There are no good studies linking nitrosamines to breast cancer, however, nitrosamines cause many types of cancer, most notably along the gastrointestinal tract.
Theoretically, nitrosamines could be contributing to the non-estrogen dependent breast cancers as this chemical causes DNA mutations in general, according to mammalian laboratory studies.
There are obvious sources of nitrosamines; nitrite preserved food like luncheon meats, bacon or smoked fish. But there are non obvious sources as well; some beers contain nitrosamines from the brewing process, tobacco products and any food item that is in a non breathing container. Sodium nitrate is used as a preservative specifically against botulinum toxin.
Another non obvious source is our own stomachs. Under a reduced acidic environment, our stomachs turn nitrates to nitrites. This is important to remember as many foods have nitrates and many people are unfortunately on acid blockers. (there's a good reason our stomach PH is supposed to be highly acidic, and messing with the PH leads to all sorts of dangerous consequences).
Ascorbic acid or vitamin C, reduces the amount of nitrites produced from nitrates. Take plenty of vitamin C, just not with sodium benzoate as this combination leads to benzene, another known carcinogen.
Beer as it turns out is a big source of nitrosamines which forms in the malting process. The beers with the highest levels tend to be the cheaper domestic types. Stick with higher quality and imported beers and consider drinking it with limes or lemons for the ascorbic acid.
Green tea with a high catechin concentration has been also shown to inhibit nitrosamine formation. Green tea with low catechin concentration may actually promote nitrite formation.
It is important to use ample green tea leaves and drink only the first two infusions if you want the benefit of reducing nitrosamine formation. I prefer high quality Japanese green teas like Sencha or Gyokuro. For instant green tea, I use Matcha powder which is a powdered form of sencha or gyokuro, both have high catechin levels.