Dose Escalation For INtraprostatic LEsions

External beam radiotherapy (RT) in combination with androgen deprivation therapy (ADT) is a standard definitive treatment for localized prostate cancer. It confers long-term oncologic outcomes equivalent to those of radical prostatectomy. The current standard approach for prostate RT consists of uniform irradiation of the entire gland. This technique employs computed tomography (CT) images for planning, in which disease is not readily apparent. Multiparametric magnetic resonance imaging (mpMRI) of the prostate allows for accurate visualization of clinically significant tumour foci within the prostate gland. It thereby permits selective escalation of dose to tumours within the gland - a so-called focal boost or "microboost" - with the aim of improving treatment efficacy. The current standard approach to prostate RT involves delivery of treatment in daily fractions, five days per week, over four weeks. Prostate cancer appears to exhibit an uncommon fractionation sensitivity among solid tumours. A consequence of this is that hypofractionation - that is, delivery of RT with larger fraction sizes - may further improve the therapeutic ratio. A number of large-scale studies have shown promise for an ultra-hypofractionated approach (that is, larger than 5-Gy fractions) in the treatment of localized prostate cancer. There is an unmet need for innovations in radiotherapy that further reduce the risk of relapse without increasing toxicity or compromising health-related quality of life and that reduce the treatment burden. Both focal intraprostatic boosts - informed by mpMRI findings - and ultra-hypofractionation represent promising approaches to achieve this objective. To date, the optimal regimen that combines these two innovations remains to be explored. In this trial, we will investigate in a single-arm prospective cohort the safety of an mpMRI-defined focal boost technique with a convenient and radiobiologically compelling ultra-hypofractionated radiotherapy regimen (consisting of three fractionas delivered over one week) for localized unfavourable intermediate and high-risk localized prostate cancer. This is a single-arm prospective trial conducted in patients with unfavourable intermediate-risk or high-risk localized prostate cancer. Treatment consists of ultra-hypofractionated intensity-modulated, image-guided prostate radiotherapy delivered to 27 Gy in 3 fractions (1 fraction per day every other day, over a 5 day period) to uninvolved regions, with up to 39 Gy in 3 fractions delivered to mpMRI-defined intraprostatic lesions, assuming normal tissue dosimetric criteria can be safely met. Radiotherapy will be combined with concurrent/adjuvant androgen deprivation therapy (6 months duration in patients with intermediate-risk disease and 24 months for patients with high-risk disease). There will be follow-up assessments for toxicity, quality of life, and biochemical control. The primary endpoint will be prevalence of grade ≥ 2 genitourinary or gastrointestinal toxicity at 12 months. Primary Objective The primary objective is to determine whether, in a population of men with unfavourable intermediate-risk or high-risk localized prostate cancer receiving a combination of three-fraction ultra-hypofractionated RT and adjuvant androgen deprivation therapy, an mpMRI-defined focal boost can be delivered with acceptable GU and GI toxicity at 1 year of follow-up. Secondary Objectives The secondary objectives are to evaluate, in a population of men with unfavourable intermediate-risk or high-risk localized prostate cancer, the effects of an mpMRI-defined focal boost on: * Health-related quality of life * Biochemical disease-free survival