Department of Radiation Oncology, Jewish General Hospital, Montréal, Québec, Canada; Medical Physics Unit, McGill University, Montréal, Québec, Canada.
Department of Radiation Oncology, Jewish General Hospital, Montréal, Québec, Canada; Medical Physics Unit, McGill University, Montréal, Québec, Canada.
Phys Med. 2019 Jun;62:105-110. doi: 10.1016/j.ejmp.2019.04.015. Epub 2019 May 11.
In this work we use Monte Carlo simulations to investigate change in Computed tomography (CT) X-ray energy spectra between exposures in air and within CT dose index (CTDI) phantom. While the results of these simulations will be relevant when measuring CTDI with any dosimeter, we apply the appropriate beam quality change correction for CTDI measurements using XR-QA2 model GafChromic™ film.
Dose profiles were measured with film strips, sandwiched between acrylic rods cut in half, placed within CTDI phantoms and scanned before and after irradiation with document scanner in reflective mode. Reference dosimetry system was calibrated in terms of air kerma in air, which was converted into absorbed dose using ratio of mass-energy absorption coefficients water-to-air for a given beam quality, following the AAPM TG-61 protocol.
Beam qualities for all film positions within CTDI phantom show beam softening for HVLs above 6 mm Al and beam hardening for HVLs bellow 6 mm Al. Calculated CTDI values using HVL in air for all CTDI positions, and those calculated using the appropriate calibration curves based on beam quality correction show for Head CTDI phantom differences ranging from 0.3% to 2.1% and for Body CTDI phantom from 2.5% to 5.7%.
We describe method for CTDI measurements using radiochromic film dosimetry protocol corrected by the beam quality change within the phantom. Our results show differences in CTDI measurements of up to 5.7% when compared to using film calibration curves for beam quality in air.
在这项工作中，我们使用蒙特卡罗模拟来研究空气和 CT 剂量指数（CTDI）体模内曝光之间的 CT 射线能量谱的变化。虽然这些模拟的结果在使用任何剂量仪测量 CTDI 时都是相关的，但我们将 XR-QA2 模型 GafChromic™胶片的适当束质变化校正应用于 CTDI 测量。
使用胶片条进行剂量分布测量，将胶片条夹在切成两半的丙烯酸棒之间，放置在 CTDI 体模内，在照射前后用反射模式的文档扫描仪进行扫描。参考剂量测量系统是根据空气比释动能校准的，空气比释动能是在给定束质下，使用水-空气质量能量吸收系数的比值转换为吸收剂量，遵循 AAPM TG-61 协议。
CTDI 体模内所有胶片位置的束质都显示 HVL 大于 6mm Al 时的束质软化和 HVL 小于 6mm Al 时的束质硬化。使用空气 HVL 计算所有 CTDI 位置的 CTDI 值，以及使用基于束质校正的适当校准曲线计算的 CTDI 值，对头 CTDI 体模的差异范围为 0.3%至 2.1%，体 CTDI 体模的差异范围为 2.5%至 5.7%。
我们描述了一种使用放射性铬胶片剂量计协议进行 CTDI 测量的方法，该协议通过体模内的束质变化进行校正。我们的结果显示，与使用空气束质校准曲线相比，CTDI 测量的差异高达 5.7%。
