Teletherapy

In teletherapy (Greek tele: far), the radiation source is located at a spatial distance from the tumour. In the process, mostly tumours inside the body are irradiated through the skin from outside the patient. This case is referred to as percutaneous radiation therapy (percutaneous - through the skin). Various radiation qualities can be used.

Photons (ultra-hard X-rays)

The treatment beam is generated by a linear accelerator. Like in an X-ray machine, negatively charged particles – electrons - are generated first and then strongly accelerated. By a subsequent sudden deceleration of the electrons, photons in the form of so-called X-ray bremsstrahlung (German: "braking radiation") are produced. These high-energy photons are used for the irradiation of tumours inside the body.

When the photon beam enters the body, initially the dose increases and then it slowly decreases as the beam weakens. Irradiation with photons is the most widely used method in modern radiation therapy.

Electrons

Electrons generated in the linear accelerator can also be directly used for irradiation. Because their dose falls off sharply in the depth of the tissue, electrons are used for superficial irradiation.

Protons and heavy ions

With a few exceptions (proton beam therapy for chordomas (a rare type of bone tumour) of the skull base and choroidal melanoma of the eye), irradiation with protons and heavy ions is currently not a standard therapy yet and is still being researched as part of clinical studies.

Physical properties

Unlike photons and electrons, protons and heavy ions do not release their energy until deep in the tissue within the so-called Bragg peak. Along their path, however, only little radiation energy is deposited in the tissue and right after the Bragg peak the dose immediately drops off sharply again.

By selecting the appropriate particle energy, the dose maximum can be precisely targeted to the tumour. These special properties of protons and heavy ions allow irradiating the tumour with a high dose while protecting the surrounding tissue as well as possible.

Protons barely differ from photons with regard to their effect on the irradiated tissue. Heavy ions, however, have a stronger effect on the irradiated tissue than photons when administered at the same physical dose.

Possibilities and problems

Especially for the treatment of radiation-resistant tumours that cannot yet be treated adequately with conventional photon irradiation because the limited tolerance of the surrounding organs at risk prevents increasing the irradiation dose, the irradiation with protons and heavy ions promises benefits.

However, the tightly localised energy release in the Bragg peak also involves problems: Already slight deviations in the couch position of the patient can result in considerable deviations between the desired and actual dose distribution in the diseased tissue. Much the same applies to the irradiation of tissues with variable and inhomogeneous density as the position of the Bragg peak also depends on the density of the tissue in question.

Gamma radiation (cobalt-60)

Telecobalt machines employing the gamma emitter cobalt-60 as radiation source were commonly used for radiation therapy in the past. Strictly speaking, gamma quanta are photons. In contrast to X-radiation, however, they are formed during the radioactive decay of atomic nuclei. In today's modern radiation therapy, this type of irradiation is of hardly any importance.