ESTRO 2025 - Abstract Book

S4132

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

ESTRO 2025

is time-consuming. The aim of this study was to determine the intrafraction changes in bladder (BV) and rectum volume (RV) during an oART workflow on the Varian Ethos system.

Material/Methods: Forty-nine fractions from ten previously treated oART patients with prostate cancer were analysed. The bladder and rectum were contoured in two cone beam computed tomography (CBCT) scans for each fraction, the initial CBCT1 at the start of the treatment session, and the verification CBCT2 after the adaptation process. All patients were instructed to drink 500 mL of water, empty bladder, and rectum 60 minutes prior to treatment. Patients were also recommended a diet to minimise gas within the gastrointestinal tract. A paired t-test was applied to evaluate the statistical significance of differences in BV and RV. Results: The median time required for the adaptation workflow (AT) was 26 min (SD 6 min) and ranged from 16 to 47 min. The mean BV measured on the CBCT1 images was 182 cm³ (SD=96 cm³), while the BV measured in the CBCT2 images was 227 cm³ (SD=115 cm³) (p < 0.001). The mean RV measured in the CBCT1 and CBCT2 images was 61 cm³ (SD=15 cm³) and 63 cm³ (SD=20 cm³), respectively, (p > 0.05). In 67% of the cases, intrafractional variability in BV was ≤60 cm³. Large changes in BV (>100 cm³) were observed in 12.5% of the cases. These cases were analysed in detail. In one case, such a large difference in BV resulted from the change of the doctor conducting the oART and the two-fold extension of the AT to 39 min. In the remaining cases, the average AT was comparable and the attending physician did not change. The volume differences in these patients on other days of treatment were small. The patients admit not to follow the preparation protocol by drinking too much fluid. Conclusion: Significantly larger differences were observed in changes in the BV compere to RV. The results show the importance of appropriate preparation of patients for each fraction. Due to observed cases of increased diuresis during some fractions, currently we do not recommend administering fluids an hour before treatment. The other crucial issue is a well-trained adaptation team to make the whole process of adaptation as fast as reasonably possible. Digital Poster Impact of thermoplastic masks on whole body CT-MRI image fusion quality in total marrow lymph-node irradiation Aurora Mantovani, Damiano Dei, Nicola Lambri, Giovanni Savini, Carmela Galdieri, Andrea Bresolin, Ciro Franzese, Stefano Tomatis, Marta Scorsetti, Pietro Mancosu Radiotherapy and Radiosurgery Department, IRCCS Humanitas Research Hospital, Rozzano, Italy Purpose/Objective: Total Marrow Lymph-Node Irradiation (TMLI) is an intensity-modulated radiotherapy technique targeting bones and lymph nodes while sparing surrounding healthy tissues. Accurate lymph-node delineation benefits from whole-body MRI (WB-MRI), as WB-CT with contrast media is often infeasible for this patient population. However, the extended duration of WB-MRI (approximately 1 hour) and the weight of coils pose challenges for patient immobilization. Standard CT-based immobilization, which uses three thermoplastic masks to stabilize the entire body, may be difficult to maintain during MRI. This study evaluates the impact of thermoplastic masks on CT-MRI image fusion quality, using rigid and elastic registration methods. Keywords: intrafraction, adaptive radiotherapy, prostate 3712