Our bodies are made up of trillions of cells. When our cells get old or damaged, they die and are replaced by new cells. Cancer occurs when old or damaged cells don’t die and new cells reproduce. These extra cells may then build up and form tumors.

Cells can change, or mutate, for different reasons. Mutations can be passed down from parents to children or they can result from exposure to certain things known to cause cancer, like smoking, certain chemicals, overexposure to the sun, and other pollutants and gases. And, although there are numerous risk factors that may cause cancer, it can also develop for unknown reasons.

Though cancer can appear in places all over the body, we are going to focus on the lungs.

How Lung Cancer Starts

Lung cancer starts in the cells of the lung, and as it progresses, it can grow into surrounding structures, and spread into the body's small, bean-shaped masses of tissue called lymph nodes that surround the bronchi. The lymph nodes are part of the lymphatic system, which helps to filter out harmful substances in the body. When cancer cells are found in the lymph nodes, there's a chance that the cancer may be in other places in the body as well. Each stage of lung cancer is determined by the size of the tumors, and if or where they have spread to within the body.