Radiation therapy of malignant tumours
There are basically two different types of radiation therapy: Irradiation from outside (teletherapy) with gamma radiation generated mostly by electron accelerators, and internal radiation therapy (brachytherapy) via radioactive sources which are administered at the surface or inside the body (see Lu-177).
The FRM II has a facility for teletherapy of malignant tumours using highly effective fast neutrons. Neutrons do not penetrate deep into the human body due to its high water content. Therefore, only patients suffering from superficial tumours, e.g., in the head and neck area, breast cancer and melanoma metastases are deemed suitable for irradiation using neutrons. The small penetration depth is offset by the very high efficiency of neutron beams: It is possible to locally suppress the growth of tumours that are resistant to other forms of radiation and which can no more be treated using other therapies (mostly palliative cases).
Aim of radiation therapy is to cause an irreparable break in the genetic material of the cell – the double strand of the DNA helix – and to damage it to such an extent that the cell triggers cell death. In the case of fast neutrons, due to the high ionization density, one hit, resulting in an irreversible double-strand break is often sufficient. The cells’ own repair mechanism is absolutely vital for all living organisms, since they are exposed to natural ionizing radiation from space or from the earth and to many other deteriorating influences. A healthy cell is therefore permanently engaged in DNA repair; most tumour cells are capable of this to a lesser extent, and it is this fact that healthy cell have an advantage and that side effects can be mastered. Radiation damage is caused because radiation breaks down atoms into ions and electrons, i.e. they ionize. This ionization affects all components of the cell, but DNA is the largest and most important molecule that controls all vital processes in the cell at any given moment.
Due to their energy spectrum, fission neutrons (energy 0.1 to 10 MeV) exhibit the highest relative biological effectiveness of all neutron beams used to treat cancer. In close collaboration with the Department of Radiation Oncology at the TUM – as was the case in the old FRM – a facility has been set up that provides advantages for clinical applications:
- The FRM II has a special and worldwide unique system which allows for the provision of fast fission neutrons at a beam tube. The system, a so-called neutron converter, consists of two uranium plates located approximately 1 m from the fuel element in the heavy water moderator. The thermal neutrons in the moderator induce fission processes which release fast reactor neutrons (fission neutrons) and gamma radiation.
- With about 3.2 • 108 nfission cm-2 s-1, the neutron beam is very intense and, through the introduction of filters, extremely flexible for use in reducing, e.g., the gamma component of the radiation or suppressing low-energy neutrons.
- The maximum irradiation field, with 20 • 30 cm, is very large, which also contributes to a shorter irradiation time. At the same time, the diseased tissue can be irradiated more evenly over the whole area.
- A variable aperture system, a so-called multileaf collimator, makes it possible to match the irradiation field to the shape of the tumour, thereby protecting healthy tissue as well as possible.