Kirov Assen S, Caravelli Gregory, Palm Asa, Chui Chen, LoSasso Thomas
Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, 1275 York Avenue, New York, New York 10021, USA.
Med Phys. 2006 Oct;33(10):3690-9. doi: 10.1118/1.2229425.
The higher sensitivity to low-energy scattered photons of radiographic film compared to water can lead to significant dosimetric error when the beam quality varies significantly within a field. Correcting for this artifact will provide greater accuracy for intensity modulated radiation therapy (IMRT) verification dosimetry. A procedure is developed for correction of the film energy-dependent response by creating a pencil beam kernel within our treatment planning system to model the film response specifically. Film kernels are obtained from EGSnrc Monte Carlo simulations of the dose distribution from a 1 mm diameter narrow beam in a model of the film placed at six depths from 1.5 to 40 cm in polystyrene and solid water phantoms. Kernels for different area phantoms (50 x 50 cm2 and 25 x 25 cm2 polystyrene and 30 x 30 cm2 solid water) are produced. The Monte Carlo calculated kernel is experimentally verified with film, ion chamber and thermoluminescent dosimetry (TLD) measurements in polystyrene irradiated by a narrow beam. The kernel is then used in convolution calculations to, predict the film response in open and IMRT fields. A 6 MV photon beam and Kodak XV2 film in a polystyrene phantom are selected to test the method as they are often used in practice and can result in large energy-dependent artifacts. The difference in dose distributions calculated with the film kernel and the water kernel is subtracted from film measurements to obtain a practically film artifact free IMRT dose distribution for the Kodak XV2 film. For the points with dose exceeding 5 cGy (11% of the peak dose) in a large modulated field and a film measurement inside a large polystyrene phantom at depth of 10 cm, the correction reduces the fraction of pixels for which the film dose deviates from dose to water by more than 5% of the mean film dose from 44% to 6%.
与水相比，放射胶片对低能散射光子具有更高的灵敏度，当射野内射束质变化显著时，这可能导致显著的剂量误差。校正此伪影将为调强放射治疗（IMRT）验证剂量测定提供更高的准确性。通过在我们的治疗计划系统中创建一个笔形束核来专门模拟胶片响应，开发了一种校正胶片能量依赖响应的程序。胶片核是通过EGSnrc蒙特卡罗模拟获得的，模拟对象是在聚苯乙烯和固体水模体中，从1.5至40 cm的六个深度处放置的胶片模型中，直径1 mm窄束的剂量分布。生成了不同面积模体（50×50 cm2和25×25 cm2聚苯乙烯以及30×30 cm2固体水）的核。通过窄束照射聚苯乙烯时的胶片、电离室和热释光剂量测定（TLD）测量，对蒙特卡罗计算的核进行了实验验证。然后将该核用于卷积计算，以预测开放野和IMRT野中的胶片响应。选择聚苯乙烯模体中的6 MV光子束和柯达XV2胶片来测试该方法，因为它们在实际中经常使用，并且可能导致较大的能量依赖伪影。从胶片测量值中减去用胶片核和水核计算的剂量分布差异，以获得实际上无胶片伪影的柯达XV2胶片的IMRT剂量分布。对于大的调强野中剂量超过5 cGy（峰值剂量的11%）的点以及在10 cm深度的大聚苯乙烯模体内的胶片测量，校正将胶片剂量偏离水剂量超过平均胶片剂量5%的像素比例从44%降低到6%。
