Scalchi P, Francescon P
Department of Medical Physics, Vicenza Hospital, Italy.
Int J Radiat Oncol Biol Phys. 1998 Mar 1;40(4):987-93. doi: 10.1016/s0360-3016(97)00894-8.
Metal oxide semiconductor field-effect transistor (MOSFET) detectors were calibrated to perform in vivo dosimetry during 6-MV treatments, both in normal setup and total body irradiation (TBI) conditions.
MOSFET water-equivalent depth, dependence of the calibration factors (CFs) on the field sizes, MOSFET orientation, bias supply, accumulated dose, incidence angle, temperature, and spoiler-skin distance in TBI setup were investigated. MOSFET reproducibility was verified. The correlation between the water-equivalent midplane depth and the ratio of the exit MOSFET readout divided by the entrance MOSFET readout was studied. MOSFET midplane dosimetry in TBI setup was compared with thermoluminescent dosimetry in an anthropomorphic phantom. By using ionization chamber measurements, the TBI midplane dosimetry was also verified in the presence of cork as a lung substitute.
The water-equivalent depth of the MOSFET is about 0.8 mm or 1.8 mm, depending on which sensor side faces the beam. The field size also affects this quantity; Monte Carlo simulations allow driving this behavior by changes in the contaminating electron mean energy. The CFs vary linearly as a function of the square field side, for fields ranging from 5 x 5 to 30 x 30 cm2. In TBI setup, varying the spoiler-skin distance between 5 mm and 10 cm affects the CFs within 5%. The MOSFET reproducibility is about 3% (2 SD) for the doses normally delivered to the patients. The effect of the accumulated dose on the sensor response is negligible. For beam incidence ranging from 0 degrees to 90 degrees, the MOSFET response varies within 7%. No monotonic correlation between the sensor response and the temperature is apparent. Good correlation between the water-equivalent midplane depth and the ratio of the exit MOSFET readout divided by the entrance MOSFET readout was found (the correlation coefficient is about 1). The MOSFET midplane dosimetry relevant to the anthropomorphic phantom irradiation is in agreement with TLD dosimetry within 5%. Ionization chamber and MOSFET midplane dosimetry in inhomogeneous phantoms are in agreement within 2%.
MOSFET characteristics are suitable for the in vivo dosimetry relevant to 6-MV treatments, both in normal and TBI setup. The TBI midplane dosimetry using MOSFETs is valid also in the presence of the lung, which is the most critical organ, and allows verifying that calculation of the lung attenuator thicknesses based only on the density is not correct. Our MOSFET dosimetry system can be used also to determine the surface dose by using the water-equivalent depth and extrapolation methods. This procedure depends on the field size used.
对金属氧化物半导体场效应晶体管（MOSFET）探测器进行校准，以便在6兆伏治疗期间，在常规设置和全身照射（TBI）条件下进行体内剂量测定。
研究了MOSFET的水等效深度、校准因子（CFs）对射野大小、MOSFET方向、偏置电源、累积剂量、入射角、温度以及TBI设置中 spoiler-皮肤距离的依赖性。验证了MOSFET的可重复性。研究了水等效中平面深度与出口MOSFET读数除以入口MOSFET读数的比值之间的相关性。将TBI设置中的MOSFET中平面剂量测定与人体模型中的热释光剂量测定进行了比较。通过电离室测量，在存在软木塞作为肺替代物的情况下，也验证了TBI中平面剂量测定。
MOSFET的水等效深度约为0.8毫米或1.8毫米，具体取决于传感器哪一侧面对射束。射野大小也会影响这个量；蒙特卡罗模拟允许通过污染电子平均能量的变化来驱动这种行为。对于5×5至30×30平方厘米的射野，CFs随方形射野边长的函数呈线性变化。在TBI设置中，将spoiler-皮肤距离在5毫米至10厘米之间变化时，CFs的变化在5%以内。对于通常给予患者的剂量，MOSFET的可重复性约为3%（2标准差）。累积剂量对传感器响应的影响可忽略不计。对于0度至90度的射束入射角，MOSFET响应的变化在7%以内。传感器响应与温度之间没有明显的单调相关性。发现水等效中平面深度与出口MOSFET读数除以入口MOSFET读数的比值之间具有良好的相关性（相关系数约为1）。与人体模型照射相关的MOSFET中平面剂量测定与TLD剂量测定在±5%以内一致。非均匀模型中的电离室和MOSFET中平面剂量测定在±2%以内一致。
MOSFET特性适用于6兆伏治疗相关的体内剂量测定，无论是在常规设置还是TBI设置中。使用MOSFET进行的TBI中平面剂量测定在存在肺（最关键的器官）的情况下也是有效的，并且可以验证仅基于密度计算肺衰减器厚度是不正确的。我们的MOSFET剂量测定系统还可用于通过使用水等效深度和外推方法来确定表面剂量。此过程取决于所使用的射野大小。
