Cancer patients and
their families

What is radiation oncology? Here, patients and their families can find a comprehensive guide of radiotherapy treatments in Belgium.


Healthcare Professionals and radiation oncology

As a medical professional, you're familiar with the basics of radiotherapy. Visit the following pages for a more elaborate approach on the topic.


Healthcare Policy makers and radiation oncology

Radiation oncology is not like other medical specialties. This section deals with existing quality initiatives and cost-effectiveness.


Home > Glossary > I - P

I - P

 Immobilisation Mask PET-CT
 In vivo dosimetry NMR/MRI/KST      Planning
 Isodose Nomenclature Portal Image
 Leafs OAR Prone
 Linear Accelerator Palliation Proton Therapy



Precise radiotherapy requires a patient to move as little as possible while on the treatment table during the treatment session. Different tools and systems are used to immobilise a patient: thermoplastic sheets, cushions, supports, etc. In many cases these devices are individually adapted to take the form of the patient. See also 'Mask'.

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Intensity modulated radiotherapy. This means that the radiotherapy beams are not homogeneous beams or rays. They contain subzones of more and less strong irradiation. By applying these beams from a multitude of angles, IMRT helps to sculpt the dose around the target volume, even if this target volume has a irregular form or is partially hidden behind more dense tissues in the human body. (Source:

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In vivo dosimetry

The radiation oncologist prescribes a certain radiotherapy dose, and the physicist calculates the treatment (treatment plan). In order to check if what was planned is effectively given, an extra security step is taken: at the start of the treatment, a radiation detector can be placed against the body to check if the expected dose at that point is really given. The radiation oncology team has also other techniques to check the correct delivery of the dose. (Source: Medical Physics)

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The treatment planning system (TPS) calculates (based on the simulation CT scan) a distribution of the dose in the patient. In a graphical presentation of this theoretical dose distribution in the patient, the radiation oncologist can make appear 'isodose lines' which are lines that indicate regions of the human body that receive the same dose.

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The beam produced by a radiotherapy machine (accelerator) does not always have the size/form that is suitable to irradiate a given tumor or patient. Therefor it is possible to slide into the beam a number of 'metallic bars', called leafs, that will locally stop the irradiation. These leafs can move in and out the radiation beam, and can continue moving during the treatment sessions.

By doing so, they can shape the form of the beam, and also create zones where the beam is 'stronger' than in other zones.

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Linear accelerator

A linear accelerator (LINAC) is the device most commonly used for external beam radiation treatments for patients with cancer. The linear accelerator is used to treat all parts/organs of the body. It delivers high-energy x-rays to the region of the patient's tumor. These x-ray treatments can be designed in such a way that they destroy the cancer cells while sparing the surrounding normal tissue.

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Immobilisation of the patient during treatment is very important to obtain a reliable and precise treatment. Even a very collaborative patient can not stay still enough for some precise treatments, like in the head or the neck area. A mask is the term used for a thermoplastic sheet that is heated to become flexible, then placed on the head/neck/shoulder area of the patient during the simulation procedure. It is then moulded to take the precise anatomical form of the patient. When it cools down, it gets rigid again. Modern masks are skin-friendly, hygienic, easily moldable, solid when cooled down, and do not interfer with the treatment beams.

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Magnetic Resonance Imaging. This medical exam is obtained in the department of medical imaging. It is better suited then a classical CT scan for some tumors or some areas of the body. MRI scanners use strong magnetic fields and radiowaves that form images of the body. Without using ionising irradiation, information both on anatomy and on function are available.

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See MRI.


Belgian physicians encode a specific code for each medical act performed. Each code is linked to a set of rules, and also an amount of money the physician or the hospital will receive for performing the medical act. The nomenclature is the whole set of codes.

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Organ At Risk. During preparation of a radiotherapy treatment, the medical team will use the images obtained during the simulation procedure to calculate the treatment. To do this, the medical team will have to indicate on these images which areas of the body of the patient absolutely need to be irradiated (the 'target volume'), and which other organs nearby should be avoided so far as possible. These other organs are the 'organs at risk', even if they are not really 'at risk' because in many cases it will be possible to limit the irradiation dose on these organs to a great extent.

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The goal of a medical treatment like radiotherapy can be to cure the tumor. But when this is not possible or only partially possible, the goal of the treatment can be to increase or maintain the quality of life of the patient or to avoid future degradation of this quality of life. These 'supportive treatments' are also called palliative treatments. Specifically considering radiotherapy, one can for instance propose a radiation treatment to reduce pain, or to reduce a tumoral swelling, or to avoid a bleeding. Other palliative goals exist. Palliative treatments are possible also in patients who still have a reasonable life expectancy.

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Positron Emission Tomography. This examination, which is performed in the department of nuclear medicine, is not a part of  radiotherapy (it is not a treatment). It is a diagnostic procedure that verifies the exact location of a disease in the body and if the disease may have spread. With a PET-CT procedure a radio-active product is injected and will circulate everywhere in the body (via the blood). It will then settle down in specific areas, like for instance tumours that use a lot of sugar to grow. A scan is made to register where exactly in the body this product was found. During a PET scan, a classical CT scan is made at the same time.

In fact the PET scan measures a function. In the example above radio-active sugar was used, that indicates where growing tissue uses this sugar. Other tracers and markers are available to measure other functions, like for instance where tumours are stimulating the body to grow new blood vessels, and many other functions.

For more information, see Wikipedia.

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A photon is an elementary particle that forms the basis of the radio-active beams used in external beam radiotherapy (other types of external beam radiotherapy use electrons or other particles). It is also the physical entity of light and other electromagnetic waves, which come in a large variety of wavelenghts, determining the physical behaviour of the photons. A photon has a specific energy.

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See TPS (Treatment Planning System).

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Portal image

A portal image is a radiographic image obtained with a portal imaging device just before, or during the execution of a radiotherapy treatment. A portal imaging device uses photons  to make  a two-dimensional image of the patient in treatment position  on the table. The image is used to verify the correct positioning of the patient, and of organs within the patient, relative to the predefined positioning. Several two-dimensional images can be combined to obtain 3-dimensional information.

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This refers to the position of the patient on the treatment table: supine (on the back) or prone (on the belly).

A prone position during a breast irradiation can in some patients help to reduce the radiation dose to heart and lungs.

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Proton Therapy

Proton therapy (also called proton beam therapy) is a type of radiation treatment that uses protons rather than x-rays to treat cancer. A proton is a positively charged particle that is part of an atom, the basic unit of all chemical elements, such as hydrogen or oxygen. At high energy, protons can destroy cancer cells. See also hadron therapy.

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