Chemo is by no means an effective treatment for cancer because of the collateral damage. You should not have to subject yourself to that to get healthy.
Vitamin D deficiency is linked to many illnesses including cancer because most people are not giving their bodies what it needs to maintain optimal health. If you're not running at 100% capacity, then production suffers, right? It's like a lack of sleep will reduce your awareness, focus, etc. Just common sense to me.