Vitamin D is fat soluble, and thus you can overdose on it. High dose frequently is not recommended, and most high dose supplements will have warning labels about vitaminosis.
I've also been told to be careful with any vitamin supplements while on chemo, and to consult a cancer dietician before conducting you're own science... the reason, as I understand it as a patient, is that high oxidation can actually keep cancer cells strong and healthy enough to create chemoresistance.
Something like 40,000-100,000IU/day for several months.
That's 8-20 pills per day of the most potent D3 commonly available.