Tumor growth refers to the uncontrolled and abnormal increase in the size of a mass or lump, which is formed by an accumulation of cells. This term usually applies to cancerous tumors but can also be used for benign (non-cancerous) tumors as well. Tumor growth occurs when normal cells divide uncontrollably and form new abnormal cells that do not die or stop growing. The mass of these cells forms a lump, which is called a tumor. As the cancer cells continue to grow, they can invade nearby tissues and organs, leading to more health problems. Tumor growth rate refers to how quickly the tumor is increasing in size over time.