S2739
RTT - Patient preparation, immobilisation, and verification protocols
ESTRO 2026
not reach statistical significance (p=0.12). Conclusion: The preliminary data strongly suggest that this “void- and-no-fluid” protocol proved highly effective in achieving its goal, as it can provide clinically acceptable bladder volume stability within a single oART session. The data indicates that diuretic intake did not significantly influence intrasession bladder volume stability. Therefore, this simple protocol may be considered for incorporation into routine prostate oART workflows. Keywords: adaptive radiotherapy, bladder volume, prostate Digital Poster 3599 Improving imaging accuracy for non-coplanar surface-guided radiotherapy with cube calibration Michelle Alexis Karlik 1 , Linda Croton 1 , Ryan Smith 2 , Clem Conheady 2 , Jeremy Ruben 2 1 Alfred Radiation Oncology, Alfred Health, Melbourne, Australia. 2 Alfred Radiation Oncology, Alfred Health, Melbourne, Australia Purpose/Objective: Cube calibration introduces a superior method of refining the Surface Guided Radiation Therapy (SGRT) isocentre to accurately align with the Linear Accelerator (LINAC) imaging isocentre (1). This study evaluates the impact of cube calibration on the accuracy of SGRT for non-coplanar patient treatment position verification when compared to stereoscopic x- ray imaging. Material/Methods: An end-to-end (E2E) anthropomorphic Max-HD head phantom was used and set to a planned treatment position with cone beam computed tomography (CBCT). The SGRT system was calibrated using the plate method (plate-SGRT). The accuracy of SGRT was measured for comparison with stereoscopic imaging, which is standard practice in Stereotactic Radiation Therapy (SRT) patient position verification. Patient positional verification was then performed using stereoscopic x-ray imaging and SGRT. The couch 0 degree position represented the phantoms position baseline setup position, and imaging was then performed for 13 planned couch angles (90 degree to 270 degree) at 15 degree intervals. The phantom position shifts reported by each system (X-ray & SGRT) were recorded. The SGRT system was recalibrated using the cube calibration process (cube-SGRT) and the patient position verification imaging was repeated. Results: The vector difference from the reference baseline
Dawid Bodusz, Iwona D ę bosz-Suwi ń ska, Anna Podgórska, Katarzyna Wieczorek, Bogumi ł a Gurgul, Jerzy Wydma ń ski Radiotherapy Department, Maria Sklodowska-Curie National Research Institute of Oncology, Gliwice Branch, Gliwice, Poland Purpose/Objective: The study investigated the effectiveness of a new, simple, and clearly defined “void-and-no-fluid” bladder preparation protocol to ensure anatomical stability, which is critical for target coverage and organ-at-risk preservation during online adaptive radiotherapy (oART) of prostate cancer patients. This protocol was implemented because conventional bladder preparation methods, which rely on fixed time points and prescribed water intake, are often suboptimal in the oART setting, as continued bladder filling during image acquisition, contouring, and plan adaptation could potentially lead to substantial increases in
intrasession volume. Material/Methods:
A prospective registry of 21 patients who underwent prostate radiotherapy between May and June 2025 was analysed (101 fractions). The institutional protocol strictly defined the patient’s preparation requirements, maximising the likelihood of a stable bladder volume: patients were required to void 2 hours before treatment and, critically, abstain from fluid intake from that time until radiation therapy (especially beverages such as tea or coffee). BV was contoured on pre-adaptation and post-adaptation CBCT acquired within the oART workflow. The following metrics were evaluated: (i) difference between planned BV and actual BV for a given fraction, and (ii) intrafraction BV change between pre- and post- adaptation CBCT. Differences in intrasession bladder volume change ( Δ BV) between fractions with and without diuretic use were evaluated using linear regression with robust standard errors to account for correlation between patients. Results: The median pre-adaptation BV was 158.6 ml (range, 42.9–747.7 ml), increasing to a median of 219.4 ml (range, 44.8-774.4 ml) on post-adaptation CBCT. The median difference between planned and delivered BV within a treatment fraction was 57.2 ml (range, 1.8– 560.5 ml), with a 5th–95th percentile interval of 8.8– 282.3 ml. The median change in intrafraction BV between the pre- and post-adaptation scans was 16.6 ml (range, 10.6 - 107.9 ml), with fifth- to 95th percentile limits of 1.0 to 61.7 ml, indicating that most fractions demonstrated modest intrasession variability. The diuretic use showed higher median Δ BV (19.7 vs. 12.8 ml) and greater variability; however, the adjusted mean difference of +9.7 ml (95% CI –2.6–22.0 ml) did
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