What is lung cancer?

Lung cancer starts in the windpipe (trachea), the main airway (bronchus) or the lung tissue. Cancer that starts in the lung is called primary lung cancer.

There are 2 main types of primary lung cancer:

  • non small cell lung cancer
  • small cell lung cancer

The most common type is non small cell lung cancer.

Primary lung cancer differs from cancer that spreads to your lungs from somewhere else in your body. This is called secondary lung cancer.

The lungs

The lungs are part of the breathing system, which is also called the respiratory system. The breathing system includes:

  • the nose and mouth
  • windpipe (trachea)
  • airways to each lung (left and right bronchus)
  • lungs
Diagram of the windpipe, airways and lungs

The right lung is divided into 3 sections called the upper, middle and lower lobes. The left lung is divided into 2 sections called the upper and lower lobes.

The pleura (covering of the lungs)

The pleura or pleural membranes are 2 fibrous sheets of tissue. They cover the lungs and help to protect them. 

The gap between the pleura is called the pleural space. The pleura make a lubricating fluid. The fluid keeps the membranes moist, so they slide easily over each other as we breathe in and out.

Lung cancer cells can spread to the pleura. The cancer cells inflame the pleural membranes and they then make too much fluid. The fluid collects in the pleural space and stops the lung from expanding fully. This can make you feel breathless. Build up of fluid between the pleura is called a pleural effusion.

The lymph nodes

Lymph nodes (also called lymph glands) are close to the lungs and airways. They are part of the lymphatic system. This a network of tubes and glands in the body that filters body fluid and fights infection and illnesses like cancer.

Lymph fluid circulates through body tissues. Cancer cells may be released from a lung tumour and trapped in nearby lymph nodes. 

When you are diagnosed, your surgeon or cancer specialist (medical oncologist) checks your lymph nodes for cancer cells.

How the lungs work

Your lungs have a system of tubes that carry oxygen in and out as you breathe. 

The windpipe divides into two tubes, the right bronchus and left bronchus. These split into smaller tubes called secondary bronchi. They split again to make smaller tubes called bronchioles. The bronchioles have small air sacs at the end called alveoli.

In the air sacs, oxygen passes into your bloodstream from the air breathed in. Your bloodstream carries oxygen to all the cells in your body. At the same time carbon dioxide passes from your bloodstream into the air sacs. This waste gas is removed from the body as you breathe out.

Where lung cancer starts

Lung cancer can start in any part of the lungs or airways.

How common is lung cancer?

Lung cancer is the third most common cancer in the UK. Around 48,500 people are diagnosed in the UK each year.

Who gets lung cancer?

Lung cancer is more common as you get older. Around 45 out of 100 people (around 45%) diagnosed with lung cancer in the UK are aged 75 and older.

In the UK each year:

  • around 25,300 men are diagnosed
  • around 23,300 women are diagnosed

More than 70 out of 100 (more than 70%) of lung cancer cases in the UK are caused by smoking.

Other causes or risk factors include:

  • exposure to radon gas
  • exposure to certain chemicals in the workplace
  • a family history of lung cancer
  • certain cancer treatments for other types of cancer

Related cancers

A rare cancer called mesothelioma starts in the protective sheets that cover the lungs (pleura). This is very different to lung cancer.

Cancer can spread into the lung from other parts of the body. This is called secondary lung cancer.