The contact X-ray collimator has a function of scattering electrons to improve the uniformity. The bottom of the contact X-ray collimator needs to be directly in contact with the patient’s skin. Otherwise, the uniformity is significantly different from the distance from the contact X-ray collimator. The changes, although designed to take into account the risk of compression of the patient, are often equipped with a compressible buffer spring, and must be very careful when using a motor to drive the lifting treatment bed.
In addition, the electron energy scattered by the inner wall of the contact X-ray collimator is lower than that of the electron directly emitted from the radiation source, thus increasing the surface dose, and the maximum absorbed dose point will move toward the surface, reducing the slope of the range end dose drop. The average energy of the electrons scattered by the electron X-ray collimator wall into the electron beam current is about 40% lower than the energy of the incident electron beam current. Therefore, a shallow “hot spot” is formed in the edge of the field. To reduce this effect, a high-density material is generally applied to the edge portion of the electron X-ray collimator, such as a 1 mm metal tungsten as a bushing.