A Single-Institution Retrospective Review Intensity-Modulated Accelerated Partial Breast Irradiation Using a Radiopaque Filament and Breath-Hold to Facilitate Reduced CTV to PTV Margin.

Accelerated Partial Breast Irradiation (APBI) with external beam IMRT has emerged as an excellent treatment strategy for select early-stage breast cancer patients. Our institutional preference for APBI consists of 3000cGy in 5 non-consecutive fractions with IMRT per the Florence protocol. At our institution, this has recently evolved to include deep-inspiration breath-hold (DIBH) for motion management, along with the preferred use of radiopaque filament marker (FM) for target delineation and daily CBCT-based image guidance. The CTV to PTV margin has been reduced from 10-mm to 5-mm for such patients, in light of reduction in uncertainty of target localization and uncertainty of motion. We hypothesize a reduction in CTV-to-PTV margins from 10 mm to 5 mm with implementation of DIBH and filament marker for IMRT-based APBI will result in non-inferior local control compared to recently published contemporary data. This study was conducted through a retrospective chart review of 16 stage 0 and 57 stage 1 breast cancer patients including 60 "acceptable" and 13 "cautionary" patients per ASTRO APBI consensus guidelines treated at our institution from March 2021 through June 2023 with 5-fraction IMRT APBI, using DIBH for motion management and lumpectomy cavity delineation using a FM. Standard 10-mm margin was used to expand from lumpectomy cavity to CTV for all patients. A reduced 5-mm expansion from CTV to PTV was utilized (compared to 10-mm CTV to PTV expansion in Florence protocol) in light of motion management and target localization techniques described above. Cropping of PTV from skin surface and/or chest wall followed Florence guidelines. With a median follow up of 23.1 months, local control was 100% for patients treated with reduced 5-mm CTV to PTV margins. There were no cases of subacute radiation pneumonitis. Compared to the results of "standard" 10-mm CTV to PTV expansion, the use of 5-mm CTV to PTV margin reduced the mean PTV for these patients by approximately 112 cc (> 80% absolute volume reduction). For patients treated with 5-fraction external beam APBI, the use of DIBH for motion management and filament marker for target localization appears to allow for safe reduction in CTV-to-PTV expansion without reduction in local control. Absolute reduction in PTV with this approach is substantial, with potential implications for cosmetic outcome or other late effects. Longer term follow up will be needed to confirm the durability of these local control and toxicity results and may offer potential for improved cosmesis results. [ABSTRACT FROM AUTHOR]