Purpose/Objective(s):
In gynecologic external beam radiotherapy (RT), an internal target volume is traditionally required to account for significant target displacement driven by unpredictable bladder filling. Adaptive RT can minimize these margins by reoptimizing the plan based on anatomy-of-the-day; however, in hypofractionated regimens, bladder variability remains a critical challenge. Significant volume shifts alter organ-at-risk (OAR) geometry, compromising constraints and increasing daily plan reoptimization complexity. Anatomic stability is central to the HERA trial (NCT06538337), a prospective single-institution study of adjuvant adaptive stereotactic body RT (aSBRT) for endometrial and cervical cancers. We evaluated whether a standardized institutional ultrasound-guided (USg) filling workflow achieves reproducible CT-based bladder volumes for patients treated with aSBRT on the HERA trial.
Materials/Methods:
We conducted a retrospective review of eight patients enrolled on the prospective HERA trial. All followed an institutional USg bladder filling protocol prior to CT simulation (Sim CT) and each treatment. At simulation, a US-estimated volume of 200-250 cc was targeted, with final volumes tailored to individual tolerance. Point-of-care US confirmed threshold compliance before each fraction. Bladder volumes were contoured on Sim CT and five daily pre-treatment cone-beam CT (CBCT) scans per patient (40 total fractions). Consistency was quantified using intra-patient coefficient of variation (CoV), mean absolute/percentage differences, and paired t-tests.
Results:
Mean bladder volume on Sim CT was 378.5 ± 152.0 cc (range 187.0-588.2 cc). Mean daily bladder volume on CBCT was 346.9 ± 145.2 cc (169.8-569.4 cc). CBCT volumes were systematically and significantly lower than Sim CT volumes, with a mean absolute decrease of 31.6 cc (8.7%;p = 0.0054). Despite this shift, intra-patient reproducibility remained high, with mean CoV of 12.4% ± 10.2% (1.9%-33.4%). High stability was observed across the cohort, with 75% of patients (6/8) maintaining an individual mean CoV <15%. 85% of fractions (34/40) showed <20% deviation from Sim CT volume, and 70% (28/40) were within 15%.
Conclusion:
Among patients enrolled on the HERA trial undergoing adjuvant aSBRT for gynecologic cancers, implementation of a standardized USg bladder filling workflow resulted in reproducible CT-based bladder volumes across treatment fractions. Although pre-treatment CBCT volumes were systematically lower than Sim CT, likely due to shorter US-to-treatment intervals, intra-patient CBCT volumes remained highly consistent during the treatment course. In a hypofractionated adaptive workflow, this reproducibility supports stable OAR constraints, minimizes extent of daily plan reoptimization, and improves confidence in cumulative dose delivery. Ongoing analyses will evaluate whether this stability translates into improved adaptive workflow efficiency and reduced OAR exposure variability.
