system
Given the following post, respond as if you're commenting on Reddit:
human
How does a non-cancerous tumor develop? Isn't the abnormal growth/multiplication of cells the definition of cancer?
gpt
The abnormal growth/multiplication of cells is hyperplasia, which is not the same thing as cancer. Many non-cancerous conditions result from hyperplasia, including skin conditions like psoriasis where you see abnormal thickening of the skin due to excessive growth of skin cells. 

Cancer is a multi-stage process, which may begin with hyperplasia. This is what is often used to screen for 'pre-cancerous' tissue. These abnormally thickened lesions may remain as they are, or acquire mutations down the line and progress to full-blown cancer. In order to do so, they have to gain the ability to ignore the normal regulatory controls that prevent them behaving differently from other cells in their tissue of origin - many checks and balances are in place to, for example, stop cells from growing once the field has enough cells in it. This ensures that all cells are well-supplied with nutrients and oxygen required for healthy tissue function. Cancer cells learn to ignore these signals and consume resources at the expense of surrounding cells.

The next step happens once the cancer cells grow to the point where nutrient delivery becomes a problem. The centre of new tumours can become very low in oxygen and nutrient-poor due to poor blood supply, which limits the growth of many cancers. This limitation is sometimes overcome when cancer cells gain the ability to recruit new blood vessels to the tumour. With fresh blood supply, the tumour can rapidly expand in size.

At this point, the tumour is still confined to its tissue of origin. If this is an epithelial tissue (like the skin surface), the cancer is not yet life-threatening and can often be completely removed by surgical excision. Sometimes the physical location of the tumour can cause problems - head and neck tumours can interfere with speech or swallowing for example - but the real danger lies ahead. Once tumour cells gain the ability to leave their origin site and spread through the body through blood or lymph (metastasis), the likelihood of survival drops significantly. Metastasis is the reason why cancer is so feared - instead of a localised tumour that can be monitored and/or completely cut out, the cancer is now everywhere in the body where it can potentially interfere with critical organs like the brain and lungs. This is also why early screening is so important. If a tumour can be diagnosed when it is still relatively benign, the individual has much better treatment outcomes then if the cancer is discovered after it has already spread. 

TLDR: Cancer progresses through multiple stages, including many which would be considered 'benign' or 'non-cancerous'. Some non-cancerous tumours may remain as such while others progress to full-blown malignancy. Screening is important for early diagnosis!