Lung cancer is cancer that has developed in the lungs. There is an uncontrolled growth of cancer cells that starts in the lungs, which then cluster together to form a tumor that can later spread to other parts of the body.

It is the third most common cancer in the United States after skin cancer, which is the most common followed by prostate cancer in men and breast cancer in women.

It is the most lethal of all cancers. Worldwide, lung cancers cause the highest number of deaths compared with all other types of cancers.

The lungs are exposed to different types of pollutants and harmful substances present in the air that we breathe in. This makes them most vulnerable to cancer and other lung diseases.

Lung cancer most commonly develops inside the lungs but rarely may also develop in the chest wall.

About the structure and function of lungs

When you inhale, air enters through your nose or mouth and travels through the windpipe (called Trachea) to enter your lungs.

The trachea divides into two branching tubes called bronchi, which enter each lung and divide into smaller bronchi, which again divide to form smaller branches called bronchioles. The bronchioles terminate into tiny air sacs called alveoli.

An exchange of gases takes place in the alveoli. Oxygen from the inhaled air is absorbed into the blood and carbon dioxide from the blood is removed from the blood and thrown out through the air when you exhale. This is the main function of the lungs, that of taking in oxygen and getting rid of carbon dioxide.

About lung cancer

Lung cancers typically start in the cells lining the bronchi and in the bronchioles or alveoli. These are the epithelial cells, which line the airways and secrete mucus to lubricate and protect the lung.

What happens is that the normal lung cells mutate into cancer cells. This alters their natural growth and life cycle, resulting in uncontrolled cell division and rapid cell proliferation. These rapidly dividing cells do not carry out the functions of normal lung cells.

Tobacco smokers are at the greatest risk of lung cancer, though it can also develop in nonsmokers. The risk increases with the duration and number of cigarettes smoked. However, quitting smoking, even after having smoked for many years, can significantly reduce the risk of developing lung cancer.

Smoking and lung cancer statistics tell us that 85% of cases are due to tobacco use and 10% to 20% of lifelong smokers develop this cancer. The fact is that though smoking causes cancer, most smokers do not develop cancer, and not all cancer patients are tobacco smokers.

Lung cancer disease is not just a privilege of smokers. It can affect anyone. As a matter of fact, most such patients have either stopped smoking years earlier or never smoked.

There are other causes and risk factors for lung cancer. If you harbor any of these potential risks, you should undergo screening and if necessary diagnostic tests. A positive diagnosis not only confirms cancer but also informs the oncologist of the right treatment option for you.

Types of lung cancer

There are 2 main types: Non-small cell lung cancer (NSCLC) and Small cell lung cancer (SCLC)

Non-small cell lung cancer (NSCLC)

About 80% to 85% of cases are NSCLC. The main subtypes of NSCLC are

  • Adenocarcinoma (35–40%),
  • Squamous cell carcinoma (25-30%), and
  • Large cell carcinoma (25-30%).

These three subtypes originate from different types of lung cells but their treatment and prognoses (outlook) are mostly similar. Hence they are grouped together under NSCLC.


Adenocarcinomas originate in the epithelial cells lining the airways, which secrete mucus and are found more in the outer sections of the lungs.

Adenocarcinoma develops mainly in present smokers or those who used to smoke and now have given up smoking.

However, it is also the most common type of lung cancer seen in people who have never smoked. It is more common in women than in men, and it mostly develops in younger people.

Adenocarcinoma usually starts in the peripheral parts of the lung and is more likely to be detected before it undergoes metastasis. It, therefore, tends to have a better outlook than the other types.

Usually, doctors treat adenocarcinoma with surgery, radiation, and oral targeted agents. Presently, immunotherapy now has become the most important systemic treatment for advanced and metastatic adenocarcinoma.

Squamous cell carcinoma

Squamous cell carcinomas, also called epidermoid carcinomas, originate in the squamous cells that line the inner surface of the airways of the lungs. They are usually caused by smoking for a long duration and are often found in the central part of the lungs, near the main airway (bronchus). They are, therefore usually detected at an early stage.

Treatment can include surgery, chemotherapy, radiation, and immunotherapy.

Large cell (undifferentiated) carcinoma

Large cell carcinoma can develop anywhere in the lung. It tends to grow and spread rapidly, which can make its treatment difficult and prognosis worse.

