Dart-CBCT: Dose Accumulation for Robust Tangents Using CBCT(Focus: Evaluating Open Beam Proportion in 5-Fraction Tangential Breast)
Abstract
Purpose
Hybrid tangential breast radiotherapy commonly uses a high proportion of open fields to enhance robustness against inter-fraction anatomical variation, potentially limiting planning flexibility. In five-fraction hypofractionated breast radiotherapy, the necessity of this constraint is unclear. This study evaluates the dosimetric impact of reducing the open field proportion from 80% to 70% using CBCT-based dose accumulation and describes the development and initial application of a robustness-informed delivered dose assessment framework.
Methods
CBCT-based dose accumulation was implemented in RayStation 2023B using deformable image registration to map daily CBCT images to the planning CT. A phantom study characterized baseline accumulated dose differences arising from CBCT density assignment and dose recalculation, establishing expected methodological variability independent of anatomical change. Two strategies for CBCT HU-to-density correction were evaluated and their impact on accumulated dose metrics assessed. Following validation, 20 patients treated with five-fraction tangential breast radiotherapy with daily CBCT imaging are being retrospectively analyzed. For each patient, the clinically delivered plan (≥80% open beam weight) is compared with a re-planned treatment using a 70% open field proportion. Accumulated dose is evaluated for target volumes and organs at risk, including the heart, lungs, humeral head, and skin.
Results
Phantom analysis defined the baseline accumulated dose variability inherent to the CBCT-based workflow. CT-based density assignment was more accurate than bulk density assignment and was therefore adopted for patient analysis. Preliminary accumulated dose results compare target coverage and organ-at-risk doses between clinical and 70% open field plans. Across six preliminary patients, delivered accumulated dose was within 3% of planned target coverage, and average heart dose differences were within 4 cGy. Replanning with only 70% of total beam weight from open segments did not compromise delivery robustness in cases analyzed to date.
Conclusion
A validated CBCT-based dose accumulation workflow demonstrates feasible robustness with reduced open field hybrid planning.