2205 - Direct-to-Unit Adaptive Single Fraction Partial Breast Irradiation Using CT-Guidance: An In-Silico Study

Purpose/Objective(s): Standard accelerated partial breast irradiation (APBI) regimens require significant time commitment from patients, physicians, and clinic staff, involving a simulation scan followed by five to ten treatments. Recent trials have investigated the safety and feasibility of using one to three fractions to deliver APBI, but still require an additional visit for simulation. We conducted an in-silico study to evaluate the feasibility of using CT-guided adaptive stereotactic radiation to deliver simulation-free, direct-to-unit (DTU) APBI in a single fraction.

Materials/Methods: Ten patients previously treated with partial breast irradiation with HyperSight cone beam computed tomography (H-CBCT) imaging were included in this study. Previously acquired H-CBCTs were used to simulate single session sim-free adaption sessions. The use of online auto-contouring and an advanced practice radiation therapist (APRT) were leveraged for initial pre-plan target and organs-at-risk (OARs): ipsilateral breast, contralateral breast, lung, heart and the surgical cavity. The prescription was 20 Gy to the surgical cavity and 5 Gy to the planning target volume (PTV), defined as a 1.5 cm expansion from the surgical cavity. A physician and physicist reviewed contours prior to plan generation and optimization, then reviewed the generated online plan for dosimetric and safety constraints. A simulated APBI treatment was delivered in a single fraction. Average time for each step, from planning image acquisition to treatment delivery, was calculated.

Results: DTU APBI plans were successfully created and delivered for all ten patients in-silico. From initial H-CBCT acquisition to completion of treatment, mean (range) end-to-end workflow time was 30.8 minutes (21.0-39.4). Primary dosimetric and coverage constraints were met for all plans. Plan creation and optimization was the most time-intensive workflow component, with mean of 11.0 minutes (6.8-15.4).

Conclusion: DTU, single-fraction APBI is clinically feasible in-silico and could provide significant time-saving advantages for patients, reducing the total number of visits needed to complete adjuvant breast radiation while maintaining safety and target coverage goals. A prospective clinical trial is in development.