Part III | Chapter 10 206 and May 2021 were included. The bladder and bladder wall were delineated for each treatment fraction on the position verification (PV) MRI scan (650 fractions in total). The accumulated dose to the bladder (wall) was reconstructed on a reference MRI scan using a contour-guided version of the in-house developed DIR algorithm ‘EVolution’. Bladder (wall) dose-volume histogram (DVH) parameters, including V10-37Gy (in cm3 and %), Dmean, D1cm3, and D5cm3, were extracted from the accumulated dose file. The bladder (wall) dose parameters were correlated with a clinically relevant increase (³ 10 points from baseline) in International Prostate Symptom Score (IPSS) and/or start of alpha-blocking medication (e.g. tamsulosin) using logistic regression analysis. Thirty-nine patients (30%) experienced a clinically relevant IPSS increase and/or started with alpha-blockers. Bladder D5cm3, V10-35Gy (in %), and Dmean and Bladder wall V10-35Gy (in cm3 and %) and Dmean were correlated with the outcome (odds ratios 1.04-1.33, p-values 0.001-0.044), with increasing odds with higher dose levels or larger volumes. Corrected for baseline characteristics (including age and baseline toxicity), 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 DVH parameters generally showed a larger area under the curve (AUC) compared to whole-bladder DVH parameters. To achieve a reduction in dose received by the bladder wall, new treatment planning constraints could be applied and/or PTV margin reduction can be pursued (i.e. by improving delivery accuracy). Based on these results, we have suggested to include bladder wall V25Gy in the treatment planning constraints and to aim for a reduction in Dmean to the bladder (wall). Future research should validate these findings and assess the clinical effects of a lower (actual delivered) dose to the bladder wall. Part II. MRI-guided radiotherapy for locally recurrent prostate cancer Part II of this thesis focussed on MRI-guided radiotherapy for the treatment of locally recurrent prostate cancer after primary radiotherapy treatment. Recurrence rates after primary prostate cancer radiotherapy treatment are very low in patients with low-risk disease. However, a significant proportion (up to 50%) of the high-risk patients develop a (biochemical) recurrence in the first 10 years after treatment. Nowadays, most patients are still treated with (deferred) androgen deprivation therapy (ADT), a systemic treatment that causes significant side effects and which negatively impacts quality of life. Since the recurrent lesion is often confined to the prostate and mostly located at the site of the primary index lesion (the largest and most aggressive lesion), local therapy is an attractive option. The primary aim of local therapy is to postpone the initiation of toxic systemic treatment. Advancements in imaging, such as the availability of multiparametric MRI (mp-MRI) and prostate-specific membrane positron emission tomography CT (PSMA-PET/CT), have led to the introduction of focal salvage therapies. With focal salvage treatment, only a part of the prostate (i.e. the tumour or prostate quadrant containing the tumour) is treated. Compared to whole-gland salvage treatment, the treated volume is reduced, thereby decreasing the risk of (severe) side effects.
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