Cancerous cells can easily be destroyed by radiations due to :
Radiation therapy uses a special kind of high-energy beam to damage cancer cells. These high-energy beams, which are invisible to the human eye, damage a cell’s DNA, the material that cells use to divide. These radiations can damage both normal cells as well as cancer cells.
But radiation affects cancer cells more than normal cells. Cancer cells are constantly growing and multiplying, two activities that can be slowed or stopped by radiation damage. And as cancer cells are less organized than healthy cells, it's harder for them to repair the damage, done by radiation.
So cancer cells are more easily destroyed by radiation, while healthy, normal cells are better able to repair themselves and survive the treatment.