S1943
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
Digital Poster 3678
Evaluation of Superficial Dose Accumulation in Breast Radiotherapy: Impact of Fraction Sampling and Superficial Volume Composition Using Velocity™ CHENHSIN WU, CHIENHSUN Chen Department of Radiation Oncology, Kaohsiung Municipal United Hospital, Kaohsiung, Taiwan
Purpose/Objective: To investigate the dosimetric uncertainty of
accumulated dose in deep inspiration breath hold (DIBH) breast radiotherapy and identify risk factors associated with dose deterioration, focusing on: 1) the effect of dose accumulation sampling frequency (5 fractions vs. 20 fractions) and 2) the correlation between superficial target volume composition and dose coverage deviation. Material/Methods: Twenty-six breast cancer patients were treated with DIBH using the Varian Halcyon linear accelerator and the IDENTIFY™ Surface Guided Radiation Therapy (SGRT) system for patient setup and motion monitoring. Treatment utilized the Simultaneous Integrated Boost (SIB) technique, delivering a prescription dose of 52.6 Gy to the Planning Target Volume (PTV) and 45 Gy to the remaining breast/chest wall, in 20 fractions. A total of 520 daily CBCTs were registered and dose accumulated using Velocity™ software.Two accumulation strategies were compared: full-course accumulation (20Fx) and 5- fraction sampling average (5Fx). Psuperficial was defined as the CTV52.6 volume within 1 cm of the skin divided by total CTV. Paired Wilcoxon signed-rank tests compared CTV52.6 V95% deviation (accumulated – plan) between 20Fx and 5Fx. Mann-Whitney U tests assessed Skin_PTV(PTV45 intersection 5mm skin) V95% deviation based on median Psuperficial cut-off. Results: Sampling Frequency: For CTV52.6 V95% deviation, 20Fx demonstrated significantly lower variability (SD=0.0125) than 5Fx (SD=0.0148) (p=0.009), indicating improved stability. However, no significant difference in systematic bias (mean deviation) was observed between the two methods (p>0.05)Superficial Volume: Using the median Psuperficial cut-off of 12.44%, the high Psuperficial group (thinner/more superficial targets) showed a significantly worse V95% coverage deviation for the Skin_PTV region (Median Deviation=-2.16%) compared to the low Psuperficial group (Median Deviation=-0.65%) (p=<0.01). CTV total volume showed no significant correlation with Skin_PTV dose deviation.
Significant differences were observed in the mean heart dose and V30 Gy among three plans (P < 0.001), showing Good < Bad < COVMAT (P < 0.001), with mean heart doses of 20.7, 22.1, and 23.7 Gy and V30 Gy values of 17.6%, 20.7%, and 24.6% for Good, Bad, and COVMAT, respectively. No significant differences were found in PTV D95%, D2%, lung V20 Gy, V5 Gy, or spinal cord D0.03cc. The mean thyroid dose was significantly higher in BROAD-RT (P = 0.006), with higher values in Good vs COVMAT (P = 0.025) and Bad vs COVMAT (P = 0.031). Significant differences were also observed for the kidneys, liver, and stomach (P = 0.001, 0.008, and 0.015, respectively). Pairwise analyses showed that Bad delivered higher mean doses than COVMAT for the kidneys and stomach (P = 0.037 each), and higher than Good for the liver (P = 0.006). The estimated delivery times were 151 s (Good), 145 s (Bad), and 91 s (COVMAT). Both BROAD-RT plans required longer times than COVMAT (P = 0.002), with no difference between Good and Bad (P = 0.19). Conclusion: BROAD-RT using customized dynamic trajectories significantly reduced heart dose compared with coplanar VMAT, while maintaining target coverage. References: Hirashima H et al. Dynamic non-coplanar irradiation for esophageal cancer using OXRAY system (BROAD- RT): Dosimetric and feasibility study. Phys Imaging Radiat Oncol. 2025; 33: 100698.Kato K et al. JCOG1109 NExT trial. Lancet. 2024; 404: 55–66.Garant A et al. Clinical and dosimetric predictors of cardiac events after chemoradiation for esophageal cancer. Pract
Radiat Oncol. 2021; 11: e415–e425.Hiraoka S, Hirashima H et al. Integration test of biaxially rotational dynamic-radiation therapy for
nasopharyngeal carcinoma: Efficacy evaluation and dosimetric analysis. Radiother Oncol. 2025; 110879. Keywords: BROAD-RT, Heart dose, Esophageal cancer
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