Being an aggressive and fast-growing cancer, it is often diagnosed late when symptoms of shortness of breath and fatigue set in.

Sometimes, when detected at an early localized stage, it can be surgically removed along with a section of the lung or the whole lung. Other treatments include chemotherapy and radiation.

Other subtypes

There are a few other subtypes of NSCLC, such as adenosquamous carcinoma and sarcomatoid carcinoma, but they are much less common.

Small cell lung cancer (SCLC)

About 10% to 15% of all lung cancers are SCLC. This type also referred to as oat cell cancer, is the most aggressive type and tends to grow and spread very rapidly faster than large cell carcinoma.

In most people, SCLC is diagnosed when it has already spread beyond the lungs. It responds well to chemotherapy and radiation therapy. Unfortunately, many times it tends to recur.

It usually starts in the bronchi in the center of the chest and causes symptoms of hoarseness and hemoptysis.

It is treated with chemotherapy, immunotherapy, and radiotherapy, which may give relief from symptoms and prolong survival.

Surgery is rarely used in its treatment because it is often detected at advanced stages, which often makes it incurable.

Other types of lung tumors

Along with the two main types described above, other tumors can occur in the lungs.

  • Lung carcinoid tumors. Carcinoid tumors of the lung account for less than 5% of lung tumors. Most of these tumors grow slowly.
  • Other lung tumors. Other types of lung cancer, which are rare include adenoid cystic carcinomas, lymphomas, and sarcomas. Benign lung tumors such as hamartomas are rare.
  • Cancers that spread to the lungs. Cancers that originate in other organs, such as the breast, pancreas, kidney, or skin, can sometimes spread (metastasize) to the lungs. These are not lung cancers but are treated as cancer of the organs from where they have originated (primary site of cancer).

What are the stages of lung cancer?

Doctors use the two most common staging systems for cancer:

  1.  The TNM staging system stages most solid tumors
  2. The Roman Numeral System or overall staging system is used by some doctors on most types of cancers and is based on the clinical or pathologic status of the cancer.

Doctors classify the stages of NSCLC and SCLC using the TNM system.

  • T (tumor): The size of the initial tumor
  • N (node): Has cancer spread to nearby lymph nodes?
  • M (metastasis): Has cancer metastasized to distant organs, such as the brain, bones, or the other lung?

Doctors use another way to stage lung cancer through clinical or pathologic stages with numbers.

  • Stage 0 (in-situ): Cancer is located in the lung or bronchus and has not spread to other parts of the lung or outside of the lung.
  • Stage I: Cancer is bigger in size than stage 0, but hasn’t spread outside the lung.
  • Stage II: Cancer is larger than Stage I, has spread to lymph nodes inside or near the lung, or there’s more than one tumor in the same lobe of the lung.
  • Stage III: Cancer is larger than Stage II, has spread to nearby draining lymph nodes, or there’s more than one tumor in a different lobe of the same lung.
  • Stage IV: Cancer has spread to the other lung, or it has reached the fluid around the heart or lungs, or has spread to distant organs such as the brain, liver, or kidney.

Limited vs. extensive stage

Besides using stages as described above, doctors might also describe lung cancer as a limited or extensive stage. This depends on whether the area can be treated with a single radiation field.

  • Limited stage. SCLC is confined to one lung or it can be in the mediastinal lymph nodes or above the collarbone on the same side.
  • Extensive stage. SCLC is when it is spread throughout one lung or has spread to the other lung, to the lymph nodes on the opposite side of the lung, or to organs in other parts of the body.

Lung cancer survival by stage

Information from the Surveillance, Epidemiology, and End Results (SEER) database provides survival statistics for different types of cancer. These figures are based on people diagnosed with lung cancer between 2012 and 2018.

For survival statistics, the SEER database classifies cancers into localized, regional, and distant stages:

  • Localized: The cancer is localized inside the lung and has not spread outside of the lung.
  • Regional: Cancer has spread outside the lung to neighboring tissues or lymph nodes.
  • Distant: Cancer has spread to distant organs of the body, such as the brain, bones, liver, or other lung.

5-year relative survival rates for Non-Small Cell Lung Cancer (NSCLC)

Localized 65%
Regional 37%
Distant 9%
All stages combined 28%

5-year relative survival rates for Small Cell Lung Cancer (SCLC)

Localized 30%
Regional 18%
Distant  3%
All stages combined  7%