Under no circumstances. Tap water (provided it is stipulated as drinking water) can be used for human consumption, both for cooking and drinking.
Drinking water is essential, especially for people undergoing cancer treatment. It is necessary to keep the body's cells well hydrated so that they can better withstand the treatment. Patients should drink two litres of water (about eight glasses) daily. If you drink tap water regularly, you can continue to do so safely during cancer treatment.