system
Given the following post, respond as if you're commenting on Reddit:
human
Why don't we get cancer in our muscles?

My biology professor and I couldn't figure it out. At first we thought it could be to multiplying tissue, but brain cells don't multiply and we can get brain cancer. We also thought of energy usage, but our brain cells also use a lot of energy. So I thought to ask reddit what they thought of this question. Why don't we get cancer in our muscles, and only in our organs. 

*Thank you all for you input on this topic. You have taught me a lot. I will most definitely bring this research to the attention of my biology prof. Thanks Reddit!
gpt
Cancer of the muscles (or other connective tissue) is called sarcoma.  Sarcoma is rare because cells within connective tissue divide very rarely.  In your brain counterexample, while the neurons don't divide, the support cells *do* divide, and these are what develop into brain cancer.

The reason that slowly dividing cells develop into cancer less frequently has to do with how cancer arises.  A cell is cancerous if it is able to divide uncontrollably, and this requires a series of successive mutations, breaking the many redundant "safety systems" that our cells express in order to prevent this behavior.  

Here's a simple example - let's say that there are 5 genes that must be mutated to cause cancer.  You go out in the sun and get a bad sunburn, and it turns out that one of your skin cells has acquired a mutation in one of these genes.  That skin cell continues to live its life, dividing as normal.  Over time, there will be a *cluster* of skin cells that all contain this one mutation.  Years later, DNA replication in one of these cells causes another cancerous mutation, and now that cell has acquired 2 mutations.  This cell will form a colony of its own, creating more cells with both mutations.

In this way, the more rapidly a mutated cell divides, the more populated your body becomes with cells that contain this mutation - and there are more targets for further mutations that would lead to cancer.  Some of the most rapidly dividing cells form the *epithelium* of organs or other tissue - this is the layer of cells that replenishes tissue.  Cancer in this tissue (carcinoma) is much more common.