Hsi Wen C, Zhang Yunkai, Kirk Michael C, Bernard Damian, Chu James C H
Department of Medical Physics, Rush University Medical Center, 1653 West Congress Parkway, Chicago, Illinois 60612, USA.
J Appl Clin Med Phys. 2005 Spring;6(2):12-8. doi: 10.1120/jacmp.v6i2.1999. Epub 2005 May 19.
The Varian 120 multileaf collimator (MLC) has a leaf thickness of 5 mm projected at the isocenter plane and can deliver a radiation beam of large field size (up to 30 cm) to be used in intensity-modulated radiotherapy (IMRT). Often the dose must be delivered to depths greater than 20 cm. Therefore, during the commissioning of the BrainSCAN v5.21 or any radiation treatment-planning (RTP) systems, extensive testing of dose and monitor unit calculations must encompass the field sizes (1 cm to 30 cm) and the prescription depths (1 cm to 20 cm). Accordingly, the central-axis percent depth doses (PDDs) and off-axis percentage profiles must be measured at several depths for various field sizes. The data for this study were acquired with a 6-MV X-ray beam from a Varian 2100EX LINAC with a water phantom at a source-to-surface distance (SSD) of 100 cm. These measurements were also used to generate a photon beam module, based on a photon pencil beam dose-calculation algorithm with a fast-Fourier transform method. To commission the photon beam module used in our BrainSCAN RTP system, we performed a quantitative comparison of measured and calculated central-axis depth doses and off-axis profiles. Utilizing the principles of dose difference and distance-to-agreement introduced by Van Dyk et al. [Commissioning and quality assurance of treatment planning computers. Int J Radiat Oncol Biol Phys. 1993; 26:261-273], agreements between calculated and measured doses are <2% and <2 mm for the regions of low- and high-dose gradients, respectively. However, large errors (up to approximately 5% and approximately 7% for 20-cm and 30-cm fields, respectively, at the depth 20 cm) were observed for monitor unit calculations. For a given field size, the disagreement increased with the depth. Similarly, for a given depth the disagreement also increase with the field size. These large systematic errors were caused by using the tissue maximum ratio (TMR) in BrainSCAN v5.21 without considering increased field size as depth increased. These errors have been reported to BrainLAB.
瓦里安120多叶准直器（MLC）在等中心平面上的叶片厚度为5毫米，可提供大射野尺寸（达30厘米）的辐射束，用于调强放射治疗（IMRT）。通常剂量必须输送到大于20厘米的深度。因此，在BrainSCAN v5.21或任何放射治疗计划（RTP）系统的调试过程中，剂量和监测单位计算的广泛测试必须涵盖射野尺寸（1厘米至30厘米）和处方深度（1厘米至20厘米）。相应地，必须针对各种射野尺寸在多个深度测量中心轴百分深度剂量（PDDs）和离轴百分比分布。本研究的数据是使用来自瓦里安2100EX直线加速器的6兆伏X射线束，在源皮距（SSD）为100厘米的情况下用水模体采集的。这些测量数据还用于基于具有快速傅里叶变换方法的光子笔形束剂量计算算法生成一个光子束模块。为了调试我们BrainSCAN RTP系统中使用的光子束模块，我们对测量和计算的中心轴深度剂量及离轴分布进行了定量比较。利用Van Dyk等人提出的剂量差异和距离一致性原则[治疗计划计算机的调试与质量保证。国际放射肿瘤学、生物学、物理学杂志。1993年；26:261 - 273]，对于低剂量梯度和高剂量梯度区域，计算剂量与测量剂量之间的一致性分别<2%和<2毫米。然而，在监测单位计算中观察到较大误差（在20厘米深度处，20厘米和30厘米射野分别高达约5%和约7%）。对于给定的射野尺寸，误差随深度增加而增大。同样，对于给定的深度，误差也随射野尺寸增大而增大。这些较大的系统误差是由于在BrainSCAN v5.21中使用组织最大比（TMR）时未考虑随着深度增加射野尺寸也会增大所致。这些误差已报告给BrainLAB。
