Hi Ricochetsmum,
I am in a similar situation but a little bit further along. I had the op in June 2024 and my PSA has slowly increased from 0.02 to 0.21. The stage you are at of 0.13 means IF there is spread then the tumour is too small to be picked up on a scan. I was told that the earliest they can see any new tumour is in excess of 0.20.
From 0.02 through the many tests to 0.21; I was under Urology. With the latest increase to 0.21, I was given a PET CT Scan, which thankfully found nothing. However, I was then handed from Urology to Oncology. Going forward my PSA tests will be handled by Oncology. At the first meeting with Oncology we discussed Salvage Radiation Therapy. Unfortunately, I have a condition called Ulcerative Colitis, which means that hormone and radiation to my bowel area would cause me severe side effects. Given this, it was decided to keep testing my PSA at quarterly periods. If the PSA rises about 0.5 then I will have another scan in the hopes of finding the tumour and then have treatment to that specific area and possibly avoiding my bowel.
I am no doctor and the above is just my journey and what I found out along the way. If anything is inaccurate then I am sure others will post and correct me. As you seem to be someone who prefers to know stuff I would recommend looking up SRT - there are a lot of good posts on here about the topic. From what I have read, and been told, it sounds like it's not as horrible as it sounds.
Here is hoping that you never get a result above 0.20, but if you do then I so pray that your journey through SRT is a short and effective one.