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04Medicine

Radiation therapy

Radiation can treat cancer. Zoom from a treatment room down to one strand of DNA, and back to the person, to see how.

The room

Two ways in

About 135,000 people in the Netherlands were told they had cancer in 2025. One in two people will hear it at some point in their life.

Surgery, chemotherapy and immunotherapy are well known. Radiation therapy is used a lot too, but fewer people know how it works. The radiation can reach a tumour from outside the body or from inside it.

A machine outside the body

  • A linear accelerator, or linac, speeds electrons up to nearly the speed of light and turns them into X-rays.
  • A CT scan shows the tumour. Marks on the skin put you in exactly the same place each time.
  • Metal leaves shape the beam, at most 40 by 40 cm, to fit the tumour. A session takes minutes.

An MRI scanner and a linac in one

  • The MR-linac was designed at UMC Utrecht. The first patients were treated there in 2017.
  • The MRI shows the tumour during the treatment, also when it moves or sits deep in the body.
  • The aim is sharper, so less healthy tissue is hit. Sometimes a higher dose and fewer sessions are possible.

A source in or next to the tumour

  • This is called brachytherapy. It is used for the prostate, the bladder, the oesophagus and more.
  • Thin tubes are placed in the tumour area and joined to a machine with the source. Afterwards they come out. Nothing radioactive stays.
  • Or tiny seeds go into the prostate and stay for good, fading over months. For a while you are slightly radioactive.

The body

Many beams, one tumour

This is a slice through a body, drawn like a CT scan. The tumour is in the middle. Colours show where the radiation lands: blue is a little, red is the full dose.

X-rays lose strength as they go deeper, so one beam gives the skin more than the tumour. Beams from many angles cross at the tumour. Protons give most of their energy at the end of their path and then stop.

0 degrees
Tumour dose100%
Hottest healthy spot0%
Healthy tissue reached0%

The tumour

A crowd of cells

Your body makes new cells all the time. Now and then a copy goes wrong and the cell stops doing its job. The immune system usually clears these cells away, but not always.

Cells that are missed keep dividing and pile up. That is a tumour. It can harm the organs around it, and cells can travel through the blood to other places. That is called a metastasis.

weak

The cell

Time your shot

A cell goes round a cycle: it grows, copies its DNA and then divides. The dividing step is called the M phase, or mitosis. Radiation harms a cell most then, when it is at its weakest.

The cells of a tumour are not all at the same step at the same time, so one session cannot catch them all. Press Fire at different moments and see what the phase does.

    The damage levels are an idea, not exact numbers.

    The DNA

    A break in the code

    DNA holds the instructions of the cell. Radiation breaks it. A cell can repair a little damage, but when there is too much it can no longer repair itself or divide, and it dies.

    Healthy cells are hit too, but they repair themselves faster and better than tumour cells do. The treatment depends on that difference.

    Breaks right now0 of 8

    The atom

    Three kinds of rays

    Radiation therapy uses ionising radiation. Its rays carry so much energy that they can knock electrons out of atoms and change the molecules of a cell.

    Alpha, beta and gamma rays come from atoms that fall apart, and they differ in how far they get. X-rays are close relatives of gamma rays: both are made of photons.

    • Alpha. Alpha rays hardly travel at all. They only work from inside the body, for example radium-223 against cancer that has spread to the bones.
    • Beta. Beta rays get through skin, but thin metal stops them. They are used from inside the body, for example iodine-131 against thyroid cancer.
    • Gamma and X-rays. Gamma rays and X-rays go much further. They are the most used rays in radiation therapy, from outside the body and from inside.

    The weeks

    Why so many sessions?

    The dose is split over many small sessions, usually on weekdays with a rest at the weekend. This is called fractionation. A common plan is about 2 gray (the unit of dose) a day, five days a week, for several weeks.

    Healthy cells recover between sessions better than tumour cells do. And each session hits hardest the tumour cells that are dividing at that moment: different cells each day.

    30 sessions of 2 Gy
    Tumour cells left100%
    Healthy cells left100%

    The person

    How it feels

    The treatment itself does not hurt, and a session takes only a few minutes. Most people feel tired, and the skin in the treated area can react. Hair can fall out there too.

    Other complaints depend on the body area. Some people notice them during the treatment, others only months or years later. Many people say a personal bond with their team made it easier.

    Head and neck. A dry mouth, a hoarse voice, loss of taste and trouble swallowing.

    Chest. Trouble swallowing, a dry cough and shortness of breath.

    Belly and pelvis. Bowel and bladder problems, and feeling sick.

    Everyone. Tiredness and skin reactions are the most common. Hair can fall out in the treated area.

    Key takeaways

    1. Radiation damages the DNA of tumour cells. When the damage is too much, the cell can no longer repair itself or divide.
    2. Beams from many angles add up at the tumour while healthy tissue gets less. Protons stop at the tumour, and X-rays go all the way through.
    3. The dose is split over many sessions. Healthy cells recover in between better than tumour cells, and each session hits the dividing cells hardest.
    4. Different rays do different jobs. X-rays and gamma rays work from outside, while alpha and beta rays work from sources placed inside.
    5. The treatment is painless and takes minutes. Tiredness and skin reactions are the most common complaints, and the rest depends on the body area.

    Sources

    This page is based on a school research project about radiation therapy. The facts were checked against the pages below, and a few statements from the project were corrected. The pictures are drawings, and the beam, cell and weeks tools are simple models, not medical tools. The page explains the idea. It is not medical advice.