MS first, risk of BC:
In general MS patients are not at increased risk of cancer. Women with MS, however, seem to have a small excess risk of breast cancer
Our findings indicate that cancer incidence is significantly lower in female MS patients than in the general population. Female MS patients treated with glatiramer acetate showed an elevated rate of breast cancer and all MS patients treated with beta-interferons showed an elevated risk of non-breast cancers though not statistically significant (p = 0.122 and 0.072, respectively). Further study is needed to assess possible associations between long-term exposure to the novel immunomodulatory treatments in MS and rate of cancer.
We consider the potential for an increased risk of developing a poorer-prognosis breast cancer as a result of concomitant immunomodulatory effects of the previous MS treatment, particularly the effects the drugs are reported to have on regulatory T cells. Current data in the literature reflect the need for further study.
BC first, risk of MS - could be quite different, I don't know, depends on particular BC drugs used, ...