Irradiation of the acoustic neuroma using fractionated radiation therapy
Generation of rays
The development of the linear accelerator has made it possible to customise the art of radiation and its energy to the requirements of modern medicine. This way the penetration can be varied and a good field homogeneity achieved. In a 50 cm long, straight and 150 kg heavy tube, electrons are accelerated along a straight path (linear). The acceleration works through an electric, high frequency field.
At the end of the acceleration tube the electrons escape via a thin window, are deflected through a magnetic field around 90 and 270 degrees and they then meet a so-called bremstrahl target. On decelerating the electrons accrue the high energy photons and x-rays, which are then shaped into the most homogeneous beam of rays possible. The radiation intensity is varied and controlled via blades in the radiation head (like slats).
The desired radiation is generated during the treatment time. The device must be checked daily in terms of calibration.
Planning radiotherapy
Every patient receives individual therapy. The starting point for every individual therapy is the exact establishment of the target point for the radiation and the necessary dose of radiation – the so-called three dimensional model of the patient and the body region to be radiated, which with acoustic neuroma is the whole skull. Basic information is provided by computer tomographies (CTs) and magnetic resonance tomographies (MRTs) and other possible imaging procedures.
According to the size, form and position of the tumour, the radiation dosage is calculated, to achieve the most effective dosage for the tumour tissue, but a minimum dose for the surrounding healthy tissue. The radiation is then simulated on the patient model. The structure of the therapy simulator conforms to the therapeutic linear accelerator, where the radiation head is substituted by an x-ray apparatus.
The dose distribution is fine tuned via multiple optimisation cycles. At the end there is an optimised, three-dimensional radiation plan.
The radiation
Although the length of radiation is short, nevertheless it seeks absolute immobilisation of the body and the body part to be radiated. For this individually manufactured positioning aids serve, such as special mattresses and plastic masks for acoustic neuroma radiation.
At first there may be an uncomfortable feeling when placing on the facial mask, you get used to it quickly. The radiation itself is not painful.
Usually, you receive radiation 5 times a week, at the weekend and holidays there is no treatment. The daily radiation with a radiation dose of 1.8 to 2.5 Gy takes under a minute in general; in total the patient spends an average of 10 – 20 minutes in the radiation room (apply and position mask, radiation, remove mask). Radiation takes place on at least 5 and up to 35 treatment days. In this way a maximum total dose of 65 Gy is reached, which is above the tolerance of normal tissue (approx. 10 Gy), however it is possible because of fractionation and the given recovery time for the healthy tissue.
Due to the low individual doses, fractionated radiotherapy is the least taxing therapy for patients. Between the radiations the patient is completely resistant. However, this advantage is paid for by week-long treatment times. A few weeks or months after the radiation the first post-examination is carried out. Then yearly monitoring examinations are scheduled and performed.
The total planning and implementation of the therapy is carried out by a team of doctors, medical physicists, nurses and medical-technical assistants.