Hammer Cliff G, Rosen Benjamin Saul, Fagerstrom Jessica M, Culberson Wesley S, DeWerd Larry A
Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin-Madison, Madison, WI, 53705, USA.
Department of Radiation Oncology, University of Michigan, Ann Arbor, MI, 48109, USA.
Med Phys. 2018 Jan;45(1):448-459. doi: 10.1002/mp.12682. Epub 2017 Dec 22.
To determine experimentally the intrinsic energy response, k , of EBT3 GafChromic radiochromic film with kilovoltage x rays, Cs, and Co in therapeutic and diagnostic dose ranges through direct measurement with an accompanying mathematical approach to describe the physical processes involved.
The EBT3 film was irradiated with known doses using Co, Cs, and 13 NIST-matched kilovoltage x-ray beams. Seven dose levels, ranging from 57 to 7002 mGy, were chosen for this work. Monte Carlo methods were used to convert air-kerma rates to dose rates to the film active layer for each energy. A total of 738 film dosimeters, each measuring (1.2 × 1.2) cm , were cut from three film sheets out of the same lot of the latest version of EBT3 film, to allow for multiple dosimeters to be irradiated by each target dose and beam quality as well as unirradiated dosimeters to be used as controls. Net change in optical density in excess of the unirradiated controls was measured using the UWMRRC Laser Densitometry System (LDS). The dosimeter intrinsic energy response, k , for each dose level was determined relative to Co, as the ratio of dosimeter response to each beam quality relative to the absorbed dose to the film active volume at the same dose level. A simplified, single-hit mathematical model was used to derive a single-free-parameter, β, which is a proportionality constant that is dependent on beam quality and describes the microdosimetric interactions within the active layer of film. The response of β for each beam quality relative to Co was also determined.
k was determined for a wide range of doses and energies. The results show a unique variation of k as a function of energy, and agree well with results from other investigations. There was no measurable dose dependence for k within the 500-7002 mGy range outside of the expanded measurement uncertainty of 3.65% (k = 2). For doses less than 500 mGy, the signal-to-noise ratio was too low to determine k accurately. The single-free-parameter, β, fit calculations derived from the single-hit model show a correlation with k that suggests that β, at least in part, characterizes the microdosimetric interactions that determine k .
For the beam qualities investigated, a single energy-dependent k correction can be used for doses between 500 and 7002 mGy. Using the single-hit model with the single-free-parameter fit to solve for β shows promise in the determination of the intrinsic energy response of film, with β being the mathematical analog of the measured k .
通过直接测量并采用伴随的数学方法来描述所涉及的物理过程，以实验方式确定EBT3 GafChromic放射变色薄膜在治疗和诊断剂量范围内对千伏级X射线、铯源和钴源的固有能量响应系数k。
使用钴源、铯源和13个与美国国家标准与技术研究院（NIST）匹配的千伏级X射线束，对EBT3薄膜进行已知剂量的照射。为此项工作选择了7个剂量水平，范围从57至7002毫戈瑞。采用蒙特卡罗方法将空气比释动能率转换为薄膜活性层的剂量率，针对每种能量进行计算。从同一批次最新版本的EBT3薄膜中裁剪出3张薄膜片，共得到738个薄膜剂量计，每个剂量计尺寸为（1.2×1.2）平方厘米，以便每个目标剂量和束流质量能照射多个剂量计，同时将未照射的剂量计用作对照。使用华盛顿大学医学放射研究中心激光密度测定系统（LDS）测量超过未照射对照的光密度净变化。相对于钴源，确定每个剂量水平下剂量计的固有能量响应系数k，即每个束流质量下剂量计响应与同一剂量水平下薄膜活性体积吸收剂量的比值。采用简化的单碰撞数学模型推导出一个单自由参数β，它是一个比例常数，取决于束流质量，描述了薄膜活性层内的微剂量相互作用。还确定了每种束流质量相对于钴源的β响应。
在广泛的剂量和能量范围内确定了k。结果显示k随能量呈现独特变化，与其他研究结果吻合良好。在3.65%（k = 2）的扩展测量不确定度之外，500 - 7002毫戈瑞范围内k没有可测量的剂量依赖性。对于小于500毫戈瑞的剂量，信噪比过低，无法准确确定k。从单碰撞模型得出的单自由参数β拟合计算结果显示与k存在相关性，这表明β至少部分表征了决定k的微剂量相互作用。
对于所研究的束流质量，在500至7002毫戈瑞的剂量范围内，可使用单一的能量依赖性k校正。使用具有单自由参数拟合的单碰撞模型求解β，在确定薄膜的固有能量响应方面显示出前景，其中β是所测量k的数学类似物。
