S1927
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
quality assessment in prostate SABR, reproducible across a network using unified protocols. Strong correlation with established metrics validates their utility for quantitative QA. These benchmarks enable rapid, objective plan evaluation, combining conformity and gradient into a single intelligible metric supporting efficient online ART delivery. Integration of automated plan quality tools, alongside existing TPS evaluation, may further streamline on-set adaptive QA and aid timely plan approval. References: 1.Dimitriadis, Alexis, and Ian Paddick. "A novel index for assessing treatment plan quality in stereotactic radiosurgery." Journal of neurosurgery 129.Suppl1 (2018): 118-124.2.Paddick, Ian. "A simple scoring ratio to index the conformity of radiosurgical treatment plans." Journal of neurosurgery 93.supplement_3 (2000): 219-222.3.Paddick, Ian, and Bodo Lippitz. "A simple dose gradient measurement tool to complement the conformity index." Journal of neurosurgery 105.Supplement (2006): 194-201.4.SABR UK Consortium. Stereotactic Ablative Body Radiotherapy (SABR) Consortium guidelines. SABR UK Consortium (2024) Keywords: Plan Quality, Conformity, Gradient, Efficiency INEX RT – SBRT during inhale and exhale breath hold to reduce sliding OAR dose and improve the therapeutic ratio for upper abdominal cancers Ian J. Gerard, Alana Pellizzari, Teodor Stanescu, Jeff Winter, Michael M. Yan, Laura A. Dawson Radiation Oncology, Princess Margaret Cancer Centre, University of Toronto, Toronto, Canada Purpose/Objective: SBRT for abdominal cancers adjacent to critical OARs is challenging due to CTV and OAR differential movement, low OAR RT tolerance, and increasing use of re-irradiation lowering OAR tolerances. Breath hold is often used for CTV immobilization. This pilot study hypothesized that composite treatment plans optimized using both deep inhale breath hold (DIBH) and exhale breath hold (EBH) can reduce maximum doses to sliding adjacent OARs (from baseline shifts in OAR relative to CTV), while simultaneously facilitating Digital Poster 3249
were evaluated as quantitative metrics to streamline plan approval during adaption.1 Material/Methods: Twenty consecutive prostate patients receiving MRgSABR (5 × 7.25Gy, total 36.25Gy) on three 0.35T ViewRay MRIdian MRLs were retrospectively analysed. All plans were created by six dosimetrists using identical class solutions. The cohort included 13 low- intermediate (LIR) and 7 high-intermediate-risk (HIR) patients (mean PTV 75.7 ± 17.4 cc).Efficiency Index ( η ₅₀ %) quantifies the ratio of integral dose within the PTV to that within the 50% prescription isodose volume (PIV ₅₀ %). OAR η ₅₀ % was similarly calculated for bladder, rectum, and urethra. These values represent the proportion of energy “doing good” (PTV) vs the proportion “doing harm” (OARs). Paddick Conformity Index (PCI), Paddick Gradient Index (PGI), Modified Gradient Index (MGI), and Prescription Dose Spillage (PDS) were also recorded.2-4 Pearson correlation and t-tests compared η ₅₀ % to established metrics. Results: Mean η ₅₀ % was 38.2% ± 2.6% (range: 33.3–43.0%) demonstrating excellent consistency despite variations in volume, attributable to standardised planning practices. Mean OAR η ₅₀ % values were: bladder 15.1% ± 3.4%, rectum 6.8% ± 2.1%, urethra 2.6% ± 0.6%. The sum of all efficiency indices (PTV + OARs) was 62.8% ± 4.7%, with the remainder of 37.2% ± 4.7% distributed to non-critical tissue.Strong statistically significant correlations were observed between η ₅₀ % and established metrics: MGI (r = –0.975, p < 0.0001), PGI (r = –0.876, p < 0.0001), and PDS (r = –0.833, p < 0.0001). PCI showed moderate correlation (r = 0.496, p = 0.026), influenced by the narrow PCI range (0.85-0.94). LIR showed higher η ₅₀ % (39.2% ± 2.1%) compared to HIR (36.3% ± 1.9%), reflecting target complexity.Based on this network-wide baseline, proposed acceptance criteria for adaptive plans are: η ₅₀ % > 35%, OAR η ₅₀ %(Bladder) < 20%, OAR η ₅₀ %(Rectum) < 10%, OAR η ₅₀ %(Urethra) < 4%.
CTV dose escalation. Material/Methods:
Ten 5-fraction SBRT plans for two patients requiring re-irradiation of upper abdominal CTVs adjacent to sliding OARs were created on both DIBH and EBH datasets using RayStation 2023B TPS. For each patient, DIBH, EBH and composite 3-fraction DIBH/2-fraction EBH plans were calculated using deformable image
Conclusion: Efficiency indices provide standardised, objective plan
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