Wyvrn's suggestions to ensure you have no surplus glucose and err on the side of ketones is absolutely correct. As M Levac has pointed out several times cancer proliferate faster if fed sugar.
Remember FASTING improves the imapact of chemo. and it's obvious why. You are starving the cancer cells by inducing ketosis.

Nancy LC has correctly also suggested that raising 25(OH)D status to above 60ng/ml will improve prognosis.
ABSOLUTELY TRUE.

It really doesn't take much intelligence to see who has the best prognosis when it come to breast cancer survival.

But don't forget that vitamin D isn't the only proven successful way of improving your odds. Omega 3 also improves survival.
Here are some examples.
Fatty fish and fish omega-3 fatty acid intakes decrease the breast cancer risk: a case-control study.
A High Omega-3 Fatty Acid Diet Mitigates Murine Pancreatic Precancer Development.
Dietary omega-3 polyunsaturated fatty acids suppress expression of EZH2 in breast cancer cells.
Fatty fish and fish omega-3 fatty acid intakes decrease the breast cancer risk: a case-control study.
Fatty acid facts, part II: role in the prevention of carcinogenesis, or, more fish on the dish?
Generally speaking around 1.5g of EPA + DHA is required to counter the effects of omega 6 but cutting down on the Omega 6 vegetable oils is as important as increasing the omega 3 intake. Doing both sides of the balance obviously is MOST EFFECTIVE.