S1852
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
without compromising bladder or small bowel DVCs. Material/Methods: In this study, 46 patient plans across different treatment protocols were included.TreatmentProstate & Whole Pelvis SIBProstate BedCHHiP Prostate & SVFraction Scheme 68 Gy in 25#66 Gy in 33#60 Gy in 20#Number of Plans Examined 20818Using Monaco, values for VBmin were determined per patient via assessment of the minimum bladder volume required to satisfy dose constraints. Small bowel dose was inferred as dose received by that part of original bladder volume it displaces once bladder volume shrinks to VBmin.Treatments abandoned when CBCT indicated bladder under-filling, were analysed to assess if CBCT-based bladder volume was below VBmin resulting in unacceptable dosimetric impact.A contour generation method was developed to translate the calculated VBmin as a guide contour usable for CBCT matching. This would aid decision making for continuation to treatment delivery if the CBCT bladder is seen to be larger than the VBmin guide contour. Results: The mean planning bladder volume was 356 mL ( σ = 110 mL). Each protocol had a different mean VBmin: Treatment Prostate & Whole Pelvis SIB Prostate BedCHHiP Prostate & SVMean Vmin (mL)121157204 σ (mL)284142For all except three plans, VBmin values were constrained by bladder DVCs alone indicating that the bladder, not the small bowel, was the limiting factor in determining VBmin.
Conclusion: This study demonstrates that substantial reductions in bladder volume are possible, relative to the planning CT bladder volume, without major dose violations. Furthermore, use of patient-specific minimum bladder volume contours has the potential to improve the assessment of bladder volume seen on CBCTs, reducing delays for treatment delivery. References: Kong, V., Dang, J., Wan, V. & Rosewall, T. The dosimetric underpinning of bladder filling criteria for prostate image-guided volumetric modulated arc therapy. The British Journal of Radiology 96, 20230020 (2023).Moore, A. & Forde, E. A dosimetric evaluation of threshold bladder volumes for prostate cancer radiotherapy. Journal of Medical Imaging and Radiation Sciences 48, 270–275 (2017).Gorovets, D. et al. Dosimetric Impact of Inter-Fraction Variations in Bladder Filling and Rectal Emptying in Patients Undergoing Prostate SBRT. International Journal of Radiation Oncology, Biology, Physics 102, e511. issn: 0360-3016. https://doi.org/10.1016/j.ijrobp.2018.07.1445 (2018). Keywords: Bladder-filling, IGRT, Prostate quantification of couch angular motion in dynamic trajectory-based radiotherapy: preliminary data to help inform future plan optimization Eva Lee 1 , Lee MacDonald 2,3 , Alasdair Syme 2,3 , Christopher Thomas 2,4 1 Department of Physics and Atomspheric Science, Dalhousie University, Halifax, Canada. 2 Department of Medical Physics, Nova Scotia Health, Halifax, Canada. 3 Department of Radiation Oncology, Dalhousie University, Halifax, Canada. 4 Department of Radiology, Dalhousie University, Halifax, Canada Purpose/Objective: Dynamic trajectory-based radiotherapy improves plan quality1, but few publications have explored patient experience during dynamic couch rotation2. This work is the first study to measure couch kinematics during dynamic trajectories to characterize motion profiles Digital Poster 1666
Analysis of rejected CBCTs showed that some bladders had sufficient volume, indicating the need for a tool to assess bladder volume on CBCTs. The guide contours generated were effective in confirming when a bladder volume was sufficient as seen in Figure 2.
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