Titt U, Vassiliev O N, Pönisch F, Kry S F, Mohan R
The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, Texas 77030, USA.
Med Phys. 2006 Sep;33(9):3270-3. doi: 10.1118/1.2229430.
In conventional linear accelerators, the flattening filter provides a uniform lateral dose profile. In intensity modulated radiation therapy applications, however, the flatness of the photon field and hence the presence of a flattening filter, is not necessary. Removing the filter may provide some advantages, such as faster treatments and smaller out-of-field doses to the patients. In clinical accelerators the backscattered radiation dose from the collimators must be taken into account when the dose to the target volume in the patient is being determined. In the case of a conventional machine, this backscatter is known to great precision. In a flattening filter free accelerator, however, the amount of backscatter may be different. In this study we determined the backscatter contribution to the monitor chamber signal in a flattening filter free clinical accelerator (Varian Clinac 21EX) with Monte Carlo simulations. We found that with the exception of very small fields in the 18-MV photon mode, the contribution of backscattered radiation to the monitor signal did not differ from that of conventional machines with a flattening filter. Hence, a flattening filter free clinical accelerator would not necessitate a different backscatter correction.
在传统的直线加速器中，均整滤过器可提供均匀的侧向剂量分布。然而，在调强放射治疗应用中，光子射野的平坦度以及因此均整滤过器的存在并非必要。移除滤过器可能会带来一些优势，比如治疗速度更快以及患者的射野外剂量更小。在临床加速器中，当确定患者靶区体积的剂量时，必须考虑来自准直器的反向散射辐射剂量。对于传统机器而言，这种反向散射的精度是已知的。然而，在无均整滤过器的加速器中，反向散射的量可能会有所不同。在本研究中，我们通过蒙特卡罗模拟确定了无均整滤过器的临床加速器（瓦里安Clinac 21EX）中反向散射对监测电离室信号的贡献。我们发现，除了18兆伏光子模式下的非常小的射野之外，反向散射辐射对监测信号的贡献与带有均整滤过器的传统机器并无差异。因此，无均整滤过器的临床加速器并不需要不同的反向散射校正。
