Cancer cells invade nearby tissue and/or blood vessels. This is a necessary step for them to metastasize or spread to other organs.
Tumors remain small until they're able to attract their own blood supply (angiogenesis), which allows them to obtain the oxygen and nutrients they need to grow larger.
Cancer can take decades to develop. By the time a cancerous mass is detected, it's likely that 100 million to 1 billion cancer cells are present, and the original cancer may have been dividing for five, ten or even more years.
