Andy, I take immunosuppressants to keep an autoimmune disease under control. This results in a consistently low lymphocyte count. Yours is on the low side but not worryingly low. However you raise an interesting question as to whether it is to do with the cancer treatment. I have my bloods checked every three months and there was quite a notable step change in my lymphocyte count after I finished salvage radiotherapy.
1.7, 1.4, 1.3, 1.2, 1.1 (SRT) 0.37, 0.64, 0.67, 0.7, 0.67, 0.68. Prior to SRT the count has never been lower than 1.1. Lymphocytes form in the bone marrow, so I can't see how RT to the prostate bed would have any impact. Maybe it's just coincidence 🤔.
The consultant monitoring my autoimmune disease is not concerned about my current lymphocyte count so you don't have anything to immediately worry about at 0.9. Nevertheless it should be checked out.
Chris