Radiotherapy techniques

The radiation field shape can be adjusted to the target volume by aperture systems on the rotatable gantry. In the simplest case, rectangular fields are employed. In the radiotherapy of bone metastases of the spine, for example, the patient is irradiated from the back, often using only a single field (dorsal stationary field).

Two radiation fields irradiated from opposite directions are referred to as opposing fields. A typical example is the radiation therapy of the cranium with lateral opposing fields for brain metastases where one field is irradiated from the left and the other one from the right.

Multiple field technique

Multiple field techniques with three or more fields are employed often. With this technique, the movable gantry is moved to a new position after every field. In this way the radiation exposure in the incident area can be reduced while the dose in the target volume is increased due to the addition of the dose contribution from all fields.

Intensity-modulated radiotherapy (IMRT)

Intensity-modulated radiotherapy (IMRT) is a so-called highly conformal radiation technique. This means that a higher level of agreement between the target volume and the actual dose distribution in the irradiated area is achieved by means of this modern technique.

This technique uses special aperture systems consisting of movable leaves (multileaf collimators). Unlike the conventional technique, the radiation field is not irradiated with a uniform dose. The individual leaf positions are moving during the course of the radiotherapy session, so the shape of the main radiation field can be modified constantly.

In this manner, a large number of subdivisions of the main field, so called subfields, can be irradiated in a very short time (some of them only with a low dose). This allows a very accurate adjustment of the dose distribution to the area to be treated. This elaborate technique is particularly employed for complex-shaped target volumes adjacent to sensitive organs at risk.

Positioning aids

A multitude of positioning aids are used in modern radiotherapy. Rigid masks, for example, are employed for the irradiation of the head and neck region in order to immobilise the head of the patient on the treatment table for the duration of the radiotherapy session. This guarantees that the patient is always exposed in the same position during treatment and that the target volume really lies within the radiation field.

Verifying the patient's position

The patient's position is verified at regular intervals to ensure that the radiation is delivered correctly. In this process, the photons generated in the linear accelerator can also be used for creating an X-ray image. By means of the X-ray images of the regions to be irradiated (so-called field checks), the patient's position can be verified and adjusted, if necessary.