There seems to be a lack of studies on longer term regular use of melatonin and info on its use with cancer is conflicting, online.

https://www.mayoclinic.org/healthy-lifestyle/adult-health/expert-answers/melatonin-side-effects/faq-20057874 side effects, which may increase with longer term use

https://www.nccih.nih.gov/health/melatonin-what-you-need-to-know mentions cancer

but https://docs.wixstatic.com/ugd/6307d5_563b0beffa754b8199a4ba5197a97ff2.pdf naturopath websites mention melatonin for cancer and to help with cancer treatment side effects; one site referred readers to this study