The outline of the radiation field defines the radiation range. If the range specified by the machine is not consistent with the actual irradiation range, some parts of the target area will be missed, and in serious cases, important organs nearby will be irradiated.
The regular field is a symmetric field or an asymmetric field with respect to the axis of the radiation beam, and the limitation of the allowable error of the regular field is mainly realized by limiting the following parameters.
1. 1. 1. Provisions on axis deviation of radiation beam
The medical electronic linear accelerator must first ensure the isocentric accuracy (2mm) of the gantry, radiation head and the rotating shaft of the treatment table, and then ensure the deviation (2mm) of the radiation beam axis from the isocenter.
(2) Provisions on the shape of radiation field
For the shape requirements of square or rectangular radiation field, the maximum deviation between the parallelism of the opposite side and the verticality of the adjacent side shall not be greater than 0.5.
(3) the provisions of the radiation field boundary deviation
The medical electron linear accelerator indicates the range of radiation field by simulating the light field, so the allowable deviation (2mm) between the light field and the edge of the radiation field should be ensured. (the second co 1 1 imator) consists of two pairs of rectangular collimators which can be opened and closed, commonly known as diaphragm. The material is made of tungsten, lead or depleted uranium, and a square or rectangular radiation field can be formed by opening and closing a pair of upper and lower rectangular collimators. According to the symmetry of opening and closing movement, it can be divided into
A. Symmetric collimation system In this system, both upper and lower pairs of rectangular collimators perform symmetrical opening and closing movements. In order to reduce the scattering of X-rays on the side wall of the collimator and the transmission penumbra at the edge of the collimator, the inner side of the rectangular collimator should be tangent to the emission direction of the rays. The thickness of the rectangular collimator should be thick enough to ensure that the transmittance of X-rays passing through the collimator meets the national standards. The transmittance should be less than 0.5% of the central light intensity.
B. Asymmetric collimator system in clinical application, with the requirement of high treatment accuracy and the expansion of application scope, asymmetric collimator system (or independent collimator system) has been developed. This system still keeps a pair of rectangular collimators moving symmetrically, that is, moving symmetrically with respect to the center, while another pair (usually the next pair) of rectangular collimators moving asymmetrically with respect to the center. The rectangular collimators moving asymmetrically can move one to the other across the central axis. When the rectangular collimator on one side is just located at the position of the central axis of the ray, a tangential field is formed, which is used for the joint irradiation of adjacent fields in the * * * plane and non-* * plane, the tangential field irradiation of breast cancer and the tangential irradiation of rotary therapy. The boundary of the electron radiation field is determined by the electron beam limiter attached to the primary collimator. The electron beam limiter is divided into the following two types.
(1) Contact beam coater
The contact beam limiter is conical or barrel-shaped. It can be fixed or adjustable.
B. Material electron beam limiter is made of materials with low atomic number, and the selection of materials directly affects the quality of electron beam used for treatment. In order to reduce the share of bremsstrahlung produced by electron beam current in the beam limiter, materials with lower atomic number should be used. However, the use of materials with lower atomic number will generate a large number of low-energy electrons into the electron beam, so it is necessary to choose an optimal atomic number.
C the wall of the beam limiter has the function of scattering electrons, which can improve the uniformity. When in use, the bottom of the beam limiter needs to be in direct contact with the patient's skin, otherwise the uniformity will change obviously with the distance from the end face of the beam limiter. Although the danger of patients being squeezed has been considered in the design, many of them are equipped with compressible buffer springs, so care must be taken when the treatment table is driven by motor.
In addition, the electron energy scattered by the inner wall of the beam limiter is lower than that directly emitted from the radiation source, so the surface dose will increase, and the maximum absorbed dose point will move to the surface, which reduces the steepness of dose decline at the end of the range. The average energy of electrons scattered into the electron beam stream by the beam limiter wall is about 40% lower than that of the incident electron beam stream. Therefore, shallow "hot spots" will be formed at the edge of the beam field. In order to reduce this influence, the edge of the electron beam limiter is generally made of high-density materials, such as 1mm tungsten as the sleeve.
(2) Non-contact beam limiter
The important difference between non-contact beam limiter and contact beam limiter is that the contribution of electrons scattered by the beam limiter wall is greatly reduced, and on this basis, plate or diaphragm beam limiter is developed.
Using composite scattering filter or compensation foil can ensure uniform dose distribution. There is no need to use the beam limiter wall to scatter electrons. The required radiation field area is controlled by the diaphragm, and the lower part of the beam limiter keeps a certain distance from the skin, which greatly reduces the risk of patients being squeezed. In order to reduce the weight of the beam limiter, two pairs of secondary collimators in the radiation head are usually used for primary collimation. Because the lower part of the beam limiter keeps a certain distance from the skin, the penumbra is slightly larger than the complete penumbra of the skin, but at the depth of the target area, the influence of lateral scattering is far greater than that of the distance between the lower parts of the beam limiter.
Generally speaking, due to the multiple scattering of electrons in the air and the inevitable scattering of the beam limiter wall, the collimation of electron beam current can not be simply regarded as a problem of ray geometry. The cross section of the electron radiation beam coming out of the beam limiter is generally obviously larger than the cross section of the geometric electron beam flow, and its purpose is to realize the dynamic balance of electrons scattered in and out at the edge of the electron beam flow.
In order to adjust the shape and size of the electron radiation field, a device is equipped with beam limiters with different sizes.