What is the relationship between the control of cell cycle and cancer

Cancer cells do not complete the cell cycle, which means they start dividing again before the old cycle is finished. This is why a tumor of cells form. One way to test if the tumor is cancer or not is by examining it under the microscope and finding the mitotic index, which is an indicator of the rate of cell division and hence if the cells complete the cell cycle or not.

Well uncontrolled cell division can lead to cancer.