servius - Your concern is well represented many places on the net. I gleaned the following thread from a website I frequently refer to, as I find the authors are well written.

http://www.domsub.info/breastprimer.html
Lymph nodes, though certainly there for some important purpose, are the site of one of the meanest cancer problems around, and once infected, frequently spread cancer through a body like a bad chain letter.

! - Serious problems, ranging from fibroadenoma to breast cancer, are very real issues when it comes to playing fast and rough with breasts. Any damage to the inner workings of a breast places the woman at risk. Skin is made to take certain amount of abuse, and nipples and areola are made, within reason, for even more, particularly in a rug rat biting and sucking on them until they’re numb sort of way, but, you do not want to do anything that will bruise, cut, pierce or otherwise cause damage below skin level!
I'm curious however, are you purposing that breast play can lead to breast cancer, or just exacerbate it?

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ID