S2001
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
Radiotherapy, Radiotherapy and Oncology, 2021. 3. Tian, L., Superficial Dosimetry Study of the Frequency of Bolus Use in Post - Mastectomy Breast Radiotherapy, Technology in Cancer Research & Treatment, 2024. 4. Kaidar - Person, O., The Use of Bolus in the Setting of Post - Mastectomy Radiation Therapy for Breast Cancer: A Systematic Review, Radiotherapy and Oncology, 2021. 5. Bahhous, K., Zerfaoui, M., El Khayati, N., Effect of Bolus Frequency and Its Thickness in Post - Mastectomy 3D - CRT on Skin Dose, Iranian Journal of Medical Physics, 2019;16:397 - 404. Keywords: Simulation bolus, Planned bolus, Dose difference Enhancing plan quality through collimator angle optimisation in head and neck (H&N) volumetric modulated arc therapy (VMAT) planning Rhea Modi Radiotherapy Physics, University Hospitals Sussex NHS Foundation Trust, Brighton, United Kingdom Purpose/Objective: In VMAT, collimator angle influences modulation and leakage, in turn affecting PTV coverage, conformity, and OAR sparing. For complex H&N sites, where targets lie near critical structures, optimising Digital Poster 4508 collimator angles may yield significant dosimetric improvements. However, research using several collimator angles in H&N radiotherapy is limited.We evaluated whether optimising collimator angles in dual-arc and two independent single-arc H&N VMAT plans improves target coverage and/or reduces OAR dose. We hypothesised that 30° or 45° could yield superior dosimetric outcomes versus standard practice [1, 2]. Material/Methods: Twenty-one retrospective H&N dual-arc VMAT plans were carefully selected and categorised into five sites: tonsil, base of tongue, oropharynx, nasopharynx and larynx, including one large-PTV case per site to explore optimisation for such presentations. Originally at 5°, treatment plans were re-optimised in RayStationTM at 15°, 30° and 45°, using identical objectives and clinical goals. Python scripts automated plan creation, optimisation and evaluation. All parameters were standardised. Plan quality was assessed through DVHs, quality indices, clinical goals and 3D dose inspection, with statistical analysis determining the optimal collimator angle for PTV coverage and OAR
dosimetric differences between plans generated with bolus scanned during simulation versus plans where bolus is applied only at treatment. Material/Methods: Thirty post-mastectomy breast cancer patients treated at Labaid Cancer Hospital and Super Specialty Centre were included. Each underwent two CT simulations: one with a 5-mm bolus applied during scanning (Bolus Plan) and one without bolus (Non-Bolus Plan). Identical 3D conformal radiotherapy plans were generated for both datasets using the same beam parameters. Dosimetric analysis included skin surface dose (0–3 mm depth), PTV coverage (D95%, Dmean), dose homogeneity index (HI), conformity index (CI), and ipsilateral lung and heart doses. Paired t-tests assessed statistical significance (p < 0.05). Results: Surface dose in the Bolus Plan ranged from 92–101% of prescribed dose (mean 96.6% ± 2.9%), while in Non- Bolus Plans, TPS-calculated surface dose was 78–87%, but the estimated delivered dose with bolus was 108– 121%, corresponding to an average 17.5% overdose. PTV coverage (D95%) decreased from 95.5% ± 1.8% in Bolus Plans to 89.4% ± 2.5% in Non-Bolus Plans, with Dmean reduced by 3.2–4.8%, indicating slight underdosage of deeper target regions. HI increased from 0.13–0.16 to 0.19–0.22 (8–11% reduction in dose uniformity), while CI decreased from 0.91 ± 0.03 to 0.86 ± 0.04. Patients with thinner chest walls (<2.5 cm) experienced higher surface dose discrepancies (up to 25%). Ipsilateral lung V20Gy and mean heart dose showed minor changes (<3%), which were not statistically significant. All differences in surface dose, PTV coverage, and HI were statistically significant (p < 0.05). Conclusion: Omitting bolus during CT simulation results in substantial deviations between planned and delivered dose distributions, including surface dose overestimation and partial under dosage of the PTV. Variations in chest wall thickness and bolus placement exacerbate these effects. Including bolus during simulation ensures accurate modeling of dose build- up, improves homogeneity, and minimizes skin toxicity. Routine verification of bolus placement, patient immobilization, and consistent simulation practices are recommended to ensure reproducible and clinically safe dose delivery in post-mastectomy breast radiotherapy. Specially for those centers where half treatment days are bolus and others are non bolus it is recommended to do two treatment plan on different scans. References: 1. Li, F., Bolus Use in Post - Mastectomy Radiation Therapy for Breast Cancer, Translational Cancer Research, 2025. 2. Kawamoto, T., Dosimetric Assessment of Bolus for Post - Mastectomy Chest Wall
sparing. Results:
Mean PTV D95% was highest at 45° (63.54 ± 0.28 Gy) and lowest at 15° (63.32 ± 0.22 Gy). Tukey’s range test confirmed a statistically significant difference between
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