ESTRO 2024 - Abstract Book

S4380

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2024

2844

Digital Poster

Treatment duration determines intrafraction PTV margins for online adaptive RT of bladder cancer

Aymane Khouya 1 , Christoph Pöttgen 1 , Christian Hoffmann 1 , Toke Printz Ringbæk 1 , Wolfgang Lübcke 1 , Frank Indenkämpen 1 , Maja Guberina 1 , Nika Guberina 1 , Thomas Gauler 1 , Martin Stuschke 1,2 , Alina Santiago 1 1 University Duisburg-Essen, Department of Radiotherapy, Essen, Germany. 2 German Cancer Consortium (DKTK), Partner Site University Hospital Essen, University Duisburg-Essen, Essen, Germany

Purpose/Objective:

For image-guided definitive chemoradiation after trans-urethral resection of bladder cancer (TUR-B), planning target volume (PTV) margins between 10 and 25 mm are used to account for interfractional anatomical variations. Cone beam computer tomography (CBCT) based online adaptive radiotherapy (oART) can potentially allow margin reduction, since within this therapy concept only intrafractional variations need to be accounted for. We studied the influence of adaption time on the quality of the delivered oART treatment, based on calculations of the equivalent uniform dose (EUD) in the clinical target volume (CTV) on the anatomy at the dose delivery-time, under assumption of an isotropic CTV-to-PTV margin of 5 mm, CTV 5mm .

Material/Methods:

Patients with bladder carcinoma were treated after TUR-B at ETHOS ® under online adaption with tumor-directed partial-bladder target volumes, with a half-filled bladder. A CBCT was acquired at the beginning of each fraction (CBCT1). On this image dataset, the bladder and rectum were automatically contoured with artificial intelligence (AI) based models, and corrected if necessary. These structures were subsequently used to guide the deformation of CTV and PTV from the planning CT (pCT) to CBCT1. The clinical margins were 5-10 mm. Before treatment delivery, a second CBCT was taken (CBCT2), and a couch shift was applied if necessary to re-center the target in the high dose region. The EUD for the CTV was calculated with parameter a=-20. EUD values were given as percentages of the prescribed dose. The CTV 5mm was created in silico as a 5 mm isotropic reduction of the clinical PTV on CBCT1 and the pCT. The bladder on CBCT2 was created with MIM Maestro TM AI autocontouring. Based on this bladder contour, a hybrid deformation from CBCT1 to CBCT2 was made in order to propagate CTV 5mm onto CBCT2. A time-dependent model for the bladder growth during the adaption time was generated in Matlab by processing the bladder deformation from CBCT1 to CBCT2. The bladder contour modelled after 10 min adaption time was used as basis for the deformation of CTV 5mm on CBCT2. For the evaluation, a dose accumulation on the pCT was carried out with help of a contour based deformation, guided by CTV 5mm .

Results:

A total of 120 adaptive fractions from 6 patients were evaluated, 7.5% of which were for full bladder and 92.5% for partial bladder irradiation. The median bladder volume on pCT was 256.2 ml (range: 121.5 ml – 381.6 ml). The median overall clinical treatment duration was 20.6 min (range: 9.6 min – 33.3 min). The bladder volume increased during the adaption at a median rate of 1.3 ml/min.