Higgins P D, Sibata C H, Paliwal B R
Phys Med Biol. 1985 Feb;30(2):153-62. doi: 10.1088/0031-9155/30/2/004.
Experimental verification of the difference between absorbed dose in tissue and the collision fraction of kerma requires precise knowledge of the absorbed dose curve, particularly in the build-up and build-down regions. A simple method for direct measurement of contamination-free build-up for 60Co, which should also be applicable for most of the photon energies commonly employed for treatment, is presented. It is shown that the contribution from air-scattered electrons to the surface dose may be removed by extrapolating measurements of build-up to zero field size. The remaining contribution to contamination from the collimators and other source-related hardware may be minimised by measuring these build-up curves sufficiently far from the source. These results were tested by measuring the build-up using a magnet to sweep scattered electrons from the primary photon beam and by measuring the surface dose in the limit of an evacuated beam path. The relative dose at zero depth in polystyrene was found to be approximately 8.9 +/- 0.3% of the dose at the depth of maximum build-up.
对组织中吸收剂量与比释动能碰撞分数之间差异的实验验证需要精确了解吸收剂量曲线，特别是在积累区和衰减区。本文提出了一种直接测量60Co无污染积累的简单方法，该方法也应适用于大多数常用于治疗的光子能量。结果表明，通过将积累测量值外推到零射野大小，可以消除空气散射电子对表面剂量的贡献。通过在离源足够远的地方测量这些积累曲线，可以将准直器和其他与源相关硬件对污染的剩余贡献降至最低。通过使用磁铁扫掠初级光子束中的散射电子来测量积累，并在真空射线路径极限下测量表面剂量，对这些结果进行了测试。发现在聚苯乙烯中零深度处的相对剂量约为最大积累深度处剂量的8.9±0.3%。
