West Michigan Cancer Center, Kalamazoo, MI 49007, USA.
Med Phys. 2011 Oct;38(10):5432-40. doi: 10.1118/1.3633939.
For external beam in vivo measurements, the dosimeter is normally placed on the patient's skin, and the dose to a point of interest inside the patient is derived from surface measurements. In order to obtain accurate and reliable measurements, which correlate with the dose values predicted by a treatment planning system, a dosimeter needs to be at a point of electronic equilibrium. This equilibrium is accomplished by adding material (buildup) above the detector. This paper examines the use of buildup caps in a clinical setting for two common detector types: OSLDs and diodes. Clinically built buildup-caps and commercially available hemispherical caps are investigated. The effects of buildup cap thickness and fabrication material on field-size correction factors, C(FS), are reported, and differences between the effects of thickness and fabrication material are explained based on physical parameters.
Measurements are made on solid water phantoms for 6 and 15 MV x-ray beams. Two types of dosimeters are used: OSLDs, InLight∕OSL Nanodot dosimeters (Landauer, Inc., Glenwood, IL) and a P-type surface diode (Standard Imaging, Madison, WI). Buildup caps for these detectors were fabricated out of M3, a water-equivalent material, and sheet-metal stock of Al, Cu, and Pb. Also, commercially available hemispherical buildup caps made of plastic water and brass (Landauer, Inc., Glenwood, IL) were used with Nanodots. OSLDs were read with an InLight microStar reader (Landauer, Inc., Glenwood, IL). Dose calculations were carried out with the XiO treatment planning system (CMS∕Elekta, Stockholm) with tissue heterogeneity corrections.
For OSLDs and diodes, when measurements are made with no buildup cap a change in C(FS) of 200% occurs for a field-size change from 3 cm × 3 cm to 30 cm × 30 cm. The change in C(FS) is reduced to about 4% when a buildup cap with wall thickness equal to the depth of maximum dose is used. Buildup caps with larger wall thickness do not cause further reduction in C(FS). The buildup cap fabrication material has little or no effect on C(FS). The perturbation to the delivered dose caused by placing a detector with a buildup cap on the surface of a patient is measured to be 4%-7%. A comparison between calculated dose and dose measured with a Nanodot and a diode for 6 and 15 MV x-rays is made. When C(FS) factors are carefully determined and applied to measurements made on a phantom, the differences between measured and calculated doses were found to be between ±1.3%.
OSLDs and diodes with appropriate buildup caps can be used to measure dose on the surface of a patient and predict the delivered dose to depth dmax in a range of ±1.3% for 100 cGy. The buildup cap: can be fabricated from any material examined in this work, is best with wall thickness dmax, and causes a perturbation to the delivered dose of 4%-7% when the wall thickness is dmax. OSLDs and diodes with buildup caps can both give accurate measurements of delivered dose.
对于外部束体内测量，剂量仪通常放置在患者的皮肤上，并且通过表面测量来获得患者内部感兴趣点的剂量。为了获得与治疗计划系统预测的剂量值相匹配的准确可靠的测量值，剂量仪需要处于电子平衡状态。通过在探测器上方添加材料（附加物）来实现这种平衡。本文研究了在两种常见探测器类型（OSLD 和二极管）的临床环境中使用附加物帽。研究了临床使用的附加物帽和市售的半球形帽。报告了附加物帽厚度和制造材料对射野大小校正因子 C（FS）的影响，并基于物理参数解释了厚度和制造材料影响之间的差异。
在 6 和 15 MV X 射线束的固体水体模上进行测量。使用两种类型的剂量仪：OSLD，InLight/OSL Nanodot 剂量仪（Landauer，Inc.，Glenwood，IL）和 P 型表面二极管（Standard Imaging，Madison，WI）。这些探测器的附加物帽由 M3 制成，M3 是一种与水等效的材料，以及 Al、Cu 和 Pb 板材制成。此外，还使用了市售的由塑料水和黄铜制成的半球形附加物帽（Landauer，Inc.，Glenwood，IL）与 Nanodots 一起使用。OSLD 用 InLight 微星阅读器（Landauer，Inc.，Glenwood，IL）读取。使用 XiO 治疗计划系统（CMS/Elekta，Stockholm）进行剂量计算，并进行组织异质性校正。
对于 OSLD 和二极管，当没有附加物帽时进行测量时，射野大小从 3 cm×3 cm 变为 30 cm×30 cm，C（FS）的变化率为 200%。当使用与最大剂量深度相等的壁厚度的附加物帽时，C（FS）的变化率降低至约 4%。具有较大壁厚度的附加物帽不会进一步降低 C（FS）。附加物帽制造材料对 C（FS）几乎没有或没有影响。将带有附加物帽的探测器放置在患者表面上会对所传递的剂量产生干扰，测量结果为 4%-7%。对 6 和 15 MV X 射线进行了 6 和 15 MV X 射线的计算剂量与使用 Nanodot 和二极管测量的剂量之间的比较。当仔细确定 C（FS）因子并将其应用于体模上的测量时，发现测量和计算的剂量之间的差异在±1.3%之间。
具有适当附加物帽的 OSLD 和二极管可用于测量患者表面的剂量，并在 100 cGy 范围内预测深度 dmax 处的传递剂量，其精度在±1.3%之间。附加物帽：可以用本工作中检查的任何材料制成，最好的是壁厚度为 dmax，并且当壁厚度为 dmax 时，对传递剂量的干扰为 4%-7%。带有附加物帽的 OSLD 和二极管都可以对传递剂量进行准确测量。
