Department of Radiotherapy, Division Medical Radiation Physics, Medical University of Vienna, AKH Vienna, 1090 Vienna, Austria.
Med Phys. 2010 Jan;37(1):49-53. doi: 10.1118/1.3264617.
Recently, there has been an increasing interest in operating conventional linear accelerators without a flattening filter. The aim of this study was to determine beam quality variations as a function of off-axis ray angle for unflattened beams. In addition, a comparison was made with the off-axis energy variation in flattened beams.
Two Elekta Precise linear accelerators were modified in order to enable radiation delivery with and without the flattening filter in the beam line. At the Medical University Vienna (Vienna, Austria), half value layer (HVL) measurements were performed for 6 and 10 MV with an in-house developed device that can be easily mounted on the gantry. At St. Luke's Hospital (Dublin, Ireland), measurements were performed at 6 MV in narrow beam geometry with the gantry tilted around 270 degrees with pinhole collimators, an attenuator, and the chamber positioned on the table. All attenuation measurements were performed with ionization chambers and a buildup cap (2 mm brass) or a PMMA mini phantom (diameter 3 cm, measurement depth 2.5 cm).
For flattened 6 and 10 MV photon beams from the Elekta linac the relative HVL(theta) varies by about 11% for an off-axis ray angle theta = 10 degrees. These results agree within +/- 2% with a previously proposed generic off-axis energy correction. For unflattened beams, the variation was less than 5% in the whole range of off-axis ray angles up to 10 degrees. The difference in relative HVL data was less than 1% for unflattened beams at 6 and 10 MV.
Off-axis energy variation is rather small in unflattened beams and less than half the one for flattened beams. Thus, ignoring the effect of off-axis energy variation for dose calculations in unflattened beams can be clinically justified.
最近，人们对不使用均整滤过器操作常规直线加速器越来越感兴趣。本研究的目的是确定离轴射线角度对非均整射束的束质变化。此外，还比较了非均整射束的离轴能量变化。
为了能够在射束线中带有和不带有平坦化滤波器进行放射治疗，两台 Elekta Precise 直线加速器在维也纳医科大学（奥地利维也纳）进行了改装。使用自行开发的设备，可以方便地安装在龙门架上，进行 6 MV 和 10 MV 的半价层（HVL）测量。在都柏林圣卢克医院（爱尔兰都柏林），使用带有针孔准直器、衰减器和置于治疗台上的电离室的窄束几何结构，在龙门架倾斜约 270 度的情况下，进行 6 MV 的测量。所有衰减测量均使用电离室和一个 2 毫米厚的铜衬垫（2 毫米厚的黄铜）或 PMMA 微型模体（直径 3 厘米，测量深度 2.5 厘米）进行。
对于 Elekta 直线加速器的平坦化 6 MV 和 10 MV 光子射束，离轴射线角度 theta = 10 度时，相对 HVL(theta)的变化约为 11%。这些结果与之前提出的通用离轴能量校正相差在正负 2%以内。对于非均整射束，在离轴射线角度达到 10 度的整个范围内，变化小于 5%。在 6 MV 和 10 MV 时，非均整射束的相对 HVL 数据差异小于 1%。
非均整射束的离轴能量变化相当小，小于平坦化射束的一半。因此，在非均整射束中忽略离轴能量变化对剂量计算的影响在临床上是合理的。
