I have a particular interest in this as a cancer survivor, recently re-staged to stage 4 breast cancer. I had severe b12 deficiency some years back, but my levels have been corrected and always show high on tests these days. I continue to receive regular injections (every 8 weeks).
I thought some of you might find the article below interesting as it explores the effect of B12 (or its absence) on cancer. I have copied out the part pertaining to b12.
Vitamin B-12 has not been proven to be an anti-cancer agent, but there is some evidence indicating that it could be beneficial. The form of administered vitamin B-12 may be important.
Some experimental cancer studies have been carried out with various forms of vitamin B-12. Methylcobalamin inhibited tumor growth of SC-3 injected into mice , and caused SC-3 mouse mammary tumor cells to undergo apoptosis, even when stimulated to grow by the presence of growth-inducing androgen . Methylcobalamin, but not cyanocobalamin, increased the survival time of mice bearing implanted leukemia tumor cells . 5'-deoxyadenosylcobalamin and methylcobalamin, but not cyanocobalamin, were shown to be effective cytotoxic agents . Methylcobalamin also was able to increase survival time and reduce tumor growth in laboratory mice .
Laboratory mechanistic evidence for the effects of vitamin B12 were seen in a laboratory study with vitamin B-12 deficient rats. Choi et al  found that the colonic DNA of the B-12 deficient rats had a 35% decrease in genomic methylation and a 105% increase in uracil incorporation, both changes that could increase risk of carcinogenesis. In two prospective studies (one in Washington Country, Maryland and the Nurses' Health Study) a relation between lower vitamin B12 status (but not deficiency) and statistically significant higher risk of breast cancer was found [117,118]. So, there is evidence from laboratory studies, prospective cohort studies, and mechanistic studies showing that vitamin B-12 is an important nutrient for genetic stability, DNA repair, carcinogenesis, and cancer therapy."