Lombardi P, Fiorino C, Cattaneo G M, Calandrino R
Servizio di Fisica Sanitaria, H S Raffaele, Milano, Italy.
Br J Radiol. 1997 Jun;70(834):638-44. doi: 10.1259/bjr.70.834.9227259.
The results of an investigation of the accuracy of monitor unit (MU) calculation in clinical shaped beams are presented. Measured doses at the reference depth on the beam central axis (isocentre) or on a beam axis representative of the irradiated area (when the isocentre lies under a block or near the edges of the block's shadow) were compared with the expected doses when calculating MUs, by applying different methods normally used in clinical practice. Empirical (areas weighted, Wrede) and scatter summation (Clarkson) methods as well as a pencil-beam based algorithm were applied. 40 irregular fields (6 MV X-rays, CLinac, Varian 6/100), divided into six categories, were considered. Dose measurements were performed with a NE2571 ionization chamber in an acrylic 30 x 30 x 30 cm3 phantom. The depths in acrylic were converted into water-equivalent depths through a correction factor derived from TMR measurements. The method of dose measurements in acrylic was found to be sufficiently accurate for the purpose of this study by comparing expected and measured doses in open square and rectangular fields (mean deviation +0.2%, SD = 0.5%). Results show that all the considered methods are sufficiently reliable in calculating MUs in clinical situations. Mean deviations between measured and expected dose values are around 0 for all the methods; standard deviations range from 1% for the Wrede method to 0.75% for the pencil-beam method. The differences between expected and measured doses were within 1% for about 3/4 of the fields when calculating MUs with all the considered methods. Maximum deviations range from 1.6% (pencil-beam) to 3% (Wrede). Slight differences among the methods of MU calculation were revealed within the different categories of blocked fields analysed. The surprising agreement between measured and expected dose values obtained by using empirical methods (area weighted and Wrede) is probably due to the fact that the reference points were positioned in a "central" region of the unblocked areas.
本文给出了临床成形射束中监测单位（MU）计算准确性的调查结果。通过应用临床实践中常用的不同方法，将射束中心轴（等中心）或代表受照区域的射束轴上参考深度处（当等中心位于挡块下方或挡块阴影边缘附近时）的测量剂量与计算MU时的预期剂量进行比较。应用了经验法（面积加权法、弗雷德法）、散射求和法（克拉克森法）以及基于笔形束的算法。研究考虑了40个不规则射野（6MV X射线，直线加速器，瓦里安6/100），分为六类。使用NE2571电离室在30×30×30cm³的丙烯酸模体中进行剂量测量。通过从组织最大剂量比（TMR）测量得出的校正因子，将丙烯酸中的深度转换为水等效深度。通过比较开放方形和矩形射野中的预期剂量和测量剂量（平均偏差+0.2%，标准差=0.5%），发现丙烯酸中的剂量测量方法对于本研究目的而言足够准确。结果表明，所有考虑的方法在临床情况下计算MU时都足够可靠。所有方法测量剂量值与预期剂量值之间的平均偏差约为0；标准差范围从弗雷德法的1%到笔形束法的0.75%。使用所有考虑的方法计算MU时，约3/4的射野预期剂量与测量剂量之间的差异在1%以内。最大偏差范围从1.6%（笔形束法）到3%（弗雷德法）。在所分析的不同类别挡块射野中，MU计算方法之间存在细微差异。使用经验法（面积加权法和弗雷德法）获得的测量剂量值与预期剂量值之间惊人的一致性，可能是由于参考点位于未挡块区域的“中心”区域。
