Adaptive Versus Non-adaptive Radiotherapy and 2 Bladder Filling Instructions for Patients With Intermediate or High Risk Localized Prostate Cancer (ARP)

Inclusion Criteria:

* Age ≥ 18
* Pathological diagnosis of a primary prostate adenocarcinoma
* NCCN Intermediate or high risk localized prostate cancer and clinically N0 and M0 by conventional imaging
* Intermediate: one or more of cT2b-T2c, PSA 10-20, or Gleason Group 2 or 3

  * High: one or more of: cT3a, PSA \> 20, or Gleason Group 4 or 5.
  * Consented to RT with moderate hypofractionated RT to pelvic nodes (44 Gy) and prostate (60Gy) in 20 fractions using standard BC Cancer regimen
* Able to complete BC Cancer POSI questionnairesWilling to attend adaptive treatment slot timing as required by treating centre.
* Willing and able to have regular per protocol follow up and blood work
* Feasible to start protocol treatment within 120 calendar days of participant enrolment
* Continent of urine (or able to maintain continence with penile clamp/cuff during RT as needed) and able to follow bladder filling instructions.
* Baseline IPSS score \< 20 at time of enrolment.
* ECOG performance Status : 0-2.
* Nodal risk ≥ 20% using the Memorial Sloan Kettering Nodal Risk Nomogram (https://www.mskcc.org/nomograms/prostate/pre\_op).
* Negative for bone metastases on bone scan within 3 months of enrolment
* No involved nodes on Abdomen and pelvic CT or pelvic MRI within 3 months of enrolment.

Exclusion Criteria:

* Indwelling urinary catheter.
* Hip prosthesis
* Prior pelvic radiotherapy
* Unable to lie still on RT treatment couch for more than 30 minutes
* Clinical T3b or T4 tumours
* Prior transurethral resection of prostate or radical prostatectomy
* Presence of a pelvic kidney.
* Prior pelvic radiotherapy.
* Significant urinary incontinence: ie \> 1 soaked, or 2 non-soaked pads per day.
* Inflammatory bowel disease, systemic lupus erythematosis, scleroderma, or other connective tissue disorders other than rheumatoid arthritis.
* Metastatic disease identified on staging investigations.
* Any other serious intercurrent illness or medical condition judged by the local investigator to compromise the participant's safety, preclude safe administration of the planned protocol treatment, or prevent the participant from being managed according to the protocol guidelines
* Potentially fertile patient who are unwilling to employ highly effective contraception