Cancer is caused by virus. Where would the cancer appear, depends on something called viral tropism. A strong immune systems wipes up quickly the virus, while a weak one tolerates a chronic inflammation. When virus finally hits stem cells, a protection mechanism is triggered by the organism. For more details please see presentations by professor Zajicek on you tube. It is assumed that among other tasks, that stem cells also secrete a substance to sustain tissues. In order to protect itself form tissue degeneration, organism mobilizes a pull of stem cells into a new organ. In the beginning the substance deficiency is low and fully compensated by a minor tumor. With time the deficiency grows and so does the new organ, in order to compensate and as a part of a protection measure (against Cachexia). The new organ puts pressure on other organs and causes pain. Patient seeks medical help and new organ is diagnosed as cancer.