Department of Radiation Oncology, Samsung Medical Center Sungkyunkwan University School of Medicine, #50 Ilwon-dong, Kangnam-ku, Seoul 135-710, Korea.
Med Phys. 2010 Jun;37(6):2925-33. doi: 10.1118/1.3395574.
Accurate dosimetry is essential to ensure the quality of advanced radiation treatments, such as intensity modulated radiation therapy (IMRT). Therefore, a comparison study was conducted to assess the accuracy of various film dosimetry techniques that are widely used in clinics.
A simulated IMRT plan that produced an inverse pyramid dose distribution in a perpendicular plane of the beam axis was designed with 6 MV x rays to characterize the large contribution of scattered photons to low dose regions. Three film dosimetry techniques, EDR2, EDR2 with low-energy photon absorption lead filters (EDR2 WF), and GafChromic EBT, were compared to ionization chamber measurements as well as Monte Carlo (MC) simulations. The accuracy of these techniques was evaluated against the ionization chamber data. Two-dimensional comparisons with MC simulation results were made by computing the gamma index (gamma) with criteria ranging from 2% of dose difference or 2 mm of distance to agreement (2%/2 mm) to 4%/4 mm on the central vertical plane (20 x 20 cm2) of a square solid water phantom. Depth doses and lateral profiles at depths of 5, 10, and 15 cm were examined to characterize the deviation of film measurements and MC predictions from ionization chamber measurements.
In depth dose comparisons, the deviation between the EDR2 films was 9% in the low dose region and 5% in high dose region, on average. With lead filters, the average deviation was reduced to -1.3% and -0.3% in the low dose and high dose regions, respectively. EBT film results agreed within 1.5% difference on average with ionization chamber measurements in low and high dose regions. In two-dimensional comparisons with MC simulation, EDR2 films passed gamma tests with a 2%/2 mm criterion only in the high dose region (gamma < or = 1, total of 63.06% of the tested region). In the low dose region, EDR2 films passed gamma tests with 3%/3 mm criterion (gamma < or = 1, total of 98.4% of the tested region). For EDR2 WF and GafChromic EBT films, gamma tests with a 2% /2 mm criterion (gamma < or = 1) in the tested area was 97.3% and 96.8% of the tested region, respectively.
The EDR2 film WF and GafChromic EBT film achieved an average accuracy level of 1.5% against an ionization chamber. These two techniques agreed with the MC prediction in 2%/2mm criteria evaluated by the gamma index, whereas EDR2 without filters achieved an accuracy level of 3%/3 mm with the decision criteria of agreement greater than 95% of the tested region. The overall results will provide a useful quantitative reference for IMRT verifications.
精确的剂量测定对于确保诸如调强放射治疗（IMRT）等先进放射治疗的质量至关重要。因此，进行了一项比较研究，以评估在临床中广泛使用的各种胶片剂量测定技术的准确性。
使用 6 MV X 射线设计了一个在射束轴垂直平面中产生逆金字塔剂量分布的模拟 IMRT 计划，以表征散射线光子对低剂量区域的大贡献。将三种胶片剂量测定技术（EDR2、带有低能光子吸收铅滤器的 EDR2（EDR2 WF）和 GafChromic EBT）与电离室测量以及蒙特卡罗（MC）模拟进行了比较。通过计算伽马指数（gamma），将这些技术的准确性与电离室数据进行了评估，伽马指数的标准范围从剂量差异的 2%或距离的 2 毫米到中心垂直平面（20 x 20 cm2）的 4%/4 毫米。在方形水模体的中心垂直平面上，通过计算伽马指数（gamma），对二维比较结果与 MC 模拟结果进行了比较，伽马指数的标准范围从剂量差异的 2%或距离的 2 毫米到中心垂直平面（20 x 20 cm2）的 4%/4 毫米。在 5、10 和 15 cm 的深度处检查了深度剂量和横向分布，以表征胶片测量值和 MC 预测值与电离室测量值之间的偏差。
在深度剂量比较中，EDR2 胶片的平均偏差在低剂量区域为 9%，在高剂量区域为 5%。使用铅滤器后，低剂量和高剂量区域的平均偏差分别降低至-1.3%和-0.3%。EBT 胶片的平均偏差在低剂量和高剂量区域分别为 1.5%和 1.5%。在与 MC 模拟的二维比较中，EDR2 胶片仅在高剂量区域通过了 2%/2mm 标准的伽马测试（gamma < 或 = 1，测试区域的总共有 63.06%）。在低剂量区域，EDR2 胶片通过了 3%/3mm 标准的伽马测试（gamma < 或 = 1，测试区域的总共有 98.4%）。对于 EDR2 WF 和 GafChromic EBT 胶片，在测试区域中通过 2%/2mm 标准（gamma < 或 = 1）的伽马测试的总比例分别为 97.3%和 96.8%。
EDR2 胶片 WF 和 GafChromic EBT 胶片的平均精度水平为 1.5%，与电离室相比。这两种技术在 2%/2mm 标准下与 MC 预测一致，通过伽马指数进行评估，而没有滤器的 EDR2 胶片则以 95%以上的测试区域的一致性决策标准达到了 3%/3mm 的精度水平。总体结果将为 IMRT 验证提供有用的定量参考。
