Up to 30% of patients who undergo radical prostatectomy as treatment of their prostate cancer, will develop elevation of PSA levels in the post operative period, consistent with recurrence of their prostate cancer.

This prostate cancer recurrence is from either recurrent disease around the area were the prostate was removed, or alternatively be due to the prostate cancer spreading to other parts of the body before the prostate was removed.
When a urologist feels the prostate cancer is most likely due to recurrent local disease, the addition of radiotherapy to the prostate bed and pelvic area can be used to control or cure recurrent prostate cancer.   There has been several randomised trials, in particular the SWOG 8794 and EORC 22911 trials, that show early adjvuant radiotherapy (radiation within 4 months of your surgery) in patients with a high risk of local recurrence of prostate cancer, can improve long term cure and cancer free survival.  Therefore in some circumstances your urologist will suggest the addition of radiotherapy following your prostate cancer surgery.
Risk factors for local recurrence include tumour invasion outside the prostate (extracapsular extension), invasion of the prostate cancer into the seminal vesicals, or a positive surgical margin noted when the histopathologists reviews the prostate specimen under the microscope.

Side Effects of Adjuvant or Salvage Radiotherapy

Late toxicity occuring more than 90 days after completion of salvage / adjuvant radiotherapy occurs in approximately 13% of patients (Peteson etel  Radiology and Oncology 93 :2009: pages 2003 – 2006).
Late complications include scarring (stricture)of the tube which drains urine from the bladder (urethra). Chronic irritation of the bladder (cystitis) with frequency and urgency of urination and also chronic irritability of the rectum (bowel) can occur, resulting in urgency and frequency of bowel motion  with more loose stool and occasional rectal bleeding.  Impotence can also occur after salvage radiotherapy and although incontinence can occur, this is an uncommon complication.
One of the problems with radiation therapy after a radical prostatectomy is that there is no test to guarantee the cancer is only in the area of radiation field.  Therefore you may undergo radiotherapy without any benefit if the cancer has already spread to another part of the body.  Also, not all people who have a positive surgical margin will every develop recurrence of prostate cancer and some of these patients may already be cured of their cancer.  The role of adjuvant or salvage radiotherapy as a multi-modality treatment following a radical prostatectomy should be discussed with your urologist and radiation oncologist, to confirm your suitability.