system
Given the following post, respond as if you're commenting on Reddit:
human
eli5: What makes certain cancers resistant to radiation/treatment?

For example, I know someone whose daughter has medulloblastoma. She can be in radiation for years, but as soon as they stop the treatment the cancer starts to grow again. How can cancer "survive" after months or even years of constant radiation and chemo?
gpt
The problem with cancer cells is that they’re still *human* cells, and so selecting a treatment that will only kill them without just killing everything is sometimes very difficult.

Since they’re frequently dividing we often go with radiation and chemical poisons that will damage cells currently undergoing cell division a lot more than other cells.  It hits the cancer hard, but also hits your hair, digestive system, reproductive system, and immune system because they’re often dividing too.

This limits the amount and harshness of treatment we can apply.  You can’t just go scorched earth and dump 7 times the lethal dose for three months to make sure the cancer cells are all fried - the patient won’t survive.

Cancer cells that have spread to many little nooks and crannies where they can evade the worst of the treatment or that still have better than average cell repair capacity can be very difficult to successfully treat with radiation and chemical methods because they’re just as robust as the rest of your cells.