system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 radiation causes cancer, but radiation therapy is used to beat cancer?
gpt
Radiation causes DNA damage.

DNA damage causes mutations.

Mutations *can* cause cancer... but usually trigger apoptosis; cell suicide.

Tumors are a concentrated mass of cancerous cells.

In radiation therapy, *many* relatively weak beams of radiation are aimed at the tumor from all angles. They are aimed so that they ***only*** converge on the tumor; this means that the target tissues are affected tens of times more powerfully than the non-target tissues. 

Essentially, you irradiate the tumor with such an absurd dose of radiation that the DNA damage goes beyond the realm of mutations, and into the realm of, "What the fuck is that molecule anymore? Certainly not DNA."

This kills the cancer cells.