I am amazed how large cancer tumors can get and the patient still lives. A tumor is a lump but if it is not next to some vital structure it's actually just a large mass and we can live with even huge tumors. Fast growing tumors are growing uncontrolled and very metabolically active so they are sucking up energy or blood sugar from the body. Also we treat cancer with chemotherapy which makes the patient sick and causes the patient to lose their appetite while they are dealing with this growth that is sucking up the patient's energy possibly driving the patient to starvation. This makes it so important for cancer patients to eat well during treatment. This may also explain why sudden weightloss is a sign of cancer and also why cancer patients' immune system are reduced, the tumor is sucking up the energy that would make gone to the immune system.