The very good link found by fullofbeans (in a separate post today) icludes a well documented sub-link to interactions (good or bad) between some supplements (such as curcumin, black cohosh...) or foods ( grapefruit) and cancer treatment:
http://breastcancer.evidencewatch.com/
Click on the "SPECIAL ISSUES" heading & select "Drug/herbal/CAM interactions in oncotherapy"
Among the usefull pieces of data are the specific members of the P450 family of enzymes which are either inhibited or reinforced by supplements or foods & their implication in the metabolism of drugs or progression/regression of cancer cells.