Part I | Chapter 7 132 Abstract Background and purpose Magnetic resonance imaging (MRI)-guided linear accelerators (MR-Linac) enable accurate estimation of delivered doses through dose accumulation using daily MR images and treatment plans. We aimed to assess the association between the accumulated bladder (wall) dose and patient-reported acute urinary toxicity in prostate cancer patients treated with stereotactic body radiation therapy (SBRT). Materials and methods One-hundred-and-thirty prostate cancer patients treated on a 1.5 T MR-Linac were included. Patients filled out International Prostate Symptom Scores (IPSS) questionnaires at baseline, 1 month, and 3 months post-treatment. Deformable image registration-based dose accumulation was performed to reconstruct the delivered dose. Dose parameters for both bladder and bladder wall were correlated with a clinically relevant increase in IPSS (³ 10 points) and/or start of alpha-blockers within 3 months using logistic regression. Results Thirty-nine patients (30%) experienced a clinically relevant IPSS increase and/or started with alphablockers. Bladder D5cm3, V10-35Gy (in %), and Dmean and Bladder wall V10-35Gy (cm3 and %) and Dmean were correlated with the outcome (odds ratios 1.04-1.33, p-values 0.001-0.044). Corrected for baseline characteristics, bladder V10-35Gy (in %) and Dmean and bladder wall V10-35Gy (cm3 and %) and Dmean were still correlated with the outcome (odds ratios 1.04-1.30, p-values 0.001-0.028). Bladder wall parameters generally showed a larger area under the curve (AUC). Conclusion This is the first study to assess the correlation between accumulated bladder wall dose and patientreported urinary toxicity in prostate cancer patients treated with MRI-guided SBRT. The dose to the bladder wall is a promising parameter for prediction of patient-reported urinary toxicity and therefore warrants prospective validation and consideration in treatment planning.
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