Tatcher M, Glicksman A S
Quality Assurance Review Center, Rhode Island Hospital, Providence.
Int J Radiat Oncol Biol Phys. 1990 Feb;18(2):441-4. doi: 10.1016/0360-3016(90)90113-x.
X-ray beams are usually described by "MV" numbers which represent accelerating potentials (AP) and approximations to the maximum energies in the photon spectra. However, these numbers do not uniquely specify the properties of the beams. Current high energy photon dosimetry protocols specify radiation quality in terms of a measured ionization ratio which is equivalent to the ratio of the tissue-maximum ratios at depths 10 cm and 20 cm, for field size 10 cm X 10 cm [TMR)20(10]. For convenience, the American Association of Physicists in Medicine introduced a new parameter, known as the Nominal Accelerating Potential (NAP), which was derived from (TMR)20(10) and features values in MV units that are similar to those of the conventional accelerating potentials. (TMR)20(10) and Nominal Accelerating Potential may be considered to be expressions of the penetrating powers of x-ray beams. We determined (TMR)20(10) and Nominal Accelerating Potential for 460 treatment machines with stated accelerating potentials from 4 MV to 25 MV in the Quality Assurance Review Center's files of machine data from institutions that participate in cooperative clinical trials. The results demonstrate appreciable variability of the two parameters at each stated accelerating potential, with overlapping of adjacent groups of machines. It is concluded that the manufacturers' MV numbers do not reliably identify x-ray beams in terms of their depth dose properties. To promote standardization and consistency of energy specification in clinical trials as well as in general practice, we propose that x-ray beams be designated by their Nominal Accelerating Potential values as an adjunct to the use of (TMR)20(10) in radiation therapy.
X射线束通常用“MV”数值来描述，这些数值代表加速电位（AP）以及光子能谱中最大能量的近似值。然而，这些数值并不能唯一地确定射线束的特性。当前的高能光子剂量测定协议根据测量的电离比来规定辐射质量，该电离比等同于在10 cm×10 cm射野尺寸下，深度为10 cm和20 cm时的组织最大剂量比[TMR(10)]20。为方便起见，美国医学物理学家协会引入了一个新参数，称为标称加速电位（NAP），它由[TMR(10)]20推导得出，其MV单位的数值与传统加速电位的数值相似。[TMR(10)]20和标称加速电位可被视为X射线束穿透能力的表达式。我们在质量保证审查中心参与合作临床试验机构的机器数据文件中，确定了460台标称加速电位为4 MV至25 MV的治疗机器的[TMR(10)]20和标称加速电位。结果表明，在每个标称加速电位下，这两个参数都存在明显的变异性，相邻机器组之间存在重叠。得出的结论是，制造商的MV数值并不能根据其深度剂量特性可靠地识别X射线束。为了促进临床试验以及一般实践中能量规范的标准化和一致性，我们建议在放射治疗中，除了使用[TMR(10)]20之外，还应通过其标称加速电位值来指定X射线束。
