There would appear to be a growing body of evidence to the effect that long- chain omega-3 fatty acids, in particular EPA and DHA, help prevent the promotion and progression of certain cancers.
Using a chemical carcinogen-induced cancer model, researchers found that a high intake of fish oil significantly lowered the cancer incidence in animal studies as compared to animals fed either low fat diets or high corn oil diets. By implanting human tumors into immune-deficient (nude) mice, researchers have found that a high fish oil diet can slow tumor growth. These results suggest that fish oil can be used for both prevention and treatment of cancer.
Of some concern is the uncertainty surrounding vitamin E. Both vitamin E and fish oils have been found to help prevent hormone- dependent cancers, so taking both for cancer prevention is probably desirable.
You can read the latest research concerning fish oils and cancer here: