Aurelia asked rather a while ago (sorry, Aurelia!) if I'd supply a short review of folic acid and neuroblastoma incidence. Well, I'm sitting in lab waiting for something to cool off, so here goes.
Folate supplementation in pregnancy is known to reduce neural tube defects up to 70 or 90%, though only if taken in the first 28 days of pregnancy, after which the neural tube is formed and fused, i.e. it's too late. So if you're planning to become pregnant, either eat a lot of spinach or take your vitamins.
A Canadian Motherrisk study is one of the stronger pieces of evidence for folate preventing pediatric cancers. They survey NB cases before and after mandatory flour supplementation took effect, and find that the rate declines from "1.57 cases per 10,000 births before to 0.62 case per 10,000 births after folic acid fortification (P < .0001)". However, another author points out in a commentary that they would only be expected to catch about 40% of the NBs that would eventually arise. (I didn't check their calculations, but let's assume that's correct.) .62/.40 = 1.55 cases per 10,000 births. In other words, that implies no effect.
On the other hand, a recent meta-analysis looked at a bunch of studies on prenatal vitamins and cancers, and concluded that vitamins reduce NB risk by 50%. (Now, meta-analyses are notoriously flawed, because each paper is looking at something different, asking different questions of the patients, and using different samples.) So this could be true, but it could not. Who knows.
A large 'case-control' study- still interview-based, and I add quote marks because they didn't divide the mothers into groups and give half vitamins, they simply asked questions and then divided up the groups- did find an association between prenatal vitamin usage and decreased risk of brain tumor.
This study showed the same thing, but slightly more convincingly because they sample several hundred NB patients, comparing vitamin users to non-users. This German study, on the other hand, is rubbish. They do a case-control study of pediatric cancer incidence and conclude that vitamins cause cancer! Aaaaah! Eat your meat and potatoes. But. For the 'pain reliever use and bone tumor' association, for example, they have 93 (98.9% of respondents) who had cancer and didn't use pain relievers. But they only had one who did. From this they conclude that taking pain relievers lowers your risk of bone cancer by 70%, i.e. that the odds ratio is 0.33. Ah, no.
This is what's known as 'insufficient statistical power', i.e. 'Your sample's waaaay too small.' They are looking at a rare condition in 94 patients and it just so happens that only one mother took asprin. Well, to really study this, you need either a huge cohort of people who did and didn't take asprin, or a large set of bone cancer patients along with complete medication information for each mother. Neither is precisely ideal, but that's human studies for you. All those pesky ethics rules.
As far as I personally would judge, folate probably does reduce neuroblastoma risk. It makes sense, even though the mechanism isn't known, because folate clearly has something to do with proper neural development, migration, and signaling (to close the neural tube). To give a little perspective, we've used aspirin for hundreds of years until recently discovering it's a COX inhibitor. A mechanism isn't necessary to know something works.
Overall: The evidence that folate might cause cancer is weak. Prenatal vitamin use is strongly associated with lower rates of birth defects in many studies, is associated with a 90% reduction in neural tube defects, and may be associated with reduced risk for some pediatric cancers.
By the way: Methylenetetrahydrofolate reductase is apparently abbreviated 'MTHFR.'
