"NSFW" means "Not Safe For Work" which means there may be inappropriate language or imagery in what you are likely to encounter on this site. I'm not sure I can think of anything less appropriate than cancer. Check.
It's also a warning that content might be upsetting. Cancer? Upsetting? Check.
NSFW indicates salty language, adult content, sexual overtones, and terminology or phraseology that could offend sensitive types. Or even you, tough guy. We're going to be talking (and, I hope, laughing) about breast cancer, yep, all of the above, Check! and if you're already offended by the idea of laughing at cancer, well this isn't the site for you.