Radioactivity is a reason of cancer. Also, it is used to cure cancer. How?

The difference between a normal cell and a cancer cell is that a cancer cells are divided really fast. And radiation is most dangerous for cells while they are getting divided. Therefore, radiation is more dangerous for cancer cells than they are for normal cells. During radiation therapy, the affected body part can receive really high level radiation dose (upto 40 sievert) where the deathly dose is 6-7 sievert. That means a patient receives 5-7 deathly doses while being treated. But only the cancer tumour gets the 30-40 sievert dose, not the whole body. And thats how radiation therapy is used to treat cancer.