电子弧形治疗的监测单位的蒙特卡罗计算。

Radiation Medicine Program, Princess Margaret Hospital, University Health Network, Toronto, Ontario M5G 2M9, Canada.

Med Phys. 2010 Apr;37(4):1571-8. doi: 10.1118/1.3359819.

PURPOSE

Monitor unit (MU) calculations for electron are therapy were carried out using Monte Carlo simulations and verified by measurements. Variations in the dwell factor (DF), source-to-surface distance (SSD), and treatment are angle (a) were studied. Moreover, the possibility of measuring the DF, which requires gantry rotation, using a solid water rectangular, instead of cylindrical, phantom was investigated.

METHODS

A phase space file based on the 9 MeV electron beam with rectangular cutout (physical size = 2.6 x 21 cm2) attached to the block tray holder of a Varian 21 EX linear accelerator (linac) was generated using the EGSnrc-based Monte Carlo code and verified by measurement. The relative output factor (ROF), SSD offset, and DF, needed in the MU calculation, were determined using measurements and Monte Carlo simulations. An ionization chamber, a radiographic film, a solid water rectangular phantom, and a cylindrical phantom made of polystyrene were used in dosimetry measurements.

RESULTS

Percentage deviations of ROF, SSD offset, and DF between measured and Monte Carlo results were 1.2%, 0.18%, and 1.5%, respectively. It was found that the DF decreased with an increase in a, and such a decrease in DF was more significant in the a range of 0 degrees-60 degrees than 60 degrees-120 degrees. Moreover, for a fixed a, the DF increased with an increase in SSD. Comparing the DF determined using the rectangular and cylindrical phantom through measurements and Monte Carlo simulations, it was found that the DF determined by the rectangular phantom agreed well with that by the cylindrical one within +/- 1.2%. It shows that a simple setup of a solid water rectangular phantom was sufficient to replace the cylindrical phantom using our specific cutout to determine the DF associated with the electron arc.

CONCLUSIONS

By verifying using dosimetry measurements, Monte Carlo simulations proved to be an alternative way to perform MU calculations effectively for electron are therapy. Since Monte Carlo simulations can generate a precalculated database of ROF, SSD offset, and DF for the MU calculation, with a reduction in human effort and linac beam-on time, it is recommended that Monte Carlo simulations be partially or completely integrated into the commissioning of electron are therapy.

目的

使用蒙特卡罗模拟对电子放射治疗的监测单位（MU）进行计算，并通过测量进行验证。研究了驻留因子（DF）、源皮距（SSD）和治疗角度（a）的变化。此外，还研究了使用实心水矩形而不是圆柱形模体来测量需要旋转机架的 DF 的可能性。

方法

使用基于 EGSnrc 的蒙特卡罗代码生成了一个基于带有矩形切口（物理尺寸=2.6x21cm2）的 9MeV 电子束的相空间文件，并通过测量进行了验证。使用测量和蒙特卡罗模拟确定了 MU 计算所需的相对输出因子（ROF）、SSD 偏移量和 DF。电离室、射线照相胶片、实心水矩形模体和聚苯乙烯制成的圆柱形模体用于剂量测量。

结果

ROF、SSD 偏移量和 DF 的测量值与蒙特卡罗结果之间的百分比偏差分别为 1.2%、0.18%和 1.5%。结果发现，DF 随 a 的增加而减小，并且这种 DF 的减小在 a 范围为 0 度-60 度时比 60 度-120 度时更为显著。此外，对于固定的 a，DF 随 SSD 的增加而增加。通过测量和蒙特卡罗模拟比较使用矩形和圆柱形模体确定的 DF，发现通过矩形模体确定的 DF 与通过圆柱形模体确定的 DF 在 +/-1.2% 内吻合良好。这表明，使用我们特定的切口，通过简单的实心水矩形模体设置即可代替圆柱形模体来确定与电子弧形相关的 DF。