The BTEC students at Rooks Heath also need to know about the uses of X-rays and gamma rays in the treatment of cancer.
Radiation is not just used for diagnosis, but for treating cancer as well. This is called radiotherapy. High energy radiation (x and gamma rays) are used.
Radiotherapy uses the fact that ionising radiation damages cells, and high enough doses can kill them. The cells in cancerous tissue divide very rapidly. This makes them more susceptible to damage by radiation than healthy cells, so there is a higher chance that they will be killed. Even so, care has to be taken to ensure that only the malignant cancer cells, and not the surrounding healthy tissue, receive a high dose.
In order to minimise damage to healthy tissue the gamma rays need to be targeted precisely on the cancer. Areas that particularly need to be avoided are the eyes and spinal cord.
This is done by mounting the system on a ring so it can rotate around the patient, with the tumour at the centre of the rotation. In this way the tumour gets a higher dose of radiation than the surrounding healthy tissue.
Originally, radiotherapy machines consisted of a cobalt-60 source which emitted gamma rays which irradiated the tumour. Modern hospitals use linear accelerators (linacs for short) instead to produce very high energy x-ray beams, with a higher energy than the Cobalt-60 gamma rays. In the UK, medical physicists are required by law to calibrate the linacs to ensure that the best possible treatment is given.
Each treatment requires careful planning. This involves deciding which directions to irradiate the tumour from, what dose to give and, in new machines, what shape region to expose.
External beam radiotherapy or teletherapy is the most common form of radiotherapy. The patient sits or lies on a couch and an external source of radiation is pointed at a particular part of the body. Kilovoltage (“superficial”) X-rays are used for treating skin cancer and superficial structures. Megavoltage (“deep”) X-rays are used to treat deep-seated tumours (e.g. bladder, bowel, prostate, lung, or brain).
Commercially available medical linear accelerators produce X-rays and electrons with an energy range from 4 MeV up to around 25 MeV (megavoltage). The X-rays themselves are produced by the rapid deceleration of electrons in a target material, typically a tungsten alloy, which produces an X-ray spectrum via bremsstrahlung radiation. The shape and intensity of the beam produced by a linac may be modified or collimated by a variety of means. The main problem with X-ray radiotherapy is that it covers a large area of the body and may adversely affect healthy tissue.
A linac generates high energy x-rays. It rotates around the body, with the tumour at the centre of rotation, irradiating the tumour from different directions. This means the tumour receives a large dose, but the dose to healthy tissue is minimal.
The size, shape and location of the tumour are worked out using CT or MRI scans. Isodose curves, which join points that will receive the same dose of radiation, are overlaid on this X-ray CT scan of the chest.
The image below left is a picture of a linear accelerator (linac).
The treatment plan lists the directions the X-rays will come from and calculates the radiation does to the tumour and the rest of the body.
Computers help the physicists to calculate the dose. The picture above shows the beam coming from 5 linac positions to treat a tumour in the chest.
In Brachytherapy (meaning short-distance therapy), radioactive material is inserted into the body, inside or near to the tumour. This means the tumour receives a high dose while the surrounding tissues have a smaller exposure. The pellets are commonly caesium-137 or iridium-192 and are used and left in place for up to a few days. Alternatively sources emitting a particularly high dose of radiation are inserted for just a few minutes. Small sealed radioactive sources can also be permanently implanted.
Here, tiny pellets of radioactive iodine-125 have been implanted into the prostate gland.
These pellets will not be removed, but have a fairly short radioactive half-life so that after a while they will become inactive.
The Gamma Knife
A more modern approach to radiotherapy using cobalt-60 is with a Gamma Knife.
The gamma knife is not really a knife, but a way of performing brain surgery without cutting through the skin, muscle or skull (non-invasive). It uses 201 radioactive cobalt-60 sources to irradiate the brain. Cobalt-60 emits gamma rays and has a half-life of 5.26 years.
The treatment is planned using CT or MRI images, so that the sources are correctly targeted, to irradiate the tumour and avoid healthy tissue, especially sensitive regions around the eye and cochlea.
The first stage in treatment is to fit a metal frame to the skull, which is done using four screws under local anaesthetic. The rigid frame allows the radiotherapy to be performed very precisely.
The Cobalt-60 sources are positioned in a hemisphere. The patient’s head, held in the frame, is held inside a helmet with 201 holes to precisely target the radiation. When treatment starts, the patient’s head is moved inside the unit.
The gamma knife is used to treat benign and malignant tumours, blood vessel malformations, some pain conditions and some movement and psychiatric disorders. In 2006, there were three in the UK (two in London and one in Sheffield).
A medical physicist decides which angles to point the radiation from to destroy a tumour and minimise damage to other tissue. He/she analyses the images and works out how to target the linac so that the tumour receives the maximum dose but other tissues receive a small dose. Particularly sensitive organs such as the spine and gut should receive as small a dose as possible.
A medical physicist decides how to target the gamma rays to destroy the tumour and minimise damage to other tissue.