That's probably because most guys who have radiation (and all guys who have surgery) lose their ability to produce semen. You are lucky! I really only produce a couple of drops - so when there's blood there it is not diluted and very vivid.
You can probably find a lot more about
it if you use its medicalese name: hematospermia
Here's an article about
it:
www.issm.info/sexual-health-qa/what-is-hematospermia1/