S2033
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
iterative Acuros CBCT (iCBCT) or iCBCT with Metal Artifact Reduction (iCBCT-MAR) algorithm. Hounsfield Unit to electron density curves were defined in a prior study per imaging protocol on HSTB-CBCT for inhomogeneity correction. 1 39 plans calculated (RayStation 12A, Collapsed Cone Convolution v5.7) across four body sites (10 head, 9 thorax, 10 abdomen, 10 pelvis) on clinical CT-simulator (CT-sim) were registered by target to the HSTB-CBCT, contours rigidly propagated, and dose recalculated on HSTB-CBCT without scaling. Each CBCT external contour was intersected with respective CT-sim external and water density overrides applied to account for patient surface anatomy differences. Dose distributions were compared using 3D gamma analysis using 1%/1mm local criteria with 10% or 50% thresholding (10%TH, 50%TH). Percent difference in target (GTV, iGTV, or CTV) dose-volume histogram (DVH) metrics D99%, Dmean, and D1% were calculated per plan between CT-sim and HSTB-CBCT. Results Gamma passing rates (GPR) averaged 96.2±3.6% and 95.0±6.9% for 10%TH or 50%TH, respectively, with 36 (10%TH) and 32 (50%TH) out of 39 plans achieving >90% pass rates. The worst-case GPR at 10%TH was 87.7% and 71.3% at 50%TH, both in abdomen. Percent difference in target DVH metrics averaged 0.21±1.23%, 0.57±0.99%, and 0.89±1.27% for D99%, Dmean, and D1%, respectively. Only one plan had target D99% change >5% in abdomen, at -5.2% difference. The worst-case Dmean and D1% differences were in a thorax plan at 2.6% and 4.1%, respectively.
Conclusion HSTB-CBCT dose calculation accuracy compares favorably to standard-of-care CT-sim, potentially enhancing future patient care through direct CBCT calculations on c-arm linacs. Head and pelvis dose calculation demonstrated more favorable dosimetric comparisons to that of abdomen and thorax. Internal anatomy differences observed in abdomen and thorax, such as greater bowel gas volume or breathing phase differences, may have impacted calculations, which do not necessarily equate to calculation inaccuracy. With strict evaluation criteria, HSTB-CBCT dose calculations demonstrate the ability to be used as a primary dataset. References 1. Gregg KW, Arsenault T, Rezaei A, Kashani R, Henke L, Price AT. Hounsfield Unit characterization and dose calculation on a C - arm linac with novel on - board cone - beam computed tomography feature and advanced reconstruction algorithms. J Appl Clin Med Phys . 2025;26(8). doi:10.1002/acm2.70145 Digital Poster 5151 SBRT for ultra-central lung tumors: a dosimetric comparative study on organs at risk using different radiotherapy techniques Tarek Al Bitar, Ali Al Zein, Zeinab Dandash, Lara Hilal, Larry Bodgi, Racha Khodr, Tala Mobayed, Jimmy Sibaaly, Bassem Youssef
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