ESTRO 2025 - Abstract Book

S4101

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2025

Keywords: SGRT, Spirometry, Gantry-less

References: Katz, S., Arish, N., Rokach, A., Zaltzman, Y. and Marcus, E.-L. (2018). The effect of body position on pulmonary function: a systematic review. BMC Pulmonary Medicine, 18(1), p.159. doi:https://doi.org/10.1186/s12890-018-0723 4. Zou, X., Lan, L., Zheng, L., Chen, J., Guo, F., Cai, C., Hong, J. and Zhang, W. (2021). Effect of tumour and normal lung volumes on the lung volume–dose parameters of IMRT in non–small-cell lung cancer. Clinics , 76, p.e2769. doi:https://doi.org/10.6061/clinics/2021/e2769

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Digital Poster Adapt-to-shape strategy for OAR deformations only has limited clinical relevance for SBRT prostate cancer in MRI-guided online adaptive radiotherapy

Lisa Wiersema, Matthijs G Dassen, Floris Pos, Uulke A van der Heide, Tomas Janssen Radiation Oncology, The Netherlands Cancer Institute, Amsterdam, Netherlands

Purpose/Objective: On the Unity MR-Linac (Elekta AB, Stockholm, Sweden) two adaptation strategies are available: Adapt-to position(ATP) and adapt-to-shape(ATS). The average total treatment time when treating SBRT prostate patients is 25.1min for ATP versus 54.5min for ATS (1). ATP could be preferred for efficiency reasons, unless there is a clear clinical benefit of ATS. In our clinical workflow, ATP is chosen, unless there is a substantial deformation of the clinical target volume (CTV), or of the organs-at-risk (OAR) within 1 cm of the planning target volume (PTV). However, the clinical relevance of correcting OAR deformations only is not obvious, while in our clinical practice this scenario is not uncommon (58% of fractions). The aim of the current work was to determine the clinical relevance of using ATS in the case of OAR deformations only for prostate SBRT patients. Material/Methods: We selected 5 SBRT prostate patients from January 2024. All patients were treated in 5 fractions, using the same OAR constraints and PTV margins. ATS was performed to correct for OAR deformations only in 84% of the fractions, per patient, we selected one of these fractions. The main OARs that influence a prostate treatment plan are Bladder and Rectum, the tolerance in our institute for SBRT prostate patients are, Bladder V3700cGy <5cm³; Rectum V3800cGy <1cm³ and V3500cGy <1cm³. To quantify the impact of dose delivered to the OAR, we compared the dose distribution from the online performed ATS fraction (ATS(adjusted OAR)), with a simulated ATP dose distribution. The ATP dose distribution was evaluated both on the non-adjusted delineations (ATP(non-adjusted OAR)) as on the adjusted OAR delineations (ATP(adjusted OAR)). PTV coverage of both strategies was also compared. Results: For all dose metrics, the average difference of the (ATP(non-adjusted OAR)) - (ATS(adjusted OAR) is positive and (ATP(adjusted OAR)) - (ATS(adjusted OAR) is negative (table1). This implies that while (ATS(adjusted OAR)) might appear beneficial, as the dose distribution is planned on adjusted OAR, the average dose in the adjusted OAR is actually slightly higher compared to (ATP(adjusted OAR)), although a T-test showed none of these differences to be significantly different from zero(P-values 0.35 – 0.70). In both strategies the PTV coverage was adequate, according to local protocol (figure1).