ESTRO 2025 - Abstract Book

S3149

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2025

Keywords: plan of the day, autosegmentation, cervical cancer

References: [1] J. Bertholet et al. , “Patterns of practice for adaptive and real-time radiation therapy (POP-ART RT) part II: Offline and online plan adaption for interfractional changes,” Radiotherapy and Oncology , vol. 153, pp. 88–96, Dec. 2020. [2] F. Isensee, et al., “nnU-Net: a self-configuring method for deep learning-based biomedical image segmentation,” Nat Methods , vol. 18, no. 2, pp. 203–211, Feb. 2021. [3] C. Zhang et al. , “Automatic segmentation for plan-of-the-day selection in CBCT-guided adaptive radiation therapy of cervical cancer,” Phys Med Biol , vol. 67, no. 24, Dec. 2022. [4] C. Beekman, et. al, “Improving predictive CTV segmentation on CT and CBCT for cervical cancer by diffeomorphic registration of a prior,” Med Phys , Feb. 2022.

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Poster Discussion Attenuation images from Time-of-Flight PET for adaptive workflows in proton therapy Ann-Kristin Lüvelsmeyer 1,2 , Claus Maximilian Bäcker 1 , Jörg Wulff 1 , Mohammadreza Teimoorisichani 3 , Beate Timmermann 1,4 , Christian Bäumer 1,2 1 West German Proton Therapy Centre Essen (WPE), University Hospital Essen, Essen, Germany. 2 Department of Physics, TU Dortmund University, Dortmund, Germany. 3 Siemens Medical Soluctions USA Inc, Siemens Healthineers, Knoxville, USA. 4 Clinic for Particle Therapy, University Hospital Essen, Essen, Germany Purpose/Objective: In image-guided proton therapy, attenuation images from the joint reconstruction of activity and attenuation from Time-of-Flight PET data could potentially be employed for adaptive therapy. Since the attenuation allows estimation of stopping powers [1], this information could be basis for an adaptive therapy approach by recalculating the treatment plan on the images. The feasibility of such a workflow was investigated using a post irradiation radio activated phantom with modified anatomies. Material/Methods: In our envisioned workflow, the decision for treatment plan adaptation is based on the gamma index analysis of dose recalculations on attenuation images after a treatment fraction (Fig. 1). The reconstruction of attenuation images resulting from PET list-mode datasets was performed using the maximum likelihood approach for the joint reconstruction of attenuation and activity (MLAA) [2]. In order to be used for dose calculation, the attenuation images were provided with a CT-values to electron density calibration [1]. Possible anatomical changes were analysed using a cylindrical PMMA-phantom containing tissue equivalent inserts. The radioactivation of the phantom was achieved through proton irradiation with the original treatment plan with 15Gy(RBE). A therapy adaptation was investigated through the intentional modification of the phantom, which incorporates an additional air cavity and an increase in girth. The gamma index was determined using the 3%/2mm criterion and a 10% dose threshold, based on a dose distribution of a treatment plan recalculated on the attenuation images. The decline in the global gamma pass rate served as an indicator for the necessity of replanning.