I'm no longer in chemo, if that's what you mean by "active" treatment, but I put off my second vaccination by nine weeks since I tend to react to most pharmacology. I also read the following article which may or may not be good guidance:

It notes that "a multinational study found that the first dose of the Moderna vaccine had 80% efficacy and that a delay of 9-12 weeks "could enhance the program effectiveness and prevent additional infections, hospitalizations, and deaths, compared to a 4-week interval between the doses."