Kim J O, Siebers J V, Keall P J, Arnfield M R, Mohan R
Department of Radiation Oncology, Medical College of Virginia Hospitals, Virginia Commonwealth University, Richmond 23298-0058, USA.
Med Phys. 2001 Dec;28(12):2497-506. doi: 10.1118/1.1420734.
Due to the significant increase in the number of monitor units used to deliver a dynamic IMRT treatment, the total MLC leakage (transmission plus scatter) can exceed 10% of the maximum in-field dose. To avoid dosimetric errors, this leakage must be accurately accounted for in the dose calculation and conversion of optimized intensity patterns to MLC trajectories used for treatment delivery. In this study, we characterized the leaf end transmission and leakage radiation for Varian 80- and 120-leaf MLCs using Monte Carlo simulations. The complex geometry of the MLC, including the rounded leaf end, leaf edges (tongue-and-groove and offset notch), mounting slots, and holes was modeled using MCNP4b. Studies were undertaken to determine the leakage as a function of field size, components of the leakage, electron contamination, beam hardening and leaf tip effects. The leakage radiation with the MLC configured to fully block the field was determined. Dose for 6 and 18 MV beams was calculated at 5 cm depth in a water phantom located at 95 cm SSD, and normalized to the dose for an open field. Dose components were scored separately for radiation transmitted through and scattered from the MLC. For the 80-leaf MLC at 6 MV, the average leakage dose is 1.6%, 1.7%, 1.8%, and 1.9% for 5 x 5, 10 x 10, 15 x 15, and 20 x 20cm2 fields, respectively. For the 120-leaf MLC at 6 MV, the average leakage dose is 1.6%, 1.6%, 1.7%, and 1.9% for the same field sizes. Measured leakage values for the 120-leaf MLC agreed with calculated values to within 0.1% of the open field dose. The increased leakage with field size is attributed to MLC scattered radiation. The fractional electron contamination for a blocked MLC field is greater than that for an open field. The MLC attenuation significantly affects the photon spectrum, resulting in an increase in percent depth dose at 6 MV, however, little effect is observed at 18 MV. Both phantom scatter and the finite source size contribute to the leaf tip profile observed in phantom. The results of this paper can be applied to fluence-to-trajectory and trajectory-to-fluence calculations for IMRT.
由于用于实施动态调强放射治疗（IMRT）的监测单位数量显著增加，多叶准直器（MLC）的总泄漏量（透射加散射）可能超过最大射野内剂量的10%。为避免剂量学误差，在剂量计算以及将优化的强度模式转换为用于治疗实施的MLC运动轨迹时，必须准确考虑这种泄漏。在本研究中，我们使用蒙特卡罗模拟对瓦里安80叶和120叶MLC的叶端透射和泄漏辐射进行了特征描述。MLC的复杂几何结构，包括圆形叶端、叶边缘（舌槽和偏移切口）、安装槽和孔，使用MCNP4b进行建模。开展研究以确定泄漏量与射野大小的函数关系、泄漏的组成部分、电子污染、束硬化和叶尖效应。确定了MLC配置为完全遮挡射野时的泄漏辐射。在位于源皮距（SSD）95 cm处的水模体中5 cm深度处计算6和18 MV射束的剂量，并将其归一化为开野剂量。分别对透过MLC透射和从MLC散射的辐射进行剂量分量评分。对于六兆伏的80叶MLC，5×5、10×10、15×15和20×20平方厘米射野的平均泄漏剂量分别为1.6%、1.7%、1.8%和1.9%。对于六兆伏的120叶MLC，相同射野大小的平均泄漏剂量分别为1.6%、1.6%、1.7%和1.9%。120叶MLC的测量泄漏值与计算值在开野剂量的0.1%范围内相符。泄漏量随射野大小增加归因于MLC散射辐射。被遮挡的MLC射野的电子污染分数大于开野的电子污染分数。MLC的衰减显著影响光子能谱，导致6 MV时百分深度剂量增加，然而，在18 MV时观察到的影响很小。模体散射和有限源尺寸都对在模体中观察到的叶尖轮廓有贡献。本文的结果可应用于IMRT的注量到运动轨迹以及运动轨迹到注量的计算。
