our cohort with no grade 3 or higher toxicities and an acceptably small percentage of grade 2 toxicities. Given the small sample size, the number of recurrences suggests that attention to appropriate patient selection is necessary until more long-term follow-up data is available.