system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do some cancers present symptoms early while others don't often present until stage III or IV?
gpt
It's important to understand every type of cancer is a different disease, even within similar types of cancers there are subtypes that present different mutations or levels of gene expression which effect their behaviour. Therefore the tumour itself can behave differently.

You also have to consider where the tumour is in the body. The physical location of a tumour could mean it's putting enough pressure on the organ or nearby organs, to disrupt their normal function. If you're lucky a very small tumour growing in the right place could cause symptoms that warrant a screen before it has spread. 

But unfortunately if you pair a fast growing tumour subtype with a location in the body that isn't disrupting normal bodily functions, it can result in tumours that cause no symptoms until they have spread to other areas of the body like your lungs. Where you may present with a bad cough when the original tumor was on your kidney for example.

This is why diseases like pancreatic cancer are so deadly and result in one of the highest mortality rates for cancers.