S1793
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
Digital Poster 31 Dosimetric accuracy of CBCT imaging for online adaptive breast radiotherapy JUAN-FRANCISCO CALVO-ORTEGA 1,2 , SOL SAN-JOSÉ- MADERUELO 1 , SANDRA MORAGUES-FEMENIA 1 , CORAL LAOSA-BELLO 1 , MARCELINO HERMIDA-LÓPEZ 3 1 RADIATION ONCOLOGY, HOSPITAL QUIRÓNSALUD BARCELONA, BARCELONA, Spain. 2 RADIATION ONCOLOGY, HOSPITAL QUIRÓNSALUD MÁLAGA, MÁLAGA, Spain. 3 Servei de Física i Protecció Radiològica, Hospital Universitari Vall d'Hebron, Vall d'Hebron Barcelona Hospital Campus, Barcelona, Spain Purpose/Objective: To investigate the feasibility of using conventional CBCT imaging for online adaptive breast radiotherapy. Material/Methods: A Varian TrueBeam v. 2.7 linac equipped with a standard CBCT imaging system, i.e., without the Varian HyperSight technology, was used. Ten breast cases (accelerated partial breast irradiation) previously treated in our department were randomly selected for this study. For each case, a setup CBCT scan acquired during a treatment session was registered (deformable fusion) with the planning CT (pCT) scan. The default “thorax” protocol available in the linac was used. No parameter adjustments were made for this default preset. A calibration curve (HU vs. mass density) was established in the Varian Eclipse TPS for this CBCT protocol. The original structures (PTV, uninvolved ipsilateral breast, contralateral breast, heart, and lungs) were mapped from the pCT to the CBCT scan, and the original IMRT plan computed on the pCT (Plan_Ref) was recalculated on the CBCT scan, by keeping the original fluences and monitor units (Plan_Adapt). The Acuros v. 16.1 algorithm (dose-to- medium) of the Eclipse was used. To investigate the dosimetric accuracy of CBCT-based dose calculation, Plan_Adapt was compared to Plan_Ref, which was established as the reference (ground truth). Two types of comparisons were done: 1) absolute differences in Dmean, D98%, and D2% were analyzed for the PTV, while Dmean and D2% were analyzed for the organs- at-risk (OARs). Differences were expressed as percentages of the dose prescription. 2) Gamma index analysis was performed using the 2% global/2 mm criteria. Gamma passing rates (GPRs) were analyzed at the levels of 20%, 50%, and 90% of the Plan_Ref maximum dose. Results: Average dose differences were within 1.4% for all metrics. The maximum difference was 2% for the ipsilateral lung (Figure).Excellent GPRs (mean ± SD) of 98.4% ± 1.2%, 98.4% ± 1.3%, and 98.0% ± 2.0% were obtained for 20%, 50%, and 90% dose levels.
Conclusion: The accuracy of the CBCT imaging investigated in this study was within 2% and is a feasible option for dose calculation in online adaptive breast radiotherapy. Note that this accuracy assessment was based on real
patients (non-phantom-based). Keywords: CBCT, adaptive, breast
Digital Poster 38 CBCT-based dosimetric verification of accelerated partial breast irradiation treatments JUAN-FRANCISCO CALVO-ORTEGA 1,2 , SANDRA MORAGUES-FEMENIA 1 , SOL SAN-JOSÉ-MADERUELO 1 , ODIMAR CORONIL-JARAMILLO 1 , MARCELINO HERMIDA-LÓPEZ 1 1 RADIATION ONCOLOGY, HOSPITAL QUIRÓNSALUD BARCELONA, BARCELONA, Spain. 2 RADIATION ONCOLOGY, HOSPITAL QUIRÓNSALUD MÁLAGA, MÁLAGA, Spain Purpose/Objective: To estimate the actual dose distribution received by patients treated with an accelerated partial breast irradiation (APBI) scheme. Material/Methods: Ten APBI patients treated with 5 × 5.2 Gy were included. Treatment plans consisted of multiple beams of IMRT planned with the Eclipse v. 16.1 treatment planning system. Patient setup was performed for each fraction with a 6D online rigid registration between the daily CBCT scan (CBCT1) and the planning CT (pCT) scan. A post-treatment CBCT scan (CBCT2) was acquired to capture the actual treatment isocenter position with respect to the patient's anatomy. The TrueBeam “thorax” imaging protocol of a TrueBeam linac was used. The accuracy of CBCT-based dose calculation with this protocol was ±2%, as was previously found by our team.Retrospectively, the pCT and CBCT2 scans were coregistered for each treatment fraction using deformable registration. The clinical and planning target volumes (CTV and PTV, respectively) and organs- at-risk (OARs) were mapped and adjusted to the CBCT2 scan by the oncologist. In all CBCT2 scans, patient rotations from the respective pCT-CBCT1 registrations were always observed..For each fraction,
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