Hao Yao, Cai Bin, Green Olga, Knutson Nels, Yaddanapudi Sridhar, Zhao Tianyu, Rodriguez Vivian, Schmidt Matthew, Mutic Sasa, Sun Baozhou
Department of Radiation Oncology, Washington University School of Medicine, 660 S. Euclid Ave, St. Louis, MO, 63110, USA.
Department of Radiation Oncology, University of Iowa, 200 Hawkins Drive, LL-W Pomerantz Family Pavilion, Iowa City, IA, 52242-1089, USA.
Med Phys. 2021 Apr;48(4):1533-1539. doi: 10.1002/mp.14757. Epub 2021 Mar 11.
The current approach to Linac beam dosimetry verification is typically performed utilizing a three-dimensional (3D) water tank system. The 3D beam scanning process is cumbersome, labor intensive, error-prone, and costly. This is especially challenging for the new Ethos system and MR Linacs with a ring gantry. This work proposes an alternative approach to verify 6FFF beam dosimetry for Ethos, ViewRay MRIdian® Linac, and other Linacs with 6FFF beam quality using two-dimensional (2D) ion chamber arrays.
Percentage depth dose (PDD) and profiles of an Ethos, an MRIdian® Linac, and several Linacs with 6FFF beams were measured at the nominal beam current. The beam energy was detuned by changing the bending magnet current on one TrueBeam. PDDs and profiles were measured for detuned beam energies. The peak shape of the 6FFF profile was defined by a "slope" parameter and unflatness. Correlations between peak slope and unflatness metrics vs PDDs were used to evaluate the sensitivity of beam energy to beam profile changes at different field sizes and depths.
Strong correlations were found between peak slope and PDDs for all Linacs with 6FFF beam. The R-squared values in the linear regression fitting between PDD and peak slope and unflatness were 0.99 and 0.84, respectively. Both profile slope and unflatness were proportional to PDD at the 10 cm depth and the peak slope was 4.3 times more sensitive than PDD. We have identified that measurements with a shallow depth are preferred to quantify the beam energy consistency.
Our work shows the feasibility of verifying 6FFF beam quality of Ethos, MR Linac, and other Linacs by defining a profile slope measured from 2D ionization chambers array devices. This new approach provides a simplified method for performing a routine beam quality check without using a 3D water tank system while maximizing cost effectiveness and efficiency.
目前直线加速器束流剂量学验证的方法通常是利用三维（3D）水箱系统进行。三维束流扫描过程繁琐、 labor intensive、容易出错且成本高昂。这对于新型Ethos系统和带有环形机架的磁共振直线加速器来说尤其具有挑战性。这项工作提出了一种替代方法，使用二维（2D）电离室阵列来验证Ethos、ViewRay MRIdian®直线加速器以及其他具有6FFF束流质量的直线加速器的6FFF束流剂量学。
在标称束流电流下测量了一台Ethos、一台MRIdian®直线加速器以及几台具有6FFF束流的直线加速器的百分深度剂量（PDD）和剖面。通过改变一台TrueBeam上的弯曲磁铁电流来使束流能量失谐。测量了失谐束流能量下的PDD和剖面。6FFF剖面的峰值形状由一个“斜率”参数和不平整度定义。利用峰值斜率和不平整度指标与PDD之间的相关性来评估在不同射野尺寸和深度下束流能量对束流剖面变化的敏感性。
在所有具有6FFF束流的直线加速器中，均发现峰值斜率与PDD之间存在强相关性。PDD与峰值斜率和不平整度的线性回归拟合中的R平方值分别为0.99和0.84。在10 cm深度处，剖面斜率和不平整度均与PDD成正比，且峰值斜率比PDD敏感4.3倍。我们已经确定，为了量化束流能量一致性，采用浅深度测量更为可取。
我们的工作表明，通过定义从二维电离室阵列设备测量得到的剖面斜率来验证Ethos、磁共振直线加速器和其他直线加速器的6FFF束流质量是可行的。这种新方法提供了一种简化的方法，无需使用三维水箱系统即可进行常规束流质量检查，同时最大限度地提高了成本效益和效率。
