system
Given the following post, respond as if you're commenting on Reddit:
human
How does radiation "cause" cancer?
gpt
2nd year UK Medical student here. 

Cancer is unregulated cell growth. Normally cell growth and production are regulated by 2 (broad) types of genes- Oncogenes which promote growth, and Tumour Suppressor genes which slow cell growth. 

Ionising radiation will break bonds and physically alter the DNA inside cells. Often this will lead to a mutation which will cause the cell either to die, or to be killed by the body in a controlled way. 

However, if an oncogene is affected so that it is over-active, or a Tumour Suppressor gene is altered so it is not active enough, then cancer will occur. 



References: Wikipedia and Medical school (in that order)