Jermoumi Mohammed, Xie Roger, Cao Daliang, Housley David J, Shepard David M
Department of Radiation Oncology, Swedish Cancer Institute, Seattle, WA, USA.
Department of Radiation Oncology, Ironwood Cancer and Research Center, Scottsdale, AZ, USA.
J Appl Clin Med Phys. 2017 Jan;18(1):90-95. doi: 10.1002/acm2.12020. Epub 2017 Jan 3.
In this study, we evaluated the performance of an Elekta linac in the delivery of gated radiotherapy. Delivery accuracy was examined with an emphasis on the impact of using short gating windows (low monitor unit beam-on segments) or long beam hold times. The performance was assessed using a 20cm by 20cm open field with the radiation delivered using a range of beam-on and beam-off time periods. Gated delivery measurements were also performed for two SBRT plans delivered using volumetric modulated arc therapy (VMAT). Tests included both free-breathing based gating (covering a variety of gating windows) and simulated breath-hold based gating. An IBA MatriXX 2D ion chamber array was used for data collection, and the gating accuracy at low MU was evaluated using gamma passing rates. For the 20 cm by 20 cm open field, the measurements generally showed close agreement between the gated and non-gated beam deliveries. Discrepancies, however, began to appear with a 5-to-1 ratio of the beam-off to beam-on times. The discrepancies observed for these tight gating windows can be attributed to the small number of monitor units delivered during each beam-on segment. Dose distribution analysis from the delivery of the two SBRT plans showed gamma passing rates (± 1%, 2%/1 mm) in the range of 95% to 100% for gating windows of 25%, 38%, 50%, 63%, 75%, and 83%. Using a simulated sinusoidal breathing signal with a 4 second period, the gamma passing rate of free-breathing gating and breath-hold gating deliveries were measured in the range of 95.7% to 100%. In conclusion, the results demonstrate that Elekta linacs can accurately deliver respiratory gated treatments for both free-breathing and breath-hold patients. Some caution should be exercised with the use of very tight gating windows.
在本研究中，我们评估了医科达直线加速器在实施门控放射治疗时的性能。重点研究了使用短门控窗（低监测单位射束开启段）或长射束保持时间的影响，以此来检验射束输送精度。使用20厘米×20厘米的开放射野，并采用一系列射束开启和射束关闭时间段来输送辐射，以此评估性能。还对使用容积调强弧形治疗（VMAT）实施的两个立体定向体部放疗（SBRT）计划进行了门控输送测量。测试包括基于自由呼吸的门控（涵盖各种门控窗）和基于模拟屏气的门控。使用IBA MatriXX二维电离室阵列进行数据收集，并使用γ通过率评估低监测单位下的门控精度。对于20厘米×20厘米的开放射野，测量结果总体显示门控射束输送与非门控射束输送之间吻合度较高。然而，当射束关闭与射束开启时间之比为5比1时，差异开始显现。在这些狭窄门控窗下观察到的差异可归因于每个射束开启段输送的监测单位数量较少。对两个SBRT计划输送的剂量分布分析显示，对于25%、38%、50%、63%、75%和83%的门控窗，γ通过率（±1%，2%/1毫米）在95%至100%的范围内。使用周期为4秒的模拟正弦呼吸信号，测量得到自由呼吸门控和屏气门控输送的γ通过率在95.7%至100%的范围内。总之，结果表明医科达直线加速器能够为自由呼吸和屏气患者准确地实施呼吸门控治疗。使用非常狭窄的门控窗时应谨慎。
