If gamma rays are poisoning, why are they used for cancer treatment?

Powerful: Gamma ray are very powerful ionised radiation, can kill cancer cells, even penetrate certain metals , could be focused to a pin point at a deep porstion of the tissue (tumor ) with out injuring sorrounding area. Gamma knife is such a device, used to treat brain tumors, also called radio surgery.Not used on normal tissues.

Kill cancer cells.: While no test is 100% effective, gamma rays can be directed to kill mostly cancer cells. Radiation treatment for cancer consists of multiple, concentrated beams of gamma rays directed at the tumor to kill the cancer cells. These are directed at different angles in order to concentrate the rays on the cancer while minimizing damage to the surrounding normal tissue.

Gamma: Radiation causes breaks in dna which can lead to cell death. It works best when cells are actively dividing or growing, the faster the better actually, so cancer cells which divide faster than normal cells are more suceptible to the damage to their dna than your normal cells. It works even better when you dose if repeatedly which doesn't give the cell time to repair its dna.