Cancerous cells can easily be destroyed by radiations due to : -

Cancer cells have lost the normal controls of division, so they divide rapidly and continuously.
Cells are most sensitive to radiation while they are dividing, because the DNA is being replicated and the chromosomes are condensed; radiation damages the DNA and stops further division.
So radiotherapy kills rapidly dividing cancer cells more than normal cells, most of which divide slowly.
So cancerous cells are easily destroyed by radiation due to rapid cell division.