Some evidence suggests that fish oil may reduce the risk of cancer

Some evidence suggests that fish oil may reduce the risk of cancer of the prostate, colon/rectum, and breast. The latest evidence relating to breast cancer comes from a prospective study of supplement use and breast cancer risk among postmenopausal women (Brasky, Canc. Epidemiol. Biomarkers Prev 2010). Current use (but not past use) of fish oil supplements was associated with a 32% reduction in the risk of breast cancer. A specific dose or frequency of fish oil use associated with the greatest risk reduction was not determined.  Risk was greatly reduced for ductal carcinoma of the breast (the most common form of breast cancer), but not lobular carcinoma. This risk reduction was not seen with other supplements commonly taken for menopausal symptoms (black cohosh, **** quai, soy, or St. John’s wort). It is speculated that the anti-inflammatory property of fish oil may be responsible for an anti-cancer effect because chronic inflammation is associated with cancer initiation and progression. Researchers note, however, that fish oil cannot be recommended for breast cancer prevention without further study.

http://cebp.aacrjournals.org/content/19/7/1696.abstract