In an article on the effects of diet on treating disease, it mentions the possibility that the use of fish oil enhances chemotherapy for those with cancer.
Anecdotal reports have also repeatedly indicated the advantages of fish oil (an important source of the omega-3 fatty acid EPA) to enhance the effectiveness of chemotherapy in cancer patients.
However, as it states, evidence is only anecdotal and no clinical studies have been conducted to my knowledge. But it does say the reports repeatedly indicate such effects, so I’m sure someone with cancer doesn’t require the proof of a study to at least try fish oil supplements, which have such a wide range of beneifts anyways. You can also read more about fish oil benefits on cancer in the Fish Oil and Cancer category.