Appendix A: Understanding cancer

Understanding cancer

Cancer is a disease of the body's cells. It starts in our genes. Our bodies are constantly making new cells to allow us to grow, replace worn-out cells or heal damaged cells after an injury.

The process of making new cells is controlled by certain genes: the codes that tell our cells how to grow and behave. Cancers are caused by damage to these genes. These changes usually happen during our lifetime.

In a very small number of families, damaged genes may be passed through the generations. While these people will have an increased risk of developing cancer, it does not mean they will definitely get cancer.

How cancer starts

Tumours can be benign (not cancerous) or malignant (cancerous). Benign tumours do not spread to other parts of the body.

How cancer spreads

A malignant tumour is made up of cancer cells. When it first develops, a malignant tumour is usually confined to its original site. This is known as the primary site. Some tumours can become quite large within their organ of origin: for example, the lung or breast. With growth, the tumour may spread beyond the original organ boundaries and into surrounding tissues. This is called locally advanced cancer.

Sometimes, cells move away from the original (primary) cancer through the blood stream or lymphatic systems and start to grow in other body organs. When these cells reach a new site they may form another lump or mass. This is called a secondary cancer or metastasis. For example, if lung cancer spreads to the bone, it is called a bone secondary (or metastasis).

If the only place of spread is to nearby lymph nodes this is called regional nodal spread. Your cancer doctor will still refer to it as lung cancer even though it has spread to another part of your body.

The sort of treatment you are offered for cancer depends on the type of cancer, where it began and whether it has spread. Your cancer doctor will also take into account other things about you, such as your age and general health.

Treatment for cancer includes surgery, radiation treatment or chemotherapy (drug treatment). Immune therapy or targeted treatments, which are now used to treat some cancers, will become more important in the future.

Sometimes only one of these models of treatment is used for a cancer. Sometimes more than one is used.

How cancer spreads

Key Points: Understanding cancer

  • Cancer is a disease of the cells. It can grow into a tumour or spread (metastasise) to different parts of the body.
  • A metastasis (secondary cancer) keeps the name of the original cancer (for example, lung cancer that has spread to the bones is still called lung cancer).

How your lungs work

The lungs are the organs in the body's system for breathing called the respiratory system. The respiratory system also includes the nose, mouth, windpipe (trachea) and airways to each lung—known as the large airways (bronchi) and small airways (bronchioles).

The respiratory system

The respiratory system includes the upper and lower respiratory tract.

The upper respiratory tract has:

  • the nose and nasal cavity
  • the throat (pharynx)
  • the voice box (larynx).

The lower respiratory tract has:

  • the windpipe (trachea)
  • breathing tubes (bronchi and bronchioles)
  • air sacs (alveoli).

The respiratory system

The lungs

The two lungs are inside the chest, protected by the ribcage. The lungs are soft and look like two large, spongy cones.

Lungs have sections called lobes. The left lung has two lobes and the right lung has three. The lungs are separated from the stomach and liver by the diaphragm—a wide, thin muscle that helps with breathing.

Several parts of the body lie in the space between the lungs, called the mediastinum, including:

  • the heart and large blood vessels
  • the windpipe (trachea)
  • lymph glands (also known as lymph nodes).

A thin, double layer of membrane called the pleura sits around the lungs. The pleura are about as thick as plastic food wrap. Its inner layer (the visceral layer) is joined to the lungs and its outer layer (the parietal layer) lines the chest wall and diaphragm. Between the two layers is the pleural cavity, which normally holds a thin fluid. This fluid allows the two layers of pleura to slide against each other so your lungs can move smoothly against the chest wall as you breathe.


How you breathe

The lungs do not move on their own. The muscles between the ribs and the diaphragm make the chest expand and contract pulling and pushing air into and out of the lungs.

The windpipe divides into two airways. These are called the right main bronchus and left main bronchus. One goes to each lung. Within the lungs, each bronchus divides into smaller tubes called the secondary bronchus.

Each secondary bronchus divides into smaller tubes called bronchioles. Each bronchiole ends up in a tiny, bubble-like air sac. It's these air sacs (alveoli) that make the lungs spongy. When we breathe in air goes through the nose or mouth and into the throat and down the windpipe and bronchus until it reaches the alveoli.

Blood flows through very small blood vessels (capillaries) in the thin walls of the alveoli. This allows oxygen to move from the air into the blood, and carbon dioxide (a waste product) to move from blood to air to be breathed out.


Key Points: How your lungs work

  • The lungs are the body's organs for getting oxygen into the body.
  • Two lungs sit inside the chest, protected by the ribcage.
  • The left lung has two lobes and the right lung has three lobes.
  • A thin, double—layer membrane called the pleura sits around the lungs.