Producing your radiotherapy plan

CT scans or an X-ray machine called a simulator are used to plan radiotherapy treatment. It is called a simulator because it is like (simulates) a radiotherapy machine, but does not give the treatment. The pictures it creates are used, along with any scans or X-rays, to line up the area to be treated.

When the treatment is being planned, the radiographer takes some measurements of your body and perhaps the distance from the treatment couch to skin markings on your body. This is because the distance between the radiotherapy machine and the cancer is very important in working out the radiotherapy dose. The further away from the machine, the lower the radiation dose.

When all this information is put together, the planning computer can work out the best positioning of the radiotherapy beams to treat the cancer but give as little radiotherapy to healthy body organs in the area as possible.

This picture shows a typical CT scan cross section (slice). The slice shown is of the bottom of the rib cage. Note that the left on this scan is actually the right side of the body. This is what you can see

• On the left, there is a large crescent - this is the liver
• On the right, at the bottom, there is a round shape - this is the kidney
• In the centre, at the bottom, there is a lighter round shape - this is the spinal cord inside the backbone
• In the middle, there is an irregular shape, which is the pancreas with a cancer inside it

A 3D picture of the cancer can be built up when the physicist or doctor puts together a number of CT scans or MRI scans. This can be done by computer, but is often also done in the 'mind's eye' of the radiotherapist. This picture shows the 3D image. This is what you can see

• The cancer is shown in red
• The liver is shown in brown
• The spleen is shown in blue/green
• The left kidney is shown in pink
• The spine is shown in yellow

Looking at a 3D picture can be very useful, as it can be rotated and looked at from different angles. This is very helpful when planning treatment. It helps the radiotherapist to work out the best way to arrange the radiotherapy beams to cover the whole cancer and miss the major organs.

To treat cancers deep within the body, 2, 3 or 4 beams are pointed at the cancer from different angles. The area where all the beams meet is where the tumour is. This is where the highest dose of radiotherapy is given. This picture shows how 3 beams can be directed at the tumour. The picture is of the bottom of the rib cage

• On the left, there is a large crescent - this is the liver
• On the right, at the bottom, there is a round shape - this is the kidney
• In the centre, at the bottom, there is a smaller round shape - this is the spinal cord inside the backbone
• In the middle, there is an irregular shape - this is the pancreas with a cancer inside it

The straight lines are the radiotherapy beams. One beam is aimed from in front of the patient. This is the beam at the top of the picture. The other two beams are aimed from the sides at an angle so that they avoid the spinal cord and kidney.

Computers can also be used to show the view from the radiotherapy machine directly along the beam. Above you can see the 'beam's eye view'. The organs are displayed as 'wire frames' rather than as solid objects. This is very useful in making sure all the major organs have been missed, while the cancer has been hit. This is important in making radiotherapy as safe and effective as possible.

This picture shows the cancer in red and the kidney in white. As you can see, not all the red area is covered by the beam, but some of the white wire frame is in the path of the beam. So, the beam needs to be moved to include all the cancer and avoid the kidney.

Once the treatment area is lined up, marks are made on the skin. These skin marks are used by the radiographers to set up the radiotherapy machine in the same way every day.