Besides chemotherapy and surgery, radiation therapy represents an important therapeutic option in cancer therapy. About half to two thirds of all cancer patients undergo radiation therapy either in combination with the two other methods of treatment or as the sole method of treatment. All radiation therapy against cancer is based on the effect of so-called ionizing radiation. Ionizing radiation is capable of removing one or more electrons from atoms or molecules, thereby producing positively charged ions or molecules, a process called ionization. However, ionization requires a certain amount of energy that can only be produced by charged particles and highly energetic electromagnetic radiation (gamma rays, X-rays and some of UV radiation). The effect of the various types of radiation that are exploited to destroy cancer cells is the same in all cases: for one thing, ionization directly damages the genetic material, i.e. the DNA (single and double strand breaks), and further, it causes free radicals to be formed in the cell fluid, which also leads to damage of the cell DNA and to subsequent cell death.
The aim and quality criterion for Radiotherapy means to deliver a maximum, homogeneous dose of radiation to the target volume, i.e. the tumour, and to keep the dose to the healthy tissue as low as possible.
There are basically two types of radiation therapy, Brachytherapy and Teletherapy.
With brachytherapy (gr.: brachys = close or short) a radiation source is placed inside or immediately next to the tumour inside the patient's body. However, brachytherapy only covers a single figure percentage of all radiation therapy treatments.
Different to this, with teletherapy the radiation is directed to the target volume from outside through the skin and healthy tissue. Teletherapy therefore is also often called percutaneous therapy (lat.: percutaneous = through the skin). The beam is typically generated by means of a Linear Accelerator. Teletherapy using Linear accelerators comprises therapy with X-rays (or Photons) or fast Electrons. Proton Therapy is also a teletherapy mode.
Teletherapy is a fractionated therapy; this means the planned total target dose is delivered in up to 35 single fractions, usually one per day. The fractionation scheme is based on medical and radiobiological findings and expertise.
All ionizing radiation is non-tumour-specific, as it also attacks healthy tissue. This non-specific effectiveness is also an advantage of radiation therapy because in this way there is no effect of radiation resistance by mutation. The DNA is destroyed in all cells exposed to the radiation. Therefore, such treatment can only be locally targeted and is the more effective, the more precisely it is targeted to the tumour. Its tolerability is according to the same principle: The more precisely the tumours are targeted and the healthy cells are protected, the better for the patient (see above).
In the past few years, conventional radiation therapy using X-rays and electron radiation has significantly improved in its technology. This technological progress has resulted in improvements in treatment that are generally accepted today.
So-called Intensity Modulated Radiation therapy [IMRT] was developed since about 1990 and is used today quite routinely for various indications. IMRT opened up the possibility of optimum matching between the irradiated maximum dose volume and the contours of the target volume. Accordingly, higher doses could be applied to the tumour whilst sparing the surrounding healthy tissue, thus leading to an increase in the tumour control rate. Since 2000, the so-called IGRT (Image Guided Radiation Therapy) has been developed and has now been widely introduced into clinical practice.
Image guidance during irradiation allows to position and monitor the target volume of the patient relative to the beam precisely within the millimetre range. It is performed by visualization of the tumour by means of imaging systems attached to the Linear Accelerator. The radiation field size and -shape can be adjusted precisely to the tumour volume, which is often shrinking already under irradiation. At present there is intensive clinical research aimed at following the target volume during the fraction of irradiation and to apply the radiation according to the changes of the tumour.
Encouraging clinical results can be expected from this progress. For example, the 5-year survival rate for (non-small cell) lung cancer treated with radiation therapy has more than doubled with the use of modern technology. We can assume that in the future there will be further indications for radiation therapy where the cancer control rate will increase and/or the rate of complications will be reduced with the use of modern technology.
In spite of these encouraging results, there are basic limitations to the concentration and dose increase to the target volume with the use of X-rays. These limitations are based on the physical principle that X-rays penetrate the whole body.
There is proof, that heavy charged particles such as protons will be superior in this respect.