Sohn Jason W, Dempsey James F, Suh Tae S, Low Daniel A
Radiation Oncology Center, Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, Missouri 63145, USA.
Med Phys. 2003 Sep;30(9):2432-9. doi: 10.1118/1.1596785.
Application of intensity modulated radiation therapy (IMRT) using multileaf collimation often requires the use of small beamlets to optimize the delivered radiation distribution. Small-beam dose distribution measurements were compared to dose distributions calculated using a commercial treatment planning system that models its data acquired using measurements from relatively large fields. We wanted to evaluate only the penumbra, percent depth-dose (PDD) and output model, so we avoided dose distribution features caused by rounded leaf ends and interleaf leakage by making measurements using the secondary collimators. We used a validated radiochromic film dosimetry system to measure high-resolution dose distributions of 6 MV photon beams. A commercial treatment planning system using the finite size pencil beam (FSPB) dose calculation algorithm was commissioned using measured central axis outputs from 4.0x4.0 to 40.0x40.0 cm2 beams and radiographic-film profile measurements of a 4.0x4.0 cm2 beam at twice the depth of maximum dose (dmax). Calculated dose distributions for square fields of 0.5x0.5 cm2, and 1.0x1.0 cm2, to 6.0x6.0 cm2, in 1.0x1.0 cm2, increments were compared against radiochromic film measurements taken with the film oriented parallel to the beam central axis in a water equivalent phantom. The PDD of the smaller field sizes exhibited behavior typical of small fields, namely a decrease in dmax with decreasing field size. The FSPB accurately modeled the depth-dose and central axis output for depths deeper than the nominal dmax of 1.5 cm plus 0.5 cm. The dose distribution in the build-up and penumbra regions was not accurately modeled for depths less than 2 cm, especially for the fields of 2.0x2.0 cm2 and smaller. Using the gamma function with 2 mm and 2% criteria, the dose model was shown to accurately predict the penumbra. While for single small beams the compared dose distributions passed the gamma function criteria, the clinical appropriateness of these criteria is not clear for a composite IMRT plan. Further investigation of the cumulative impact of the observed dose discrepancies is warranted. We speculate that the observed differences in the penumbra regions arise from some energy dependent artifact in the radiographic-film profiles used for commissioning. In the future, radiochromic film based commissioning might provide a more accurate data set for dose modeling.
使用多叶准直器的调强放射治疗（IMRT）通常需要使用小射束来优化所输送的辐射剂量分布。将小射束剂量分布测量结果与使用商业治疗计划系统计算出的剂量分布进行比较，该商业治疗计划系统对其数据的建模是基于相对大射野的测量结果。我们只想评估半值层、百分深度剂量（PDD）和输出模型，因此我们通过使用二级准直器进行测量，避免了由圆形叶片末端和叶片间泄漏引起的剂量分布特征。我们使用经过验证的放射变色薄膜剂量测定系统来测量6 MV光子束的高分辨率剂量分布。使用有限尺寸笔形束（FSPB）剂量计算算法的商业治疗计划系统，通过测量4.0×4.0至40.0×40.0 cm²射野的中心轴输出以及在最大剂量深度（dmax）两倍深度处对4.0×4.0 cm²射野进行的X射线胶片轮廓测量来进行调试。将0.5×0.5 cm²、1.0×1.0 cm²至6.0×6.0 cm²的方形射野以1.0×1.0 cm²的增量计算出的剂量分布与在水等效体模中胶片平行于射野中心轴放置时进行的放射变色薄膜测量结果进行比较。较小射野尺寸的PDD表现出小射野的典型行为，即随着射野尺寸减小，dmax降低。FSPB能准确模拟深度超过标称dmax（1.5 cm加0.5 cm）的深度剂量和中心轴输出。对于深度小于2 cm的情况，尤其是对于2.0×2.0 cm²及更小的射野，剂量分布在剂量建成区和半值层区域的模拟并不准确。使用2 mm和2%标准的伽马函数时，剂量模型显示能准确预测半值层。虽然对于单个小射束，比较的剂量分布通过了伽马函数标准，但对于复合IMRT计划，这些标准的临床适用性尚不清楚。有必要进一步研究观察到的剂量差异所产生的累积影响。我们推测在半值层区域观察到的差异源于调试中使用的X射线胶片轮廓中的一些能量依赖性伪影。未来，基于放射变色薄膜的调试可能会为剂量建模提供更准确的数据集。
