Das I J, Harrington J C, Akber S F, Tomer A F, Murray J C, Cheng C W
Department of Radiation Oncology, Fox Chase Cancer Center, Philadelphia, Pennsylvania 19111.
Med Phys. 1994 Jun;21(6):821-6. doi: 10.1118/1.597328.
Electron beam dosimetry at low monitor unit (MU) settings is important for dosimetric applications. Dose linearity, beam flatness, and beam energies were studied at low MU settings with various dose rates for different types of linear accelerators. It is observed that for the scattering foil units, the dose/MU is a smooth function of MU for all beam energies. Discrepancies in dose/MU are highest at the lowest MU. Significant variation (5%-245%) in dose linearity is observed among various linear accelerators at low MU settings. Dose rate has no effect on the dose linearity for all energies for the scattering foil units tested. On the contrary, for the scanned beam, there is no predictable pattern as dose/MU is random in nature and varies with time and beam energy. The maximum dosimetric error is observed for the highest energy beam where the beam width is most narrow. Using film, the beam uniformity was noticed to be very poor at low MU and high energy for scanned beams. The beam uniformity and dose linearity are random at low MU due to the random nature of the scan cycle. Under the adverse conditions, the deviation in dosimetric parameters was observed up to 200 MU.
低监测单位（MU）设置下的电子束剂量测定对于剂量学应用很重要。针对不同类型的直线加速器，在低MU设置下、不同剂量率条件下研究了剂量线性、射野平坦度和束能量。观察到，对于散射箔装置，对于所有束能量，剂量/MU是MU的平滑函数。剂量/MU的差异在最低MU时最大。在低MU设置下，不同直线加速器之间观察到剂量线性存在显著变化（5%-245%）。对于所测试的散射箔装置，剂量率对所有能量的剂量线性均无影响。相反，对于扫描束，由于剂量/MU本质上是随机的且随时间和束能量变化，所以没有可预测的模式。在束宽度最窄的最高能量束中观察到最大剂量测定误差。使用胶片时，注意到对于扫描束，在低MU和高能量下射野均匀性非常差。由于扫描周期的随机性，在低MU时射野均匀性和剂量线性是随机的。在不利条件下，观察到剂量学参数偏差高达200 MU。
