ESTRO 2025 - Abstract Book

S2806

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2025

counterintuitive but it is known from SRS/SABR that steeper dose gradients are achieved by allowing target dose heterogeneity. A simultaneous integrated boost may therefore significantly improve both TCP and normal tissue sparing.

Keywords: Geometric uncertainty, margins, dose escalation

References: [1] D. Welsh, J. Wood, M. van Herk, and E. V. Osorio, “1907: Beyond the margin recipe: probabilistic and robust optimisation for cranial radiotherapy,” Radiotherapy and Oncology , vol. 194, pp. S3611–S3613, May 2024, doi: 10.1016/S0167-8140(24)02217-5. [2] X. S. Qi, C. J. Schultz, and X. A. Li, ‘An estimation of radiobiologic parameters from clinical outcomes for radiation treatment planning of brain tumor’, International Journal of Radiation Oncology*Biology*Physics, vol. 64, no. 5, pp. 1570–1580, Apr. 2006, doi: 10.1016/j.ijrobp.2005.12.022. [3] P. Pedicini et al., ‘Clinical radiobiology of glioblastoma multiforme’, Strahlenther Onkol, vol. 190, no. 10, pp. 925– 932, Oct. 2014, doi: 10.1007/s00066-014-0638-9.

2085

Digital Poster Dosimetric evaluation in lung SBRT for tumor size and lung density - A multi-treatment planning system study Indra J Das 1 , Ahtesham U Khan 1 , Mark Pankuch 2 , John McCorkindale 3 , Gocha Khelashvili 1 , Ping Xia 4 , Mohamed Abazeed 1 1 Radiation Oncology, Northwestern Memorial Hospital, Northwestern University Feinberg School of Medicine, Chicago, USA. 2 Medical Physics, Northwestern Medicine Chicago Proton Center, Warrenville, USA. 3 Department of Radiation and Cellular Oncology, Northwestern Medicine, Forest Lake, USA. 4 Radiation Oncology, Cleveland Clinic Foundation, Cleveland, USA Purpose/Objective: Lung SBRT has become a hallmark in radiation treatment of early-stage lung cancer and oligometastatic lesions in lung. Small lung lesions pose significant challenges due to very low lung density (1) and small-field dosimetry (2). Modern treatment planning systems (TPS) strive to account for these factors, but variations do exist. Due to inverse planning in VMAT, adequate coverage and dose to organs at risk (OAR) can impact clinical outcomes with toxicities (3, 4). The merit of TPS in low-density and small lesion is evaluated with advanced TPS. Material/Methods: Under IRB clearance one central lesion lung patient was anonymized and 12 studies were created with varying PTV sizes (1, 2, 3 cm diameter) and over-ridden lung density of 0.05, 0.1, 0.2 and 0.3 g/cm 3 in the range of -900 to -500 Hounsfield Unit (HU) that is often encountered in 4D and breath-hold CT-scans used in SBRT. The target density and OARs were maintained from the original plan. These 12 cases were sent to 4 advanced TPS Eclipse (AAA and Acuros), Pinnacle, RaySearch and Monaco for planning with identical dos-volume constraints with plan normalization (100% dose to 95% volume) and planning guidelines. Dose parameters, conformity index (CI), homogeneity index (HI), Gradient index (GI), MU/Gy, maximum dose, monitor unit (MU) and DVH were analyzed. Results: Most TPS met the DVH constraints with adequate target coverage reflected by CI. The HI varied significantly among the TPS as shown in Figure 1. Ideally, HI should be lowest for a good plan. HI is observed to be the lowest in Eclipse and highest in RaySearch. There was no discernible correlation with density and HI. The GI varied with lung density with higher value for lower lung density. RaySearch has the lowest GI and Eclipse the highest. The total monitor unit (MU) reflective of patient on table, body dose and beam time was analyzed and shown in Figure 2. Eclipse provided