Stage is determined by three factors, T, N and M. T represents the size of the tumor and whether it has invaded into surrounding structures such as the ribs or the heart.
N represents the lymph nodes involved by metastasis from the lung tumor.
M describes whether spread to other organs has occured. A tumor has a characteristic life cycle in most cases. As mentioned earlier, it goes through many years of increasing mutations before it becomes a cancer. In it's earliest stages, it has the appearance of a cancer under the microscope, but has not yet begun to invade surrounding tissues. This stage is rarely detected clinically.