While there seems to be agreement that diet plays some role in the development of cancer, clinical recommendations for prevention are still unclear and research data is not persuasive.
For example, the Women's Health Initiative Dietary Modification (DM) Randomized Controlled Trial,  completed in 2006, evaluated the effects of a low-fat dietary pattern on chronic disease incidence, with breast cancer and colorectal cancer as the primary outcomes. The conclusions were that a low-fat dietary pattern had no significant impacts on the development of breast or colorectal cancer, but it may reduce the incidence of ovarian cancer among postmenopausal women.
There are many conflicting recommendations as to the best diets for individuals who are already experiencing cancer and the treatment for it. Many extreme diets have been used clinically for cancer treatment, like the Gerson diet, low-carb/high fat diet, and the macrobiotic diet. No significant research studies have confirmed their therapeutic application, although a major analysis on macrobiotic diets in cancer is underway by NCI.