Devic Slobodan, Tomic Nada, Aldelaijan Saad, Deblois Francois, Seuntjens Jan, Chan Maria F, Lewis Dave
Medical Physics Unit, McGill University, Montréal, Québec H3G 1A4, Canada.
Med Phys. 2012 Aug;39(8):4850-7. doi: 10.1118/1.4736800.
Despite numerous advantages of radiochromic film dosimeter (high spatial resolution, near tissue equivalence, low energy dependence) to measure a relative dose distribution with film, one needs to first measure an absolute dose (following previously established reference dosimetry protocol) and then convert measured absolute dose values into relative doses. In this work, we present result of our efforts to obtain a functional form that would linearize the inherently nonlinear dose-response curve of the radiochromic film dosimetry system.
Functional form [ζ = (-1)[middle dot]netOD((2∕3))∕ln(netOD)] was derived from calibration curves of various previously established radiochromic film dosimetry systems. In order to test the invariance of the proposed functional form with respect to the film model used we tested it with three different GAFCHROMIC™ film models (EBT, EBT2, and EBT3) irradiated to various doses and scanned on a same scanner. For one of the film models (EBT2), we tested the invariance of the functional form to the scanner model used by scanning irradiated film pieces with three different flatbed scanner models (Epson V700, 1680, and 10000XL). To test our hypothesis that the proposed functional argument linearizes the response of the radiochromic film dosimetry system, verification tests have been performed in clinical applications: percent depth dose measurements, IMRT quality assurance (QA), and brachytherapy QA.
Obtained R(2) values indicate that the choice of the functional form of the new argument appropriately linearizes the dose response of the radiochromic film dosimetry system we used. The linear behavior was insensitive to both film model and flatbed scanner model used. Measured PDD values using the green channel response of the GAFCHROMIC™ EBT3 film model are well within ±2% window of the local relative dose value when compared to the tabulated Cobalt-60 data. It was also found that criteria of 3%∕3 mm for an IMRT QA plan and 3%∕2 mm for a brachytherapy QA plan are passing 95% gamma function points.
In this paper, we demonstrate the use of functional argument to linearize the inherently nonlinear response of a radiochromic film based reference dosimetry system. In this way, relative dosimetry can be conveniently performed using radiochromic film dosimetry system without the need of establishing calibration curve.
尽管放射变色薄膜剂量计在测量胶片相对剂量分布方面具有诸多优势（高空间分辨率、接近组织等效性、低能量依赖性），但仍需首先测量绝对剂量（遵循先前确立的参考剂量测定方案），然后将测量得到的绝对剂量值转换为相对剂量。在本研究中，我们展示了为获得一种能使放射变色薄膜剂量测定系统固有非线性剂量响应曲线线性化的函数形式所做的努力成果。
函数形式[ζ = (-1)·netOD((2∕3))∕ln(netOD)]源自各种先前确立的放射变色薄膜剂量测定系统的校准曲线。为了测试所提出的函数形式相对于所使用的薄膜模型的不变性，我们用三种不同的GAFCHROMIC™薄膜模型（EBT、EBT2和EBT3）进行了测试，这些薄膜接受不同剂量照射后在同一台扫描仪上进行扫描。对于其中一种薄膜模型（EBT2），我们通过使用三种不同的平板扫描仪模型（爱普生V700、1680和10000XL）扫描照射后的薄膜片，测试了函数形式相对于所使用的扫描仪模型的不变性。为了检验我们的假设，即所提出的函数自变量能使放射变色薄膜剂量测定系统的响应线性化，我们在临床应用中进行了验证测试：百分深度剂量测量、调强放射治疗质量保证（QA）和近距离放射治疗QA。
所得的R(2)值表明新自变量的函数形式选择能恰当地使我们所使用的放射变色薄膜剂量测定系统的剂量响应线性化。这种线性行为对所使用的薄膜模型和平板扫描仪模型均不敏感。与列表中的钴 - 60数据相比，使用GAFCHROMIC™ EBT3薄膜模型绿色通道响应测量的百分深度剂量（PDD）值在局部相对剂量值的±2%范围内。还发现调强放射治疗QA计划的3%∕(3)毫米标准和近距离放射治疗QA计划的3%∕(2)毫米标准通过了95%的伽马函数点。
在本文中，我们展示了使用函数自变量使基于放射变色薄膜的参考剂量测定系统的固有非线性响应线性化。通过这种方式，使用放射变色薄膜剂量测定系统可以方便地进行相对剂量测定，而无需建立校准曲线。
