Radiation therapy, Radiation from the outside (external or percutaneous radiation treatment)
This type of radiation treatment is by far the most common in the treatment of tumors of the central nervous system. The irradiation is generated by special therapy devices. The generated radiation is formed in the therapeutic field and penetrates the inside of the body from the outside.
Irradiation “from within”
This treatment method is rarely used. Stereotactic, radioactive, small “seeds” are placed in special, designated centers. Today’s stereotactic techniques allow a highly precise placement of the “seeds” and / or the execution of this treatment. The essential property of the radioactive substance (usually Iodine 125), the steep dose drop towards the periphery, is thereby optimally utilized. The steep dose drop means that the highest dose is given in the immediate vicinity of the “seeds”, but only a few millimeters away, but almost no dose is delivered. As a result, the surrounding tissue is maximally protected. For this treatment, however, only tumors of limited size and specific localization and only particular types of tumors (low malignant gliomas) are considered.
Which radiators are used externally in radiotherapy?
The first devices used in the application of X-ray radiation were simple X-ray therapy devices. The disadvantage of these devices was that the radiation concentrated on the surface of the body and in the depth hardly a sensible dose succeeded. Only the introduction of cobalt devices at the beginning of the 1950s opened the way to depth radiation.
Today, cobalt devices are only used very rarely. The modern technical developments have meanwhile produced more powerful, technically mature and reliable therapy devices: the linear accelerators. Ultra-hard X-rays are produced in these linear accelerators, which are capable of penetrating deep body regions and preserving the surface. Using modern computer-aided irradiation planning systems, these therapies can be individually adapted, bundled and targeted. These properties are mainly used in the irradiation of tumors of the central nervous system.
Safety of the irradiation equipment
The modern linear accelerators are technically complicated and require regular maintenance. Special quality assurance measures, which are carried out in fixed daily, weekly and annual intervals, ensure safe radiation treatment at a consistently high quality level. These extensive quality assurance measures, which are mainly carried out by physicists, in individual cases by engineers, ensure safe therapy for each patient.
Safety of irradiation
In addition, as in the aerospace industry, additional special safety mechanisms have been introduced to ensure a correct therapy sequence. This includes the computer-assisted recording of the patient data and the irradiation data such as field size, irradiation direction, single dose as well as end dose. Additional control measurements are carried out before the linear accelerator is put into operation. Thus, it is virtually ruled out in the case of modern devices that a faulty radiation treatment is carried out.
The therapy plan is checked at the beginning of the irradiation with verifications. The planned therapies are compared with an x-ray image obtained with the therapeutic irradiation field. A fine-tuning of the therapeutic fields can then be carried out. Today increasingly electronic systems are used for field verification, “portal imaging”. In addition, every step of the radiation treatment is documented in detail. With this documentation, radiation treatments can be followed down to the last detail even after decades.
What is actually a linear accelerator (for especially interested)?
Linear accelerators are technically extremely complex devices. In principle, electrons are produced in them. These particles are negatively charged and are accelerated in one direction (= linear accelerator) in a special tube with high vacuum up to the speed of light. At the end of this acceleration distance, they are deflected with magnets in a very specific direction. It is now possible to use these electrons directly, or else to use them to produce the otherwise usual electromagnetic photon irradiation. Here, the electrons are directed to a target made of a water-cooled metal. The actual photons are generated in this target. The larger the energy of the generated electrons, the higher is the energy of the photon radiation caused thereby. In principle, electrons predominantly act on the surface. They play a subordinate role in the tumors of the central nervous system. The photons capable of penetrating into deep body regions are relevant to brain tumors. The higher the energy of the photons, the greater their penetration depth.