The AFFIRM trial tests the safety and clinical feasibility of MR-guided hypofractionated focal boost radiotherapy for patients with locally advanced prostate cancer. External beam radiotherapy combined with androgen deprivation therapy is considered as the treatment of choice for patients with locally advanced non-metastatic prostate cancer with seminal vesicle invasion.The long-term results of the multicentre phase III study (FLAME trial) showed that addition of an isotoxic focal boost to the intraprostatic lesion improves biochemical disease free survival in intermediate to high-risk patients without impacting toxicity and quality of life.

This focal boost strategy is now proven for a conventional fractionation scheme (35 fractions). The current trend in radiotherapy for prostate cancer is (extreme) hypofractionation, reducing the number of fractions. For locally advanced prostate cancer, however, the data on extreme hypofractionation are scarce.

Inclusion Criteria:

• Men aged 18 years or older with histologically proven prostate carcinoma
• Imaging stage T3b (as defined on mpMRI) N0M0
• Intraprostatic lesion visible on MRI
• Capable of giving informed consent

Exclusion Criteria:

• History of radiotherapy to the pelvis or transurethral resection of the prostate (TURP)
• Contraindications for MRI according to the guidelines of the local department of Radiology, inability to lay on a treatment table for 45-60 minutes or severe claustrophobia
• Absence of pre-treatment PSMA PET CT
• WHO performance score > 2
• International Prostate Symptom Score ≥ 15
• PSA > 30
• Prostate volume >100c