Eating fish, especially fatty fish that are high in Omega-3 Fatty Acids (like salmon or tuna), 2-3 times per week is recommended for heart patients, and those affected by cardiovascular diseases. But fish oil is also seen as a preventative in breast and other cancers.
The two types of omega-3 fatty acids found in fish oil are eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA). Tests on mice found these reduced tumor size by 60-70%, with a 30% decrease in breast tumors.
A plant-based omega-3 is a-linolenic acid (ALA), which is found in soy, flaxseed, and canola oil. This was found to be less affective, and since soy contains Isoflavones, which are plant estrogens, it is not recommended to consume large amounts of soy.