Multileaf collimator

A multileaf collimator (multi- leaf collimator, MLC) is a device that is placed in the radiation therapy of a linear accelerator, to adjust the treatment beam to the shape of the object to be treated.

A collimator consists of a plurality of pairs of laminations of a material having a high atomic number, usually of tungsten. These blades can move independently and are thus able to assume any shape to the Enclosing one is located in the treatment beam volume nachzuformen ( conformal treatment).

Types of treatment

The use of a Multilamellenkollimators enables a variety of types of treatment that has a gentler and more targeted use of radiation for the patient.

Floor fields

For cocktail fields (conformal field) are accelerator and patient support brought to a particular position and during treatment will not move. The leaf positions of the collimator can be determined exactly for this alignment by a treatment planning system.

Arc irradiation

In this type of treatment is to distinguish between conformal ( Conformal Arc ) and dynamic arc radiation. Here is the patient bed in a certain position while the accelerator is moved to the center position around. In the case of conformal arc irradiation a static slat position is calculated similarly to that of a stand- field by the treatment planning system, only that in the arc irradiation, the Enclosing will be drawn from all irradiated angular positions into consideration. This will further increase in the normal case, the irradiated volume.

The dynamic arc radiation (Dynamic Conformal Arc ) compensates for this by the slats are dynamically adapted to the current angular position during rotation of the accelerator. Thus, the field size corresponds to any angular position about the equivalent of a standing field.

Intensity modulated treatment

While in the case of conformal treatment is trying to keep the radiation to the surrounding tissues as low as possible, in the case of intensity modulated treatment ( Intensity Modulated Radio Therapy - IMRT) is used the multileaf collimator to distribute evenly the radiation in the object and thus inhomogeneities in the beam passage compensate through the body.

It is this distinction between sliding window and step- and-shoot IMRT. During a sliding window IMRT treatment, the slats move, similar to a dynamic arc radiation, continuously and be driven at different speeds over the treatment area, thus to achieve differences in intensity of the radiation. In step-and -shoot IMRT, the slats are alternately moved into a certain position and following it delivered radiation until all necessary items are run through.

History

At the start of radiotherapy simple conical or round collimators was used. It is usually to lead or tungsten cylinder that is mounted in front of the Strahlenaustrittsöfnung the accelerator and in the center have a hole with a defined diameter. Depending on the shape of the volume to be irradiated, these collimators will be replaced by the diameter adapt to the real situation. For example, can be brain metastases hereby usually irradiate very good, since these are usually relatively spherical and, depending on the size, can also be treated relatively well with a specific conical collimator.

For more shapeless and even with larger objects, the conical collimator concept has its limits. Here, the object to be treated must be covered with a plurality of individual fields. Due to the circular aperture of the collimator, however thus comes in some regions overlaps between two different fields, while in other areas less radiation arrives, because they lie between two fields. One speaks here of so-called hot spots and cold spots. Surely, however, this is also a question of treatment planning and technique and the skill of the plan physician or physicist.

The principle of the conical collimator was further developed in Iriskollimator, so that is possible to realize various diameters with a collimator and then be eliminated replacing the collimators between two fields.

A further development in the direction of conformal treatment collimators have no circular opening more. For this purpose the contour of the volume to be treated is calculated by a treatment planning system for a particular accelerator and the patient bed position. This data is then transferred to a plastic block, and thus it results in a form of the contours. This image is then used to pour a collimator with lead, so that a conformal collimator. However, this is dependent on the field to be irradiated, which makes the whole concept very labor intensive.

For the IMRT treatment, there are similar methods. For this the be compensated inhomogeneity is transmitted by areas with different depths are worked out using a milling machine onto a metal block. This metal block is then subsequently introduced into the treatment beam, thus ensuring a uniform irradiation of the target volume. Again, the resulting block is specifically made ​​for a field again, and thus the total cost is relatively high.