| Ch 8 | Page 8 / 25 | |
| Cancer radiotherapy |
Common radiotherapy devices | |
They emit electrons or photons and have the same general characteristics as an X-Ray tube.
Classified according to increasing emitted energy, we can distinguish between contact-therapy devices, Cobalt accelerators and linear accelerators. The in-depth absorption is proportional to the energy of the emitted beam.
The general diagram of radiotherapy tables takes into account the various treatment needs.
They are X-ray tubes.
They deliver low energetic photon beams which are used for treating skin tumours in view of their short penetration into tissue. It is relatively easy to protect the adjacent healthy skin by placing light lead sheets in order to obtain a pleasant aesthetic result.
Such contact therapy is also used for superficial tumours of the rectum among elderly patients who are too weak to undergo rectal amputation.
Its technology is relatively simple and robust. The 60Cobalt source is a small disc which is accommodated in a movable drawer inside the lead head of the device. The source is concealed when the device is inactive. The head is situated on a 360° rotating arm (see picture.)
The beam geometry (which is only emitted when the source is in treatment position) can be modified for each patient. The use of masks, wedges and compensators gives great liberty in adapting the beam to the patient’s morphology.
The device is isolated in a lead walled room; during the session, the patient communicates with the technician via an interphone and a camera allows good visual contact with the patient whose immobility during treatment must be assessed.
The 60Cobalt emits a γ radiation beam of 1,25 MV; the maximum absorbed dose is situated around 0.5 cm under the skin. It is therefore mainly used for head and neck cancer (irradiation of relatively superficial tumours and nodes).
In fact, its use is gradually becoming rarer due to the progress of linear accelerators and the new 'multi-leaf' collimation technology.
They comprise an electron source and an electromagnet which accelerates the electrons in a deep vacuum tube (accelerator).
he electron energy is proportional to the length of the accelerator tube (cf. diagram ). The interposition of a cathode target generates photons.
The external aspect of a linear accelerator is very similar, independently of its manufacturer (cf. pictures). Modern accelerators can modify the size and shape of the beam via a multi-leaf system and can control the transmitted beam using portal imaging devices.
In clinical practice, we use photons from 4 to 25 MeV, which penetrate much deeper than 60Co photons and 8 to 30 MeV electrons.
Their absorption curves are interesting and demonstrate the possibility of obtaining deep irradiation.
With an 18 MeV accelerator, it is possible to irradiate up to 15cm deep tumours with little irradiation of healthy tissue situated on the beam path For a zone situated at a depth of 15cm, approximately 70% of the dose is delivered to the tumour. When using four field treatment, less than 50% of the dose is delivered to the neighbouring healthy tissue.
The more energetic the accelerators are, the deeper they can treat tumours (cf. curves on a further page). However the normal tissue on the beam path always receives a small but noteworthy dose for which new optimising techniques have recently been developed.
Conversely, the path of electrons is finite, depending only on their initial energy: they are therefore of great interest in the irradiation of tumours situated close to critical deeper organs such as the spinal cord.
