system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If radiation exposure causes cancer, why do they use it to treat cancer patients? Won't it just guarantee they will develop more cancer in the future?
gpt
Radiation exposure damages cells, including their DNA.  Cells with damaged DNA can become cancerous because the damage can cause them to replicate uncontrollably, which is what a cancer is.  But enough radiation exposure will kill a cell outright, including those which are already cancerous.  So doctors administer radiotherapy in a carefully targeted way such that it only attacks the cancerous cells of a tumour, killing them but leaving the surrounding cells mostly undamaged.  Because of the requirement for careful targeting, not all cancers are able to be treated with radiotherapy.