Geng Hui, Yu Siu-Ki, Lam Wai-Wang, Wong Wing-Kei Rebecca, Ho Yick-Wing, Liu Sau-Fan
Department of Radiotherapy, Hong Kong Sanatorium and Hospital, HongKong.
Med Dosim. 2011 Autumn;36(3):306-12. doi: 10.1016/j.meddos.2010.06.002. Epub 2010 Nov 19.
Tomotherapy adaptive dose calculation offers the ability to verify and adjust the therapeutic plan during the treatment. Using tomotherapy adaptive dose calculation, the planned fluence pattern can be used to recalculate the dose distribution on pretreatment megavoltage computed tomography (MVCT) images. Zipper artifacts, which appear as increased density in the central region of MVCT images, may affect the accuracy of adaptive dose recalculation. The purpose of this study was to evaluate the dosimetric effects of zipper artifacts on tomotherapy adaptive dose calculation. MVCT images of a cylindrical water phantom of 22-cm diameter were acquired on a tomotherapy system. The zipper artifacts were enclosed by a cylindrical planning target volume (PTV) contoured on these images. For comparison, artifact-free images were created by replacing the computed tomography (CT) numbers of zipper artifacts with the mean CT number of water. Treatment plans were generated by giving a uniform dose of 2 Gy to the PTV based on these modified images; it was then applied to the images that have the zipper artifacts. The impacts of different pitch ratios on the artifacts were assessed. The dose distribution differences between the 2 sets of images were compared. The absorbed dose that covered 95% volume of PTV and maximum dose, minimum dose, and mean dose of the PTV were also calculated and compared. The water phantom was scanned on the tomotherapy system twice per week for 12 consecutive weeks. The mean CT number of zipper artifacts (101 HU) was three times higher than that of water (34 HU). The CT number value and location of zipper artifacts were not affected by the pitch ratio. Gamma analysis was performed between the original and recalculated dose distributions. The discrepancies between the isodose distributions calculated by two sets of images were within 1%/1-mm tolerance. The dosimetric impact from zipper artifacts was found insignificant such that the recalculated dose was underestimated by less than 0.5%.
断层放疗自适应剂量计算能够在治疗过程中验证并调整治疗计划。利用断层放疗自适应剂量计算，可使用计划的注量模式在治疗前兆伏级计算机断层扫描（MVCT）图像上重新计算剂量分布。拉链伪影在MVCT图像的中心区域表现为密度增加，可能会影响自适应剂量重新计算的准确性。本研究的目的是评估拉链伪影对断层放疗自适应剂量计算的剂量学影响。在断层放疗系统上获取直径为22 cm的圆柱形水模的MVCT图像。在这些图像上勾勒出一个圆柱形计划靶区（PTV），将拉链伪影包含在内。为作比较，通过用水平均CT值替换拉链伪影的CT值来创建无伪影图像。根据这些修改后的图像，通过给PTV均匀给予2 Gy的剂量来生成治疗计划；然后将其应用于带有拉链伪影的图像。评估了不同螺距比伪影的影响。比较了两组图像之间的剂量分布差异。还计算并比较了覆盖PTV 95%体积的吸收剂量以及PTV的最大剂量、最小剂量和平均剂量。连续12周每周在断层放疗系统上对水模扫描两次。拉链伪影的平均CT值（101 HU）比水的平均CT值（34 HU）高三倍。拉链伪影的CT值和位置不受螺距比的影响。对原始剂量分布和重新计算的剂量分布进行了伽马分析。两组图像计算出的等剂量分布之间的差异在1%/1 mm公差范围内。发现拉链伪影的剂量学影响不显著，重新计算的剂量低估不到0.5%。
