Multi-leaf collimator
A multileaf collimator ( multi-leaf collimator , MLC ) is a device that in the radiation at a linear accelerator is attached to the treatment beam to adjust the shape of the object to be treated.
A collimator consists of several pairs of leaves made of a material with a high atomic number , usually tungsten . These lamellae can move independently of one another and are therefore able to assume any shape in order to reshape the surrounding area of a volume located in the treatment beam (conformal treatment).
Treatment types
The use of a multi-leaf collimator enables a variety of types of treatment that result in a gentler and more targeted application of the radiation for the patient.
Standing fields
With standing fields ( conformal field ), the accelerator and patient bed are brought to a specific position and not moved during treatment. The lamellar positions of the collimator are determined by a treatment planning system precisely for this one orientation.
Arc irradiation
With this type of treatment, a distinction must be made between conformal ( conformal arc ) and dynamic arc irradiation. The patient couch is in a certain position while the accelerator moves around the center position. In the case of conformal arc irradiation, the irradiation planning system calculates a static slat position similar to that of a standing field, except that with arc irradiation the surrounding area is taken into account from all irradiated angular positions. This normally increases the volume irradiated.
The dynamic arc irradiation ( Dynamic Conformal Arc ) compensates for this by dynamically adjusting the lamellae to the current angular position while the accelerator is rotating. Thus, the field size at each angular position corresponds approximately to that of an equivalent standing field.
Intensity-modulated treatment
While in the case of the conformal treatment is tried to keep the radiation on the surrounding tissue as low as possible, in the case of is intensity modulated (Treatment Intensity Modulated Radio Therapy - IMRT) of the multileaf collimator is used to distribute the radiation in the object uniformly and therefore inhomogeneities balance in the passage of rays through the body.
A distinction is made between sliding window and step-and-shoot IMRT. During a sliding window IMRT treatment, the lamellae move continuously, similar to dynamic arc irradiation, and are moved across the treatment field at different speeds in order to achieve differences in the intensity of the radiation. With step-and-shoot IMRT, the slats are alternately moved to a certain position and then radiation is emitted until all the necessary positions have been passed.
history
At the beginning of the radiation therapy, simple conical or round collimators were used. These are usually lead or tungsten cylinders that are attached in front of the accelerator's beam exit opening and have a hole with a defined diameter in the middle. Depending on the shape of the volume to be irradiated, these collimators are exchanged in order to adapt the diameter to the real conditions. For example, brain metastases can usually be irradiated very well with this, since they are usually relatively spherical and, depending on their size, can also be treated relatively well with a certain round collimator.
The circular collimator concept reaches its limits with rather misshapen or larger objects. The object to be treated must be covered with several individual fields. Due to the circular opening of the collimator, however, various fields overlap in some regions, while less radiation arrives in other regions, since these are between two fields. One speaks here of so-called hot spots and cold spots . However, this is certainly also a question of radiation planning and the technology or skills of the planning doctor or physicist .
The principle of the circular collimator has been further developed in the iris collimator , so that different diameters can be realized with one collimator, thus eliminating the need to swap collimators between two fields.
A further development in the direction of conformal treatment are collimators that no longer have a circular opening. For this purpose, the contour of the volume to be treated is calculated by an irradiation planning system for a certain accelerator and patient bed position. This data is then transferred to a plastic block and a shape of the contour is created. This image is then used to pour lead into a collimator to create a conformal collimator. However, this depends on the field to be irradiated, which makes the entire concept very labor-intensive.
Similar procedures exist for IMRT treatment. For this purpose, the inhomogeneity to be compensated is transferred to a metal block by using a milling machine to work out areas with different depths. This metal block is then also introduced into the treatment beam and thus ensures uniform irradiation of the target volume. Here, too, the resulting block is again specifically manufactured for a field and thus the total effort is relatively high.