Thomsen Mette S, Ulsø Niels
Department of Medical Physics, Aarhus University Hospital, Denmark.
Med Phys. 2002 Oct;29(10):2427-32. doi: 10.1118/1.1510129.
The properties of a new cement based material for production of compensators are presented. Broad beam attenuation of 4-20 MV x rays by slabs of the material have been measured at various field sizes and depths in a large water phantom. For comparison the attenuation of aluminum was determined at some of the photon energies and it was found that the attenuation properties of the cement based material are very close to those of aluminum. At 6 and 18 MV, a comparison of different phantoms for attenuation measurements was carried out. For this investigation the ionization chamber was placed in a 50x50x50 cm3 water phantom, a 20x20x20 cm3 water equivalent plastic phantom, and a cylindrical mini-phantom. Agreement was obtained between the measurements in the large water phantom and in the water equivalent plastic phantom. The measurements carried out with the mini-phantom in a 6 MV x-ray beam gave a higher transmission versus absorber thickness than the transmission found with the water phantom resulting in a lower value of the effective attenuation coefficient. At 18 MV x rays the difference between the measurements in the water phantom and in the mini-phantom was less. This shows that a water equivalent plastic phantom with an area comparable with the largest field size applied can be used for measurements of effective attenuation coefficients, whereas a mini-phantom cannot be used directly, especially at low photon energies.
介绍了一种用于生产补偿器的新型水泥基材料的性能。在大型水模体中，测量了该材料平板对4 - 20 MV X射线在不同射野尺寸和深度处的宽束衰减。为作比较，在部分光子能量下测定了铝的衰减，发现该水泥基材料的衰减特性与铝非常接近。在6 MV和18 MV时，对用于衰减测量的不同模体进行了比较。对于此研究，将电离室置于一个50×50×50 cm³的水模体、一个20×20×20 cm³的水等效塑料模体和一个圆柱形微型模体中。在大型水模体和水等效塑料模体中的测量结果取得了一致。在6 MV X射线束中用微型模体进行的测量相对于吸收体厚度给出了比用水模体测得的更高的透射率，导致有效衰减系数值更低。在18 MV X射线时，水模体和微型模体测量结果之间的差异较小。这表明，对于有效衰减系数的测量，可使用面积与所应用的最大射野尺寸相当的水等效塑料模体，而微型模体不能直接使用，尤其是在低光子能量时。
