Butson Martin J, Cheung Tsang, Yu Peter K N
Illawarra Cancer Care Centre, Department of Medical Physics, Wollongong, NSW 2500, Australia.
J Med Phys. 2008 Jan;33(1):24-8. doi: 10.4103/0971-6203.39421.
Solid water is often the phantom material of choice for dosimetry procedures in radiotherapy high-energy X-ray and electron beam radiation calibration and quality assurance. This note investigates variation in heat conduction that can occur for a common commercially available solid water stack phantom when a temperature differential occurs between the phantom and ambient temperature. These variations in temperature can then affect radiation measurements and thus the accuracy of radiation dosimetry. In this manuscript, we aim to investigate the variations in temperature which can occur in radiation measurement incorporated (RMI) solid water phantoms, their thermal properties and the effects on radiation dosimetry which can occur because of temperature differentials. Results have shown that the rate of temperature change at a phantom center is a complex function but appears relatively proportional to the surface area of the phantom in normal clinical usage. It is also dependent on the thermal conductivity of any material in contact with the phantom; and the nature of the phantom construction, i.e., the number and thickness of slices within the phantom. A thermal time constant of approximately 20 min was measured for a 2-cm solid water phantom slice when located on a steel workbench in comparison to 60 min when located on a wooden workbench (linac couch insert). It is found that for larger solid water stack phantoms, a transient (within 1 degrees C) thermal equilibrium exists at the center for up to 2 h, before the temperature begins to change. This is assumed to be due to the insulating properties of multiple slices within the stack, whereby very small air spaces are introduced inhibiting the heat conduction through the phantom material. It is therefore recommended that the solid water/phantom material is kept within the treatment room for closest thermal accuracy conditions or at least placed within the room approximately 10 h before dosimetry measurements. If these options are not available, a standard linear interpolation method for calculation of temperature should be used to minimize uncertainty of temperature measurements.
在放射治疗高能X射线和电子束辐射校准及质量保证的剂量测定程序中，固体水常常是首选的模体材料。本报告研究了一种常见的市售固体水叠层模体在模体与环境温度之间出现温差时可能发生的热传导变化。这些温度变化进而会影响辐射测量，从而影响辐射剂量测定的准确性。在本论文中，我们旨在研究在包含辐射测量功能的（RMI）固体水模体中可能出现的温度变化、它们的热特性以及由于温度差异可能对辐射剂量测定产生的影响。结果表明，在正常临床使用中，模体中心的温度变化率是一个复杂的函数，但似乎与模体的表面积相对成比例。它还取决于与模体接触的任何材料的热导率；以及模体结构的性质，即模体内部切片的数量和厚度。当一个2厘米厚的固体水模体切片放置在钢制工作台上时，测得的热时间常数约为20分钟，而放置在木制工作台上（直线加速器治疗床插件）时为60分钟。研究发现，对于较大的固体水叠层模体，在温度开始变化之前，中心会存在长达2小时的瞬态（在1摄氏度以内）热平衡。这被认为是由于叠层内多个切片的绝缘特性，由此引入了非常小的气隙，抑制了热量通过模体材料的传导。因此，建议将固体水/模体材料放置在治疗室内以获得最接近的热精确条件，或者至少在剂量测定测量前约10小时放置在室内。如果无法采用这些选项，则应使用标准线性插值方法计算温度，以尽量减少温度测量的不确定性。
