DISCUSSION
high‐volume, MLCs, breast cancer
BREAST CANCER, LEAKAGE
In this technical report, we have demonstrated our initial clinical experiences of benchmarking Halcyon RDS for fast, safe, and accurate CBCT‐guided treatment of APBI patients who underwent the Fast‐Forward trial of 26 Gy in five treatments, daily. Our Halcyon VMAT plans via three partial unilateral arcs with stacked/staggered MLCs and manually selected patient specific collimator angles to minimize leakage dose from MLC travelling surpassed the clinical TrueBeam plans. Halcyon APBI plans were highly conformal, homogeneous, achieved adequate target coverage, and effectively spared the adjacent critical organs (Table Achieving acceptable dose limits in regards to PTV coverage constraints and normal tissue organ constraints is of high priority. As seen in the Fast‐Forward trial, there was a significant increase in morbidity when comparing 26 Gy in five fractions with 27 Gy in five fractions. This shows a narrow therapeutic index in order to both provide adequate local control and acceptable morbidity. Therefore, the ability of Halcyon to meet the dosimetric PTV criteria of Fast‐Forward and still provide a lower ipsilateral lung dose shows that the five‐fraction regimen can be safely used on Halcyon RDS.A few things should be mentioned regarding Halcyon APBI. In addition to providing rapid APBI treatments in community centers, Halcyon RDS could potentially support academic centers with high‐volume SBRT clinics in the expansion of their SBRT program(s) or transfer of APBI patients to Halcyon for similar yet faster treatment, if needed. Moreover, in the case of a long machine (C‐arm Linac) downtime, quickly replanning APBI treatment on Halcyon would be very helpful for not delaying the treatment course for patients undergoing the Fast‐Forward trial. However, we mention a few caveats in this study. One caveat is a TrueBeam Linac with a 6DOF couch may better reproduce rotational patient setup errors in treatment delivery. However, as of now, the exact dosimetric impacts of 6DOF versus 3DOF couch corrections for a large target volume such as in APBI treatments on Halcyon are not known and it is an interest of our future investigation. Another caveat is that the current Halcyon RDS does not yet have a full package of motion management system available and clinically implemented. Thus, as of now, fully automated deep inspiration breath‐hold (DIBH) for APBI treatments is not available on our Halcyon.In summary, rapid delivery of iCBCT‐guided APBI treatments (< 10 min) is possible with the use of Halcyon RDS, potentially benefiting patients by reducing intrafraction motion error, and improving patient comfort and compliance with no patient collision issues. To provide them a faster, safer, and overall improved quality of radiation treatment, at the time of this report, we are actively recruiting APBI patients on the Fast‐Forward trial's dosing scheme on Halcyon RDS. Our ongoing study includes the automation of a RapidPlan knowledge‐based planning (KBP) model for generating high‐quality APBI plans for breast cancer patients on Halcyon RDS, similar to the one recently developed by Costa et al.