西门子PRIMUS直线加速器初级准直器和平坦滤波器对非对称场的影响
The effect of the primary collimator and flattening filter on asymmetric fields for a Siemens PRIMUS linear accelerator
摘要：
本文研究了西门子PRIMUS直线加速器高度非对称场的均匀性。平坦滤波器的半径小于主准直器的半径，从而产生影响远离准直器轴的区域中的大场的不均匀性，以及具有大偏移的不对称场。在两个钳口处于最大位置（20 cm）和另外两个钳口处于最大超程（10 cm）的情况下，对应于具有极端偏移的10 × 10场。轮廓具有朝向射束边缘减小的显著梯度，使得这些场不适合于治疗。结果表明，主准直器和平坦滤波器组件的设计对均匀性有直接影响。这可能对涉及包括这些不均匀区域的射野的治疗产生临床后果。与治疗计划系统（Philips Pinnacle）的计算结果进行比较，该计算结果在均匀平坦射束的假设下进行计算，导致严重的差异。
英文摘要：
Homogeneity for highly asymmetric fields has been studied for a Siemens PRIMUS linear accelerator. The flattening filter has a radius smaller than the primary collimator one, creating inhomogeneities that affect large fields in areas far from the collimator axis, and asymmetric fields with large offset. Profiles and absolute dose have been measured in fields with two jaws at maximal position (20 cm) and the other two at maximal overtravel (10 cm.), corresponding to 10 x 10 fields with extreme offset. Profiles have a remarkable gradient decreasing towards the beam edge, making these fields unsuitable for treatments. Results show that the design of the primary collimator and flattening filter assembly has direct consequences in homogeneity. This can have clinical consequences for treatments involving fields that include these inhomogeneous areas. Comparisons with the treatment planning system (Philips Pinnacle) calculations, that computes under the hypotheses of a uniformly flattened beam, result in severe discrepancies.
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