A linear accelerator (LINAC) is a particle accelerator used to accelerate electrically charged particles. During this process, a predetermined dose of energy is released. Linear accelerators with electron beams that generate X-ray radiation are used for the radiotherapy of cancer tumours.
Our linear accelerators feature a built-in positioning control system and enable radiotherapy to be carried out with millimetre precision. Healthy tissue is protected as much as possible and harmful side effects can be reduced to a minimum. The linear accelerator is also equipped with additional functions: The breathing-dependant control system takes into consideration the movement of the tumour during respiratory movements and the intensity-modulated radiotherapy technology allows the individual fields of radiation to be adapted in accordance with the volume of the tumour.
The linear accelerator (LINAC) has been in use for over 50 years. The basic principle has not changed during this time. Electrons are accelerated in electromagnetic fields, in which a voltage of several megavolts is created and then either used directly for therapy (particularly for surface tumours) or converted into photons (pure energy quanta). These high-energy photons penetrate the body and disperse energy depending on the irradiated tissue. A modern linear accelerator can thereby be used to irradiate any part of the body.
Progress through innovation
Since the late 1970s, radiotherapy has advanced mainly as a result of developments in computer technology. The basic technology of the linear accelerator has remained the same, but ability to plan treatment and the precision of the radiation have improved. Thanks to the use of computed tomography, it is now possible to create images of tumours within the body. The region requiring treatment can be plotted on the CT layers, making it possible to record the tumour as a three-dimensional area.
The tumour is displayed by the planning computer as a three-dimensional object, which then serves as the basis for the radiation plan created by the medical physicist. Generally, the tumour mass will be “attacked” from various directions within defined safety margins so that risk organs are omitted from the field of treatment. These fields are no longer rectangular, but are instead adapted using shields in accordance with the defined radiation volume. The aim of every radio-oncological treatment is to subject the tumour region to the highest possible radiation dose, while simultaneously protecting the risk organs to the greatest possible extent. The probability of being able to sterilise a tumour depends on its size and histology, and above all, on the dose of radiation applied. However the likelihood of side effects depends on the exposure of normal, healthy tissue to radiation. In recent years, it has become possible to control and modulate the intensity of the flow of photons within the field (intensity modulated radiotherapy, IMRT). As a result, the high-dose area can be better adapted to the defined tumour field to avoid irradiating risk organs.
More precise treatment, fewer side effects
All these new technologies require the radiotherapy to always be carried out under exactly the same conditions. The geometry of the linear accelerator is fixed and the dosage calculation is based on the assumption that the conditions are always exactly the same as those depicted in the planning computed tomography. If this deviates from the actual situation, the tumour region may receive too low a dose, or a risk organ may be exposed to too much irradiation. If the position of the patient deviates by just 1 to 2 cm, then the radiation will also be “off-target” by this distance. This inaccuracy must be assessed and included in the treatment concept. During the planning phase, a safety margin around the tumour is calculated and this area is also treated in order to ensure the most comprehensive radiation possible during all fractions. The disadvantage of this approach is a significant increase of the radiation volume, along with the corresponding side effects. The more precisely radiation therapy can be carried out, the smaller the safety margin required – making the overall treatment volume smaller.
In addition to the positioning aids, it is now possible to check the accuracy of the patient’s position and improve it with the help of a positioning control system. Two quick x-rays are taken in different directions before the radiotherapy begins. These images show the current position of the patient and are then compared with a digital reconstruction of the planning CT. The x-rays and the planning CT images are merged to directly determine any deviations between the current position and the original. The radiation table is then adjusted to compensate for these deviations. This procedure guarantees the patient is always treated in the originally planned position. It is performed as a matter of course before every radiotherapy session at Hirslanden. Our many years of experience have shown that this procedure is easy to carry out and only extends the duration of the daily fractions by approximately 1 minute. The precision of daily radiotherapy has subsequently been greatly improved so that it can now be performed with millimetre accuracy. For the Institute for Radiotherapy and the entire treatment team, radiotherapy without this system is now almost inconceivable.
Compensating respiratory movements
A completely new development is that radiotherapy can now be synchronised with the patient’s respiratory movements (respiratory gating). This development is particularly important for the treatment of tumours which change location during inhalation and exhalation. We use this technology for women suffering from left-sided breast cancer, for example. It is a known fact that the risk of a heart attack is increased for breast cancer patients for 10 to 20 years after radiotherapy. This could be caused by the effects of the radiation on the anterior surface of the heart. If radiation is applied only during the maximum inhalation phase, the breast moves away from the heart due to the increased lung volume and the heart is better protected from the effects of the radiotherapy.
It has now been shown that breathing-synchronised radiotherapy can significantly reduce the radiation exposure on the anterior surface of the heart (i.e. from an average of over 40 Gy to below 10 Gy).
These days, most patients are still treated using the linear accelerator.
The advantages of the linear accelerator
The main advantage of this technology is that larger volumes can be included and treated. A malignant tumour is known as cancer. Tumour cells from the main mass (lump which can be felt or seen in the CT/MRI scan) grow outwards into the surrounding tissue. They are also capable penetrating vessels and, for example, developing further within the lymph vessels. In this way, lymph nodes in the area around the tumour are also affected.
As a cure is only possible when all the tumour cells in the body are destroyed, it is often the case that much more than just the visible lump has to be treated. In such instances, the linear accelerator is superior to all other radiotherapy equipment.
The key advantages are:
- Targeted radiation, also of larger volumes
- Tried-and-tested over many years
- Can be combined with other forms of therapy such as chemotherapy (the benefits of this for certain tumours is now undisputed)
Our institutes are equipped with the very latest generation of linear accelerators. In addition to conventional radiotherapy, this high-tech device can also be used to perform stereotactic radiotherapy with exceptional accuracy (deviation of less than one millimetre), so that tumours close to critical areas can be very precisely treated. Thanks to this incredible precision, higher single doses of radiation can be administered, which also reduces the total number of treatments required. The provision of high doses has also reduced the treatment time from one hour to between 10 and 20 minutes.
The equipment registers movement caused by the patient breathing and with respiratory gating, allows the radiation to be applied only during defined phases of breathing so that damage to healthy tissue is reduced.