Just read a story from Reuters that Dietary Folate May Lower Ovarian Cancer Risk.

The results of a study conducted in Sweden indicate that high levels of folate obtained from food sources may protect against ovarian cancer. The benefits were found to be primarily among women who consumed at least two drinks of alcohol per week.
Previous reports have shown that dietary levels of folate, a B vitamin also known as folic acid, are inversely related to the risk of breast and colorectal cancer. In contrast, few studies have looked at the association between folate intake and ovarian cancer risk.

...

Overall, women with the highest level of folate in their diet (at least 204 micrograms/day) were 33 percent less likely to develop ovarian cancer than those with the lowest levels (less than 155 micrograms/day).

Among women who consumed more than about two drinks per day, the risk reduction seen with high folate intake was much higher -74 percent. In contrast, high folate intake provided no protection against ovarian cancer in women who consumed lesser amounts of alcohol, the investigators point out.