system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is cancer so common in some organs, and so rare in others?

You always hear about breast, skin, lung, and colon cancers for example, but things like brain, pancreatic, and stomach cancers seem to be so much more uncommon. Are some types of cells more susceptible to damage than others, or are some more resilient?
gpt
Usually cells that reproduce quickly or are subject to DNA damage are going to be more likely to become cancerous.

Cancer is when our own cells begin to replicate without stopping, recruiting resources to feed themselves. It all begins with a single cell which accumulates the right combination of mutations to do all that. Mutations occur when there are errors in DNA replication as cells divide or repair themselves, copying themselves slightly incorrectly.

Most mutations don't really cause any problems or noticeable changes, if they don't outright kill the cell. And it isn't just one mutation to cause a cell to be cancerous, it takes quite a few. But if you have trillions of cells all rolling the dice over and over eventually you will end up getting the right (or wrong) combination.

Cells that replicate more frequently naturally are rolling that dice more often. Carcinogens and things like UV light cause more damage to DNA which needs repairing, again another roll of the dice. This is why someone who smokes is more likely to develop lung cancer but isn't guaranteed, they are making many more rolls but may just never get the combination. Similarly someone who cares for themselves perfectly may get unlucky and develop cancer anyway.