As you inhale, air enters the body through the nose or mouth and travels through the windpipe, or trachea, to the lungs. Two tubes, called bronchi, connect the trachea to the lungs and branch out into multiple smaller bronchi. The bronchi divide into even smaller branches known as bronchioles. The alveoli are tiny air sacs located at the end of each bronchiole.
The alveoli contain multiple small blood vessels. These vessels handle transferring the oxygen into the circulatory system and absorbing any carbon dioxide from the blood. The carbon dioxide leaves the body as you exhale. The two primary functions of the lungs are expelling carbon dioxide from the body and taking in oxygen.
The pleura is a thin layer of tissue surrounding the lungs. The pleura offers protection by allowing the lungs to slide easily against the chest wall when expanding and contracting while breathing.
The diaphragm is a thin muscle shaped like a dome separating the abdomen from the chest cavity. As you breathe, the diaphragm forces air through the lungs as it moves back and forth.
How Lung Cancer Develops and Spreads
Now that you have got the answer to what is lung cancer, find out about how it develops and spreads quickly. Lung cancer may develop in any area of the lung, including the alveoli, the bronchioles, and the bronchi lining.
Pre-cancerous alterations in the lung are believed to be the earliest signs of lung cancer. In many cases, the lung cell DNA changes and causes uncontrolled and accelerated cell growth. At this stage, the cells do not form a tumor but they may appear abnormal when viewed in high magnification with a microscope. These cells do not show up on chest X-rays, and the individual does not have any symptoms.
As time goes by, the abnormal lung cells undergo additional gene changes, and they become cancerous. Some cancer cells produce chemicals capable of stimulating the growth of new blood vessels. As the blood vessels provide nourishment to the cancer cells, the cells grow bigger and form a mass, or tumor, that becomes visible on x-rays and other imaging tests.
Sometimes the cancer cells separate from the tumor and travel to other organs. The reason lung cancer is so often deadly is because the cells usually spread throughout the body before chest x-rays or other imaging tests can detect them.
Understanding the Lymphatic System
Lung cancer often uses the lymphatic system to spread throughout the body. This system consists of:
Lymph nodes — These clumps of cells fight infection and are connected to each other by a system of lymphatic vessels.
Lymphatic vessels — Small veins that transport lymph away from each lung.
Lymph — A clear fluid containing waste products and liquid collected from immune system cells and other body tissues.
When cancer cells enter the lymphatic vessels, they can begin to grow in the space between the lungs known as the mediastinum, as well as in the lymph nodes near the bronchi. Cancer cells that make it to the lymph nodes are more likely to spread to other organs in the body. Various treatment options are available depending on the extent of the cancerous cell growth and if the lymph nodes have been invaded by cancerous cells. We explore this topic further in the section dealing with how non-small cell lung cancer is staged.
Lung Cancer Types
Lung cancer falls into one of two major categories. The first type of lung cancer is small cell or SCLC. The second type is non-small cell or NSCLC.
Combined SCLC/NSCLC is another type of cancer that has some cells with SCLC characteristics and some cells with NSCLC characteristics. This is a rare condition.
The treatments for these two types of cancer are very different. This article specifically discusses NSCLC.
Types of NSCLC
As much as 90% of all lung cancers are categorized as NSCLC. There are three subtypes of NSCLC. Each subtype features cells that have different chemical make-ups, shapes, and sizes when viewed with a microscope. These cancer types belong to the same category because their treatment and prognosis are similar to each other.
Epidermoid (or squamous cell) carcinoma — Thirty percent of lung cancer patients suffer from this subtype of lung cancer. Cancer starts inside the flat cells lining the airways. This type of cancer is often caused by smoking and typically develops near the bronchus in the middle of the lung.
Adenocarcinoma — Adenocarcinoma makes up approximately 40% of lung cancer cases. The cells affected by adenocarcinoma normally secrete mucus and other similar substances. Although former or current smokers develop this type of cancer, it is also the most common form of cancer diagnosed in people who do not smoke. This subtype is seen more often in women and affects young people more other than other subtypes.
Adenocarcinoma typically develops in the outer portion of the lung. It is slower growing than other subtypes and physicians are often able to diagnose it before it has a chance to spread to other organs.
Adenocarcinoma in situ, previously known as bronchioloalveolar carcinoma, is a specific type of adenocarcinoma that often has a more positive prognosis than other lung cancers.
Undifferentiated (large cell) carcinoma — Approximately 15% of lung cancer patients have undifferentiated carcinoma. This type of cancer attacks all areas of the lung. It grows and spreads rapidly, which makes it more difficult to treat successfully. Large cell neuroendocrine carcinoma is a subtype of undifferentiated carcinoma that is similar to SCLC and grows quickly.
Other subtypes — Sarcomatoid and adenosquamous carcinoma, among others, are additional subtypes of NSCLC. These are less common than the other subtypes listed here.
The Characteristics of SCLC
Approximately 10% to 15% of lung cancer patients are diagnosed with SCLC, which gets its name from the size of the cells. This type of lung cancer is also known as small cell undifferentiated carcinoma or oat cell carcinoma. This type of cancer rarely develops in individuals who have never smoked cigarettes.
SCLC often begins in the middle of the chest inside the bronchi and spreads throughout the body very soon after it starts. More information about this type of cancer is found in the article titled Lung Cancer (Small Cell).
Additional Types of Cancer that Attack the Lungs
Other tumors may develop inside the lungs in addition to the two main types of cancer.
Carcinoid tumors — Less than 5% of tumors found in the lungs are categorized as carcinoid tumors. Most of them are typical carcinoid tumors, which grow slowly. The treatment for these tumors usually involves surgery to remove them. In some cases, these tumors may spread to other organs but the prognosis is usually more positive than with NSCLC or SCLC. Atypical carcinoid tumors are less common. The prognosis is somewhere between SCLC and typical carcinoid tumors. See the article titled Lung Carcinoid Tumors for additional information about atypical and typical carcinoid tumors.
Other types of tumors — Lymphomas, sarcomas, adenoid cystic carcinomas and benign tumors, including hamartomas, are not common but can develop in the lungs. The treatment for these tumors is different from the treatment for other common types of lung cancer and is not addressed here.
Other types of cancer that spread from other organs to the lung — In some cases, cancer that develops in the skin, breast, kidney, pancreas or some other organ metastasizes. Or cancer that spreads, to one or both lungs, but are not classified as lung cancers. If cancer begins in the breast and spreads to the lung, it is breast cancer. When metastatic cancer reaches the lungs, the treatment is determined by where it began. To learn more about other types of cancer, please see our articles dedicated to each type.