Bottom line: Diets high in omega-6 fat may promote breast cancer development.Int J Cancer. 2008 Oct 1;123(7):1637-43.

Swedish researches evaluated the diets of women, ages 50 years or older, who were diagnosed with invasive breast cancer, during a 10 year flollow-up study.  During that period , 430 women, out of  nearly 12,000 women, developed breast cancer (part of the large, population-based prospective, Malmö Diet and Cancer cohort) .   Diets were evaluated using a modified diet history and scientists found that those eating diets high in omega-6 PUFA were associated with breast cancer development  Scientists concluded that diets high in omega-6 PUFA may promote breast cancer development.
 
Link to Abstract:
Sonestedt E, Ericson U, Gullberg B, Skog K, Olsson H, Wirfält E.Do both heterocyclic amines and omega-6 polyunsaturated fatty acids contribute tothe incidence of breast cancer in postmenopausal women of the Malmö diet andcancer cohort?  Int J Cancer. 2008 Oct 1;123(7):1637-43.