First lesson from cancercareers.org, which a flyer from the compression bandage place told me about:
if you get a new job with health coverage, you
1) cannot be treated within six months of "opting in" for a pre-existing condition in order for it to be covered
2) your employer may make all new employees wait as much as 12 months to be covered
these times overlap, so the max is 12 months but note -- no treatments! Obviously, I will be looking into this more. I'm assuming maintenance scans don't count, although I can see an employer doing a "whaddaya mean" with annual MRIs rather than mammograms...