Gamma Knife
What is it?
How does it work?
The gamma knife is not exactly a knife but more a very precise beam of gamma rays that can be used to kill cells in a specific area, such as cancerous cells making up a tumour.[G3] This medical treatment is used for brain tumours, where accuracy in the surgery is incredibly important. On the right is the normal set up of a gamma knife, which looks similar to an MRI scan.[G4]
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The gamma rays come from the helmet, which have cobalt sources in them. The helmet is designed so the patient cannot move as this treatment is very precise (to within 0.5mm)[G5] so it is important to keep the target very still.
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The shielding is to stop the gamma radiation leaving the machine, as frequent exposure to the people working the machine could be dangerous.
The gamma knife is made up of around 200 radioactive sources (Cobalt-60), which are all focused to a central point. Click here to learn how cobalt produces gamma rays. This is done by the use of collimators, which make all of the beams travel in one direction, which narrows the radiation to one specific area.
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This essentially means that the tumour can be targeted while leaving the healthy adjacent tissue relatively unharmed. The setup is essentially a large dome of radioactive sources, where they are focused to a specific point. Click here to find out more about targetting.
A frame is fixed to the patient to prevent movement and so the positioning can be as accurate as possible. The precise beam will then kill the cancer cells, but not instantly, the effect takes place over a few months.[G6] The cells in the beam of the gamma knife are normally able to repair, apart from those where the beams intersect and the radiation dose is higher. [G7]
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Image 1: Set up of a gamma knife. Source (Indicure)
Image 2: Close up of gamma knife sources. Source (UPMC)
Safety
The intensity of the radiation is high in order to kill the cancerous cells, however because it is so focused it will do minimal damage to other surrounding cells. Normally this procedure is used on people with serious illnesses where the risk of the radiation is far outweighed by the effectiveness of the procedure.
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There are normally minimal side effects after having this treatment although some patients may experience claustrophobia.
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The dosage received by the target from this treatment is 15Gy, which is 120,000,000 bananas. This is very high, however this does not affect the whole body and the dose received drops off very sharply from the target area. The radiation dose drops off quickly to 1% (120,000 bananas as shown on the scale) in the brain. [G8]
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It is difficult to calculate the overall radiation dose to a patient as it will vary on size of tumour and the size of the patient. Click here to find out more about how the dosage is calculated.
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