No link to studies or anything of the sort, but the theory behind Water Fasting is that the body will autolyse tissue to feed itself while you're eating nothing and all organs of elimination will be solely dedicated to detox. I am obviously not a medical person, but from what I have read, high fat, high sugar, and high protein --> IGF1 all can cause cancer to grow faster. So, if you go the fasting route, I would say either water, or better green juice - leafy greens/vegetables/lemon green apples carrots etc etc. I am thinking green juice may be better in your case because if you are heavily toxic, too much detox too fast may be --- too much? LOL. Basically, too many toxins will be activated and will need processing on their way out and this may overload your system. A green juice fast will give you all the benefits of a water fast, but slower -- but then, that would mean you will be able to fast for longer.
Also, check out Gerson Therapy - they have developed a vegan detox program that has helped many cancer patients.