How long does radiation stay active in your body ,killing cancer cells

One of the RAs said that it continues for up to six months. Other sources in PubMed indicate that while radiation damages the DNA and that damage can take weeks or months to cause cell death, radiation Doesn't continue to damage the DNA of more cells. Not much help, sorry.

The radiation goes through your body, and it does damage to the cells. . The cells it damages can days to years to actually die. Some people have had their PSA drop little by little over two or three years. Other people have it drop so much at once that the little changes don’t matter. After salvage radiation, my PSA went to undetectable right away.

You have no radiation left in your body after treatment. Now, if you have something like Pluvicto, it’s a different story.

This is something you can easily find with a search on the web.

In my case (60 gy of photon SBRT to the prostate split over 20 fractions), I think my radiation oncologist said the radiation would continue to be effective and spread slightly for about 6 weeks after the end of treatment, but I wouldn't swear to the exact number. As @jeffmarc mentioned, while the radiation immediately damages the cancer cells and prevents them from continuing to reproduce, it takes a while for the cells to die out.

For some types of cancer, the radiation spread is a bad thing (e.g. brain cancer, where you want to damage as little non-cancerous tissue as possible), but for the prostate, it can be helpful. Even a biopsy is basically just sticking a needle into a haystack, so it's easy to miss some cancer, and anything outside the prostate that's still small won't show up on PSMA-PET, CT, or MRI scans. The radiation spread gives you a bit of a safety margin that you don't get with a prostatectomy (e.g. it will hit any undetected cancer cells nearby in the bladder, rectum, etc, that a prostatectomy would miss), but the downside is that you can also end up with some mild burning in those locations.

This paper titled - “In Prostate Cancer, ADT After RT Better Than Before RT” - that was presented at the American Society for Radiation Oncology (ASTRO) 2020 Annual Meeting (http://www.medscape.com/viewarticle/940049) discusses what happens after radiation treatments end.

It’s not that radiation is still in your body weeks after radiation; it’s that the radiation damage done to DNA continues long after treatments end. And because prostate cancer is so heterogeneous, it’s important to have DNA damage occurring in the short, medium, and longer term.

This paragraph from that paper explains why —> “… radiation damage to DNA can continue long after the radiotherapy itself has been completed. So by keeping the androgen receptor inhibited or suppressed by hormone therapy, you can suppress that DNA repair mechanism for months, and this is why [I think] adjuvant ADT is a very important component to kill prostate cancer cell…”

Hope that helps.

(I had 28 sessions of proton beam radiation (70 Grays total), and started 6 months of ADT just 1 week prior to radiation treatments so that most of my ADT would occur post-radiation.)