My only advice is that only you know when you are ready. I think sex is just as much mental as it is physical. I think if you trust him and you can see yourself spending some time with radiotherapy then you might be ready. I think the question to ask is how will sex change your relationship?

Will you feel closer to each other? Do you feel pressured? Do you want to have sex with him? Do you have means to have safe sex? Is he just after sex or does he want to be with you? I think if you feel good about the answers to those questions.

I had sex after two weeks of meeting my wife. I had some gf's before her but I never felt connected enough before to have sex. After meeting my wife it just felt right. It hard to explain, but I just new I was ready. I married her 3 years later and have been married for 5 years. I'm not saying having sex with someone means you will get married to them, but I think it would suck if you had sex and then the guy lost interest in you afterwards. Good luck and make sure it feels right to you.