S1896
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
Material/Methods: We used data from 12 patients with painful, mechanically stable thoracic (n=8) and lumbar (n=5) metastases without malignant spinal cord compression. The patients were previously treated with dose-intensified SBRT. We used dCT acquired at a curved couch top; dCT images were not older than 4 weeks before SBRT. All patients were in the supine position on dCT, most (n=11) with their arms above the head. Target volumes and organs-at-risk were delineated on dCT using the same guidelines as applied to planning CT (pCT). We employed two dose levels with simultaneous integrated boost using VMAT – 40 Gy/5 and 20 Gy/5 to the high-dose PTV encompassing the CT- and/or MRI-based GTV and conventional-dose PTV encompassing the uninvolved parts of the affected vertebra, respectively. SBRT treatment plans were generated on dCT and transferred to pCT after their rigid co-registration, in which only translational motion was applied. We compared dose-volume parameters for the PTVs and organs-at-risk. Treatment plans were graded as per protocol, acceptable variation and unacceptable variation. Results: Treatment plans based on dCT met all treatment planning objectives. The mean difference (standard deviation [SD]) between dCT and pCT treatment plans was small for the high-dose PTV in V107% (1.2% [2.0%]) and V100% (-4.7% [7.5%]), all per protocol or acceptable variation. Mean D95% to the conventional- dose PTV was smaller in pCT treatment plans by 1.2 Gy (SD 1.5 Gy); in 3 targets D95% was smaller than 18 Gy (unacceptable variation) by 0.4 Gy, 0.8 Gy and 2.5 Gy, the latter due to 50 roll rotation on dCT. In 4 pCT treatment plans spinal cord PRV D0.1 cm3 exceeded the constraint by 0.7 Gy (roll 20, pitch 40), 3.5 Gy (60 roll, 70 pitch), 6.8 Gy (2 vertebrae irradiated; 10 roll, 60 pitch) and 9.7 Gy (100 roll, 90 pitch), and thus unacceptable variation. D0.1 cm3 in the cauda equina differed by 0.2 Gy on average (SD 0.6 Gy), all per protocol. Conclusion: Diagnostic CT-based dose-intensified SBRT for thoracic and lumbar metastases is technically feasible. High- dose targets in close proximity to the spinal cord in combination with large rotations seen on dCT require the use of positional devices and a 6DOF. Keywords: Sim-free SBRT, vertebral metastases, diagnostic CT
Digital Poster 2743 Distance - colored dose–volume histograms and proximity - based metrics for spatially informed plan evaluation Hideharu Miura 1,2 , Shuichi Ozawa 1,2 , Masahiro Kenjo 1 , Minoru Nakao 1,2 , Masanori Ochi 1 , Masayuki Kagemoto 1 1 Radiation oncology, Hiroshima High-Precision Radiotherapy Cancer Center, Hiroshima, Japan. 2 Radiation oncology, Hiroshima University, Hiroshima, Japan Purpose/Objective: Dose–volume histograms (DVHs) are valuable tools for summarizing the dose delivered to organs at risk (OARs); however, they lack spatial information about where the dose is distributed within these structures. This limitation hinders meaningful toxicity risk interpretation and restricts the utility of analytics in treatment planning. This study aims to present a distance - colored DVH (DC - DVH) that intuitively embeds distance - to - PTV information into the familiar DVH, and to quantify spatial dose conformity using the Spatial Volume at Risk (SVAR) to improve OAR toxicity interpretation and plan comparison. Material/Methods: DICOM - RT dose and structure data were analyzed using Python, converting PTV and OAR contours into volumetric masks at the in - plane dose - grid resolution and CT slice thickness to ensure voxel - wise correspondence. For each OAR dose voxel, the minimum Euclidean distance to the PTV surface was computed using an efficient nearest-surface distance computation, and cumulative DVHs were constructed using the standard dose–volume calculation method. Distance information was encoded by mapping the mean target - to - OAR distance within each DVH dose bin onto a perceptually ordered colormap to produce DC - DVHs while preserving the standard DVH layout (Figure 1). SVAR quantified the proportion of an OAR receiving at least X Gy within Y mm of the PTV. The method was applied retrospectively to ten prostate VMAT plans prescribed 78 Gy in 39 fractions, evaluating rectum and bladder SVAR across dose levels from 10 to 70 Gy and distance - to - PTV thresholds ranging from 5 to 35 mm.
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