If you post the findings from your pathology that might shed some additional light on what's going on. Specifically, if there were any signs of residual cancer in the margins or extra capsular extension that suggests the new PSA more likely is local, right in the area where the prostate was. If the pathology shows nothing of that sort, then it is a greater possibility that some cells may have escaped the immediate area where they will be harder to hit with salvage radiation.
In general, a slightly elevated psa will show up after surgery long before any kind of metastatic cancer can be seen on a scan, and that makes it a matter of guesswork whether there is anything really there, or if it is there, where exactly it is hiding, and how to go after it.
By the way, I think the term is "systemic" meaning system-wide rather than "systematic". See here for the differences in treating systemic vs local cancer. This article is about
breast cancer, but the same principles apply.abcnews.go.com/Health/OnCallPlusTreatment/difference-local-systemic-therapy/story?id=3643074#.T0gBZvXJYx4