Cochran B, Kim G, Rice R
San Diego State University, San Diego, CA.
UCSD Medical Center, La Jolla, CA.
Med Phys. 2012 Jun;39(6Part18):3830. doi: 10.1118/1.4735632.
Measure photon beam attenuation characteristics of the Varian EXACT, Varian IGRT treatment couches and two CIVCO Universal extensions; and assess accuracy of Varian's Eclipse treatment planning system in modeling the beam attenuation of each couch.
Dose Measurements were taken using a Farmer ion chamber inside a cylindrical acrylic phantom. They were taken for two photon energies (6MV and 15MV), at three different field sizes, for various gantry angles through the couch. EXACT couch was tested both with the adjustable rails pushed to the outside of the couch, and pushed to the center of the couch. IGRT couch was divided by thickness into three sections. Effects on surface dose were measured using a Tissue Maximum Ratio or Percent Depth Dose curve. CIVCO extensions were CT scanned for inclusion in the planning system. All measurements were modeled in the Eclipse treatment planning system for comparison.
Maximum attenuation was measured at 6MV with the smallest field for all couch setups. The EXACT couch produced a maximum attenuation of 19.2% and a surface dose increase of 47% of Dmax through the couch. Maximum attenuation for the IGRT couch and both CIVCO extensions were between 6-8%. Through the couch, the IGRT couch showed a surface dose increase of 29% of Dmax,; the CIVCO extensions produced a surface dose increase of 33% and 7% of Dmax. Maximum difference between measured and planned dose was 7%, for the EXACT couch, 1.9% for the IGRT couch and 2.4% for both CIVCO extensions.
These treatment couches were shown to affect delivered and surface dose. Including the couch structure during treatment planning can account for the couch attenuation in most cases, but some attenuation values are underestimated in the planning software. Largest effects are for oblique treatment angles using low energy and small field sizes.
测量瓦里安EXACT、瓦里安IGRT治疗床以及两个CIVCO通用延长装置的光子束衰减特性；并评估瓦里安Eclipse治疗计划系统在模拟每个治疗床的束衰减方面的准确性。
使用圆柱形丙烯酸模体中的 Farmer 电离室进行剂量测量。在两种光子能量（6MV和15MV）下，针对三种不同的射野尺寸，通过治疗床在不同机架角度进行测量。EXACT治疗床在可调节轨道推到治疗床外侧和推到治疗床中心两种情况下都进行了测试。IGRT治疗床按厚度分为三个部分。使用组织最大剂量比或百分深度剂量曲线测量对表面剂量的影响。对CIVCO延长装置进行CT扫描以纳入计划系统。所有测量都在Eclipse治疗计划系统中进行建模以作比较。
在所有治疗床设置中，6MV且射野最小时测得最大衰减。EXACT治疗床产生的最大衰减为19.2%，通过治疗床时表面剂量增加到Dmax的47%。IGRT治疗床和两个CIVCO延长装置的最大衰减在6 - 8%之间。通过治疗床，IGRT治疗床表面剂量增加到Dmax的29%；CIVCO延长装置表面剂量分别增加到Dmax的33%和7%。EXACT治疗床测量剂量与计划剂量的最大差值为7%，IGRT治疗床为1.9%，两个CIVCO延长装置均为2.4%。
这些治疗床显示会影响输出剂量和表面剂量。在治疗计划期间纳入治疗床结构在大多数情况下可考虑治疗床衰减，但计划软件中一些衰减值被低估。最大影响出现在使用低能量和小射野尺寸的斜向治疗角度时。
