With all your respect, there is some confusion here. Extraprostatic extension means the tumour has breached the capsule and spread to adjacent tissue. Intraprostatic incision means that the surgeon has cut through the tumour, either due to lack of a distinct plane between capsule and connective tissue or due to lack of expertise. They both result in cancer cells being left behind and the outlook is the same.
A biochemical recurrence means that conventional imaging has not picked up significant volume of cancer because it's likely to be at the microscopic level, but oncologists believe the latter hence the offer of RT. The gents on the forum that had RT as a result of rising PSA had this on the knowledge they have residual disease in the prostate bed. There is little uncertainty that there is cancer when the PSA rises. Also even with a complete removal of the prostate there are prostate cells left behind in the bladder neck and urethra. Prostate doesn't stop at a specific point where the urethra starts, there is a transition region.
I didn't want to disclose this, but I am in fact an oncologist, who has recent had a prostatectomy, so I hope I have a fair understanding of the biology :)
Have a good evening.