A benign tumour is generally not dangerous as they grow usually within a membrane in one space. They can however grow really big in a short space of time and can cause pressure on neighbouring blood vessels which can be dangerous.
Metastatic or malignant tumours are dangerous and cancerous. After they grow, some cells break off and travel in the bloodstream to a different area of the body (usually the main organs) and forms a secondary tumour there. This keeps happening until the cancer has spread to all of the body.
