Flynn Ryan T, Hartmann Julia, Bani-Hashemi Ali, Nixon Earl, Alfredo R, Siochi C, Pennington Edward C, Bayouth John E
Department of Radiation Oncology, University of Iowa, Iowa City, Iowa 52245, USA.
Med Phys. 2009 Jun;36(6):2181-92. doi: 10.1118/1.3125663.
Imaging dose from megavoltage cone beam computed tomography (MVCBCT) can be significantly reduced without loss of image quality by using an imaging beam line (IBL), with no flattening filter and a carbon, rather than tungsten, electron target. The IBL produces a greater keV-range x-ray fluence than the treatment beam line (TBL), which results in a more optimal detector response. The IBL imaging dose is not necessarily negligible, however. In this work an IBL was dosimetrically modeled with the Philips Pinnacle3 treatment planning system (TPS), verified experimentally, and applied to clinical cases. The IBL acquisition dose for a 200 degrees gantry rotation was verified in a customized acrylic cylindrical phantom at multiple imaging field sizes with 196 ion chamber measurements. Agreement between the measured and calculated IBL dose was quantified with the 3D gamma index. Representative IBL and TBL imaging dose distributions were calculated for head and neck and prostate patients and included in treatment plans using the imaging dose incorporation (IDI) method. Surface dose was measured for the TBL and IBL for four head and neck cancer patients with MOSFETs. The IBL model, when compared to the percentage depth dose and profile measurements, had 97% passing gamma indices for dosimetric and distance acceptance criteria of 3%, 3 mm, and 100% passed for 5.2%, 5.2 mm. For the ion chamber measurements of phantom image acquisition dose, the IBL model had 93% passing gamma indices for acceptance criteria of 3%, 3 mm, and 100% passed for 4%, 4 mm. Differences between the IBL- and TBL-based IMRT treatment plans created with the IDI method were dosimetrically insignificant for both the prostate and head and neck cases. For IBL and TBL beams with monitor unit values that would result in the delivery of the same dose to the depth of maximum dose under standard calibration conditions, the IBL imaging surface dose was higher than the TBL imaging surface dose by an average of 18%, with a standard deviation of 8% (p = 2 x 10(-6)). The IBL can be modeled with acceptable accuracy using a standard TPS, and accounting for IBL dose in treatment plans with the IDI method is straightforward. The resulting composite dose distributions, assuming similar imaging doses, are negligibly different from those of the TBL. The increased IBL surface dose relative to the TBL is likely clinically insignificant.
通过使用成像束线（IBL）、不使用均整滤波器且采用碳而非钨作为电子靶，可以在不损失图像质量的情况下显著降低兆伏级锥形束计算机断层扫描（MVCBCT）的成像剂量。与治疗束线（TBL）相比，IBL产生的keV范围的X射线注量更大，这导致探测器响应更优化。然而，IBL的成像剂量不一定可以忽略不计。在这项工作中，使用飞利浦Pinnacle3治疗计划系统（TPS）对IBL进行了剂量学建模，通过实验进行了验证，并应用于临床病例。在定制的丙烯酸圆柱模体中，通过196次离子室测量，在多个成像野大小下验证了200度机架旋转时的IBL采集剂量。使用3D伽马指数对测量和计算的IBL剂量之间的一致性进行了量化。针对头颈部和前列腺患者计算了代表性的IBL和TBL成像剂量分布，并使用成像剂量纳入（IDI）方法纳入治疗计划中。使用MOSFET对4名头颈部癌患者的TBL和IBL的表面剂量进行了测量。与百分深度剂量和剖面测量相比，IBL模型在剂量学和距离接受标准为3%、3 mm时，伽马指数通过率为97%，在5.2%、5.2 mm时通过率为100%。对于模体图像采集剂量的离子室测量，IBL模型在接受标准为3%、3 mm时，伽马指数通过率为93%，在4%、4 mm时通过率为100%。使用IDI方法创建的基于IBL和TBL的调强放射治疗（IMRT）治疗计划之间的差异，在前列腺和头颈部病例中，剂量学上均无显著意义。对于在标准校准条件下监测单位值会导致在最大剂量深度处给予相同剂量的IBL和TBL射束，IBL成像表面剂量比TBL成像表面剂量平均高18%，标准差为8%（p = 2×10⁻⁶）。使用标准TPS可以以可接受的精度对IBL进行建模，并且使用IDI方法在治疗计划中考虑IBL剂量很简单。假设成像剂量相似，由此产生的复合剂量分布与TBL的复合剂量分布差异可忽略不计。相对于TBL，IBL表面剂量的增加在临床上可能无显著意义。
