Department of Radiation Oncology, Asahi University Hospital, Gifu, Japan.
Department of Radiology, Gifu University, Gifu, Japan.
J Appl Clin Med Phys. 2021 Sep;22(9):271-279. doi: 10.1002/acm2.13384. Epub 2021 Aug 10.
We evaluated the effect of changing the scan mode of the Elekta X-ray volume imaging cone beam computed tomography (CBCT) on the accuracy of dose calculation, which may be affected by computed tomography (CT) value errors in three dimensions.
We used the electron density phantom and measured the CT values in three dimensions. CT values were compared with planning computed tomography (pCT) values for various materials. The evaluated scan modes were for head and neck (S-scan), chest (M-scan), and pelvis (L-scan) with various collimators and filter systems. To evaluate the effects of the CT value error of the CBCT on dose error, Monte Carlo calculations of dosimetry were performed using pCT and CBCT images.
The L-scan had a CT value error of approximately 800 HU at the isocenter compared with the pCT. Furthermore, inhomogeneity in the longitudinal CT value profile was observed in the bone material. The dose error for ±100 HU difference in CT values for the S-scan and M-scan was within ±2%. The center of the L-scan had a CT error of approximately 800 HU and a dose error of approximately 6%. The dose error of the L-scan occurred in the beam path in the case of both single field and two parallel opposed fields, and the maximum error occurred at the center of the phantom in the case of both the 4-field box and single-arc techniques.
We demonstrated the three-dimensional CT value characteristics of the CBCT by evaluating the CT value error obtained under various imaging conditions. It was found that the L-scan is considerably affected by not having a unique bowtie filter, and the S-scan without the bowtie filter causes CT value errors in the longitudinal direction. Moreover, the CBCT dose errors for the 4-field box and single-arc irradiation techniques converge to the isocenter.
我们评估了改变 Elekta X 射线容积成像锥形束 CT（CBCT）扫描模式对剂量计算准确性的影响，这种影响可能会受到三维 CT 值误差的影响。
我们使用电子密度体模测量了三维 CT 值。将各种材料的 CT 值与计划 CT（pCT）值进行了比较。评估的扫描模式分别为头颈部（S 扫描）、胸部（M 扫描）和骨盆（L 扫描），使用了各种准直器和过滤系统。为了评估 CBCT 的 CT 值误差对剂量误差的影响，使用 pCT 和 CBCT 图像进行了蒙特卡罗剂量计算。
与 pCT 相比，L 扫描在等中心处的 CT 值误差约为 800 HU。此外，在骨材料中观察到纵向 CT 值分布的不均匀性。S 扫描和 M 扫描的 CT 值相差±100 HU 时，剂量误差在±2%以内。L 扫描的中心处的 CT 误差约为 800 HU，剂量误差约为 6%。在单野和两平行对向野的情况下，L 扫描的剂量误差发生在射束路径中，在 4 野箱和单弧技术的情况下，最大误差发生在体模的中心。
我们通过评估在各种成像条件下获得的 CT 值误差，展示了 CBCT 的三维 CT 值特征。结果表明，L 扫描受到没有独特的蝴蝶结滤线器的显著影响，没有蝴蝶结滤线器的 S 扫描会导致纵向 CT 值误差。此外，4 野箱和单弧照射技术的 CBCT 剂量误差向等中心收敛。
