I posted this in advanced prostate cancer - but I think it is valuable enough to bring here also:
This is well worth reading. It's rather dense and while non-conclusive on the mechanisms at work - the results are what's interesting. A common "supplement" Melatonin has been found to have multiple cancer benefits in preventing or slowing the spread of metastatic cancer, and in making treatment-resistant cancers once again sensitive to treatment.
Thoughts and comments are welcome!
He seems to argue that absorbing ROS is its (unproven) benefit - yet ROS is exactly how cancers are destroyed in our bodies. He also says it is an anti-estrogen, which may be beneficial in breast cancer, but detrimental in prostate cancer. Lots of nice pictures and hot air, but no clinical evidence. Why risk it?
If this research is so questionable why has the National Institute of Health accepted it for inclusion in its database?
Pubmed accepts anything in a qualified peer-reviewed journal, without evaluating any of them for content. That doesn't mean that every study or article is of equal value. In that review article, he is just giving his considered opinions - Level 5 (lowest level) in levels of evidence.
Can a substance that prevents ROS from forming, prevent a cancer in the first place? Maybe. But ROS is also part of the apoptosis mechanism that damaged cells use to self-destruct so that they will not become cancerous. The cell's DNA damage-repair mechanism (BRCA, p53, etc.) is the determinative instrument - if it is intact, ROS-damaged cells will be destroyed. If it is not intact, ROS-damaged cells will be come cancerous. It gets more complex when one realizes that the immune system uses ROS to kill foreign invaders, including cancerous cells. For people who already have cancer, there is no evidence that restricting ROS is beneficial, and it may be harmful.
Looks good on paper. Melatonin, like many things, has a dual nature. Antioxidant at lower doses and pro-oxidant at higher doses. A couple of years ago I worked out the dose based on numerous NIH research studies. It was nice because they all loosely agreed. I derived 3 mg to 5 mg for the antioxidant/pro-oxidant threshold. I accounted for human variations and figured that >20 mg would be pro-oxidant.
I would "guess" that, if it does something, it is probably the higher pro-oxidant doses that are going to be effective. Not lower dose ROS absorption.
BUT, there is a conspicuous lack of RCTs. My PSA recently became measurable so I might possibly see if melatonin appears to have an effect on my PSA. Many other things to try though: Xtandi, radiation, pseudo-BAT, even fasting and keto. I think I'll hit on something helpful that at least temporarily controls my cancer; whether in the SOC world or outside of it. If I am successful, melatonin will have to continue to wait.
I haven't found any good RCT prostate cancer data but there are numerous RCTs concerning sleep and melatonin. Looking through them it looks like it generates a very slight improvement in sleep quality, duration, and latency.
Non-Toxic Cancer Treatments or Support to Conventional Therapy
ADT with psa increasing.
one or more things that surprised you about prostate cancer
My daily drugs & drugish regimen 
Prostate Cancer Will Recur in 40%-50% of Patients After Initial Treatment
