Sun Nuclear Inc., 425-A Pineda Court, Melbourne, Florida 32940, USA.
Med Phys. 2010 Feb;37(2):477-84. doi: 10.1118/1.3284529.
Accurate modeling of beam profiles is important for precise treatment planning dosimetry. Calculated beam profiles need to precisely replicate profiles measured during machine commissioning. Finite detector size introduces perturbations into the measured profiles, which, in turn, impact the resulting modeled profiles. The authors investigate a method for extracting the unperturbed beam profiles from those measured during linear accelerator commissioning.
In-plane and cross-plane data were collected for an Elekta Synergy linac at 6 MV using ionization chambers of volume 0.01, 0.04, 0.13, and 0.65 cm3 and a diode of surface area 0.64 mm2. The detectors were orientated with the stem perpendicular to the beam and pointing away from the gantry. Profiles were measured for a 10 x 10 cm2 field at depths ranging from 0.8 to 25.0 cm and SSDs from 90 to 110 cm. Shaping parameters of a Gaussian response function were obtained relative to the Edge detector. The Gaussian function was deconvolved from the measured ionization chamber data. The Edge detector profile was taken as an approximation to the true profile, to which deconvolved data were compared. Data were also collected with CC13 and Edge detectors for additional fields and energies on an Elekta Synergy, Varian Trilogy, and Siemens Oncor linear accelerator and response functions obtained. Response functions were compared as a function of depth, SSD, and detector scan direction. Variations in the shaping parameter were introduced and the effect on the resulting deconvolution profiles assessed.
Up to 10% setup dependence in the Gaussian shaping parameter occurred, for each detector for a particular plane. This translated to less than a +/- 0.7 mm variation in the 80%-20% penumbral width. For large volume ionization chambers such as the FC65 Farmer type, where the cavity length to diameter ratio is far from 1, the scan direction produced up to a 40% difference in the shaping parameter between in-plane and cross-plane measurements. This is primarily due to the directional difference in penumbral width measured by the FC65 chamber, which can more than double in profiles obtained with the detector stem parallel compared to perpendicular to the scan direction. For the more symmetric CC13 chamber the variation was only 3% between in-plane and cross-plane measurements.
The authors have shown that the detector response varies with detector type, depth, SSD, and detector scan direction. In-plane vs. cross-plane scanning can require calculation of a direction dependent response function. The effect of a 10% overall variation in the response function, for an ionization chamber, translates to a small deviation in the penumbra from that of the Edge detector measured profile when deconvolved. Due to the uncertainties introduced by deconvolution the Edge detector would be preferable in obtaining an approximation of the true profile, particularly for field sizes where the energy dependence of the diode can be neglected. However, an averaged response function could be utilized to provide a good approximation of the true profile for large ionization chambers and for larger fields for which diode detectors are not recommended.
准确的束流轮廓建模对于精确的治疗计划剂量学非常重要。计算得到的束流轮廓需要精确地复制在机器调试过程中测量到的轮廓。有限的探测器尺寸会对测量到的轮廓产生干扰，从而影响到最终的建模轮廓。作者研究了一种从线性加速器调试过程中测量到的束流轮廓中提取未受干扰的束流轮廓的方法。
在 Elekta Synergy 直线加速器上以 6 MV 的能量使用体积为 0.01、0.04、0.13 和 0.65 cm3 的电离室和表面积为 0.64 mm2 的二极管，对平面内和平面外数据进行了采集。探测器的杆垂直于射束并指向龙门架的反方向。在 SSD 为 90 至 110 cm 的范围内，对 10 x 10 cm2 的射野深度从 0.8 到 25.0 cm 进行了测量。相对于边缘探测器获得高斯响应函数的形状参数。从测量的电离室数据中对高斯函数进行反卷积。将边缘探测器的轮廓作为真实轮廓的近似值，将反卷积数据与之进行比较。还在 Elekta Synergy、Varian Trilogy 和 Siemens Oncor 直线加速器上使用 CC13 和边缘探测器收集了其他射野和能量的数据，并获得了响应函数。作为深度、SSD 和探测器扫描方向的函数，比较了响应函数。引入了形状参数的变化，并评估了其对最终反卷积轮廓的影响。
对于每个探测器在特定平面上，高达 10%的设置依赖性出现在高斯形状参数中。这导致 80%-20%半影宽度的变化小于 +/- 0.7 mm。对于 FC65 型等大体积电离室，其腔长与直径之比远小于 1，扫描方向导致平面内和平面外测量之间的形状参数差异高达 40%。这主要是由于 FC65 腔测量的半影宽度的方向差异所致，与探测器杆垂直于扫描方向相比，在平行于扫描方向获得的轮廓中，半影宽度可以增加一倍以上。对于更对称的 CC13 腔，平面内和平面外测量之间的变化仅为 3%。
作者表明，探测器的响应随探测器类型、深度、SSD 和探测器扫描方向而变化。与平面内相比，平面外扫描可能需要计算方向相关的响应函数。对于电离室，响应函数整体变化 10%会导致反卷积后的半影与边缘探测器测量的轮廓略有偏差。由于反卷积带来的不确定性，在获得真实轮廓的近似值时，边缘探测器将更可取，特别是对于二极管能量依赖性可以忽略的射野大小。然而，平均响应函数可以为大体积电离室和不推荐使用二极管探测器的较大射野提供真实轮廓的良好近似值。
