The human body absorbs B12 from the colonies of bacteria that produce it in our digestive tract. We don't need to eat any B12 to get enough of this vitamin, as long as we are healthy enough to have two things happening:
1. the bacteria produce it in sufficient amounts, and
2. the body is absorbing this B12 properly.
Unless you or someone you know are having clear symptoms of B12 deficiency (http://en.wikipedia.org/wiki/Vitamin_B12_deficiency
), you would not need to worry about it.
Regarding the true cause of cancer, I am interested in why this question is along the same topic of another question you asked in another post, Anne. Many things in a person's lifestyle or background may cause cancer to develop, even if the current diet is healthy.
I hope this answer helps. Please, let me know if this has helped you. Don't hesitate to ask any other questions you have here.