Your lungs are 2 sponge-like organs found in your chest. Your right lung is divided into 3 sections, called lobes. Your left lung has 2 lobes. The left lung is smaller because the heart takes up more room on that side of the body.
When you breathe in, air enters through your mouth or nose and goes into your lungs through the trachea (windpipe). The trachea divides into tubes called the bronchi (singular, bronchus), which enter the lungs and divide into smaller branches called bronchioles. At the end of the bronchioles are tiny air sacs known as alveoli.
Many tiny blood vessels run through the alveoli. They absorb oxygen from the inhaled air into your bloodstream and pass carbon dioxide from the body into the alveoli. This is expelled from the body when you exhale. Taking in oxygen and getting rid of carbon dioxide are your lungs’ main functions.
A thin lining layer called the pleura surrounds the lungs. The pleura protects your lungs and helps them slide back and forth against the chest wall as they expand and contract during breathing.
Below the lungs, a thin, dome-shaped muscle called the diaphragm separates the chest from the abdomen. When you breathe, the diaphragm moves up and down, forcing air in and out of the lungs.
Lung cancers can start in the cells lining the bronchi and parts of the lung such as the bronchioles or alveoli.
Lung cancers are thought to start as areas of pre-cancerous changes in the lung. The first changes in the genes (DNA) inside the lung cells may cause the cells to grow faster. These cells may look a bit abnormal if seen under a microscope, but at this point they do not form a mass or tumor. They cannot be seen on an x-ray and they do not cause symptoms.
Over time, the abnormal cells may acquire other gene changes, which cause them to progress to true cancer. As a cancer develops, the cancer cells may make chemicals that cause new blood vessels to form nearby. These blood vessels nourish the cancer cells, which can continue to grow and form a tumor large enough to be seen on imaging tests such as x-rays.
At some point, cells from the cancer may break away from the original tumor and spread (metastasize) to other parts of the body. Lung cancer is often a life-threatening disease because it tends to spread in this way even before it can be detected on an imaging test such as a chest x-ray.
The lymph system is one of the ways in which lung cancers can spread. This system has several parts:
Lung cancer cells can enter lymphatic vessels and begin to grow in lymph nodes around the bronchi and in the mediastinum (the area between the 2 lungs). Once lung cancer cells have reached the lymph nodes, they are more likely to have spread to other organs of the body as well. The stage (extent) of the cancer and decisions about treatment are based in part on whether or not the cancer has spread to the nearby lymph nodes in the mediastinum.
There are 2 major types of lung cancer:
(If a lung cancer has some cells with characteristics of SCLC and other cells with characteristics of NSCLC it is called a combined small cell/non-small cell cancer. This is uncommon.)
These 2 types of lung cancer are treated very differently. This document focuses on non-small cell lung cancer.
About 85% to 90% of lung cancers are non-small cell lung cancer (NSCLC). There are 3 main subtypes of NSCLC. The cells in these subtypes differ in size, shape, and chemical make-up when looked at under a microscope. But they are grouped together because the approach to treatment and prognosis (outlook) are often very similar.
Squamous cell (epidermoid) carcinoma: About 25% to 30% of all lung cancers are squamous cell carcinomas. These cancers start in early versions of squamous cells, which are flat cells that line the inside of the airways in the lungs. They are often linked to a history of smoking and tend to be found in the middle of the lungs, near a bronchus.
Adenocarcinoma: About 40% of lung cancers are adenocarcinomas. These cancers start in early versions of the cells that would normally secrete substances such as mucus. This type of lung cancer occurs mainly in current or former smokers, but it is also the most common type of lung cancer seen in non-smokers. It is more common in women than in men, and it is more likely to occur in younger people than other types of lung cancer.
Adenocarcinoma is usually found in outer parts of the lung. It tends to grow slower than other types of lung cancer, and is more likely to be found before it has spread outside of the lung.
People with a type of adenocarcinoma called adenocarcinoma in situ (previously called bronchioloalveolar carcinoma) tend to have a better outlook (prognosis) than those with other types of lung cancer.
Large cell (undifferentiated) carcinoma: This type of cancer accounts for about 10% to 15% of lung cancers. It can appear in any part of the lung. It tends to grow and spread quickly, which can make it harder to treat. A subtype of large cell carcinoma, known as large cell neuroendocrine carcinoma, is a fast-growing cancer that is very similar to small cell lung cancer (see below).
Other subtypes: There are also a few other subtypes of non-small cell lung cancer, such as adenosquamous carcinoma and sarcomatoid carcinoma. These are much less common.
About 10% to 15% of all lung cancers are small cell lung cancer (SCLC), named for the size of the cancer cells when seen under a microscope. Other names for SCLC are oat cell cancer, oat cell carcinoma, and small cell undifferentiated carcinoma. It is very rare for someone who has never smoked to have small cell lung cancer.
SCLC often starts in the bronchi near the center of the chest, and it tends to spread widely through the body fairly early in the course of the disease.
Along with the 2 main types of lung cancer, other tumors can occur in the lungs.
Lung carcinoid tumors: Carcinoid tumors of the lung account for fewer than 5% of lung tumors. Most are slow-growing tumors that are called typical carcinoid tumors. They are generally cured by surgery. Some typical carcinoid tumors can spread, but they usually have a better prognosis than small cell or non-small cell lung cancer. Less common are atypical carcinoid tumors. The outlook for these tumors is somewhere in between typical carcinoids and small cell lung cancer.
Other lung tumors: Other types of lung tumors such as adenoid cystic carcinomas, hamartomas, lymphomas, and sarcomas, are rare and are treated differently from the more common lung cancers. They are not discussed in this document.
Cancers that spread to the lungs: Cancers that start in other organs (such as the breast,pancreas, kidney, or skin) can sometimes spread (metastasize) to the lungs, but these are not lung cancers. For example, cancer that starts in the breast and spreads to the lungs is still breast cancer, not lung cancer. Treatment for metastatic cancer to the lungs is based on where it started (the primary cancer site).
Referenced from American Cancer Society (cancer.org)