Department of Radiation Oncology, E-Da Hospital, Kaohsiung 824, Taiwan ; Department of Medical Imaging and Radiological Science, Central Taiwan University of Science and Technology, Taichung 411, Taiwan.
Biomed Res Int. 2013;2013:461801. doi: 10.1155/2013/461801. Epub 2013 Oct 24.
An analytical and experimental study of split shape dose calculation correction by adjusting the position of the on-axis round leaf end position is presented. We use on-axis corrected results to predict off-axis penumbra region dosimetric performance in an intensity-modulated radiation therapy treatment planning system.
The precise light-field edge position (X(tang.p)) was derived from the on-axis 50% dose position created by using the nominal light field for geometric and mathematical manipulation. Leaf position (X(mlc.p)) could be derived from X(tang.p) by defining in the treatment planning system for monitor unit calculation. On-axis offset (correction) could be obtained from the position corresponding to 50% of the central axis dose minus the X(mlc.p) position. The off-axis 50% dose position can then be derived from the on-axis 50% dose position.
The monitor unit calculation of the split shape using the on-axis rounded leaf end MLC penumbra region could provide an under-or overdose of 7.5% per millimeter without an offset correction. When using the on-axis rounded leaf end offset correction to predict the off-axis dose, the difference between the off- and on-axis 50% dose position is within ±1.5 mm.
It is possible to achieve a dose calculation within 0.5% error for an adjusted MLC leaf edge location in the treatment planning system with careful measurement and an accurate on-axis offset correction. Dose calculations located at an off-axis spilt shape region should be used carefully due to noncorrectable errors which were found to be up to 10%.
提出了一种通过调整轴上圆形叶片末端位置来校正分裂形状剂量计算的分析和实验研究。我们使用轴上校正结果来预测强度调制放射治疗计划系统中离轴半影区域的剂量学性能。
从使用名义光场创建的轴上 50%剂量位置中得出精确的光场边缘位置（X(tang.p)），以便进行几何和数学操作。叶片位置（X(mlc.p)）可以通过在治疗计划系统中定义用于监测单位计算来从 X(tang.p)中得出。轴上偏移（校正）可以从与中央轴剂量的 50%对应的位置减去 X(mlc.p)位置获得。然后可以从轴上 50%剂量位置得出离轴 50%剂量位置。
在没有偏移校正的情况下，使用轴上圆形叶片末端 MLC 半影区域的分裂形状进行监测单位计算可能会导致每毫米 7.5%的过度或不足剂量。当使用轴上圆形叶片末端偏移校正来预测离轴剂量时，离轴和轴上 50%剂量位置之间的差异在±1.5mm 以内。
通过仔细测量和准确的轴上偏移校正，可以在治疗计划系统中实现调整后的 MLC 叶片边缘位置的剂量计算误差在 0.5%以内。由于发现不可纠正的误差高达 10%，因此应谨慎使用位于离轴分裂形状区域的剂量计算。
