The Ottawa Hospital Cancer Centre, Ottawa K1H 8L6, Canada.
Med Phys. 2010 Jan;37(1):322-8. doi: 10.1118/1.3271364.
To measure the effect of the treatment couch on dose distributions and to investigate the ability of a modern planning system to accurately model these effects.
This work measured the dose perturbation at depth and in the dose buildup region when one of two treatment couches, CIVCO (formerly MED-TEC) or Medical Intelligence, was placed between a photon beam source (6, 10, and 18 MV) and the phantom. Beam attenuation was measured in the center of a cylindrical acrylic phantom with a Farmer type ion chamber at multiple gantry angles. Dose buildup was measured in Solid Water with plane parallel ion chambers (NACP-02 and PTW Markus) with the beam normal to both the phantom and couch surfaces. The effective point of measurement method as described [M. R. McEwen et al. "The effective point of measurement of ionization chambers and the build-up anomaly in MV x-ray beams," Med. Phys. 35(3), 950-958 (2008)] was employed to calculate dose in the buildup region. Both experiments were modeled in XiO. Images of the treatment couches were merged with images of the phantoms such that they were included as part of the "patient" image. Dose distributions calculated with superposition and fast superposition algorithms were compared to measurement.
The two treatment couches have different radiological signatures and dissimilar water equivalent thicknesses (4.2 vs 6.3 mm.) Maximum attenuation was 7%. Both couches caused significant loss of skin sparing, the worst case showing an increase in surface dose from 17% (no couch) to 88% (with couch). The TPS accurately predicted the surface dose (+/-3%) and the attenuation at depth when the phantom was in contact with the couch. For the open beam the TPS was less successful in the buildup region.
The treatment couch is not radio-transparent. Its presence between the patient and beam source significantly alters dose in the patient. For the most part, a modern treatment planning system can adequately predict the altered dose distribution.
测量治疗床对剂量分布的影响，并研究现代计划系统准确建模这些影响的能力。
本工作测量了当光子束源（6、10 和 18 MV）和模体之间放置两种治疗床之一（CIVCO（前身为 MED-TEC）或 Medical Intelligence）时，深度处和剂量堆积区域的剂量扰动。在圆柱形亚克力模体的中心使用 Farmer 型电离室，在多个机架角度下测量束衰减。在光束垂直于模体和治疗床表面的情况下，使用平面平行电离室（NACP-02 和 PTW Markus）在 Solid Water 中测量剂量堆积。采用 [M.R.McEwen 等人，“电离室的有效测量点和 MV X 射线束中的堆积异常”，《医学物理学》35(3)，950-958（2008 年）] 中描述的有效测量点方法计算堆积区域的剂量。XiO 中模拟了这两个实验。将治疗床的图像与模体的图像合并，使它们作为“患者”图像的一部分。将叠加和快速叠加算法计算的剂量分布与测量值进行比较。
两种治疗床具有不同的射线学特征和不同的水等效厚度（4.2 毫米对 6.3 毫米）。最大衰减为 7%。两种治疗床都会导致明显的皮肤保护丧失，最严重的情况是表面剂量从无治疗床时的 17%增加到有治疗床时的 88%。TPS 准确预测了表面剂量（+/-3%）和与治疗床接触时的深度衰减。对于开束，TPS 在堆积区域的效果较差。
治疗床不是射线透明的。它在患者和束源之间的存在会显著改变患者体内的剂量。在大多数情况下，现代治疗计划系统可以充分预测改变后的剂量分布。
