小射野输出因子测量对调强放疗剂量学的影响。

The effect of small field output factor measurements on IMRT dosimetry.

作者信息

Azimi Rezvan, Alaei Parham, Higgins Patrick

机构信息

Department of Therapeutic Radiology-Radiation Oncology, University of Minnesota, Minneapolis, Minnesota 55455, USA.

出版信息

Med Phys. 2012 Aug;39(8):4691-4. doi: 10.1118/1.4736527.

DOI:10.1118/1.4736527

PMID:22894393

Abstract

PURPOSE

To evaluate how changes in the measured small field output factors affect the doses in intensity-modulated treatment planning.

METHODS

IMRT plans were created using Philips Pinnacle treatment planning system. The plans were optimized to treat a cylindrical target 2 cm in diameter and 2 cm in length. Output factors for 2 × 2 and 3 × 3 cm(2) field sizes were changed by ±5%, ±10%, and ±20% increments from the baseline measurements and entered into the planning system. The treatment units were recommissioned in the treatment planning system after each modification of the output factors and treatment plans were reoptimized. All plans were delivered to a solid water phantom and dose measurements were made using an ionization chamber. The percentage differences between measured and computed doses were calculated. An Elekta Synergy and a Varian 2300CD linear accelerator were separately evaluated.

RESULTS

For the Elekta unit, decreasing the output factors resulted in higher measured than computed doses by 0.8% for -5%, 3.6% for -10%, and 8.7% for -20% steps. Increasing the output factors resulted in lower doses by 2.9% for +5%, 5.4% for +10%, and 8.3% for +20% steps. For the Varian unit no changes were observed for either increased or decreased output factors.

CONCLUSIONS

The measurement accuracy of small field output factors are of importance especially when the treatment plan consists of small segments as in IMRT. The method proposed here could be used to verify the accuracy of the measured small field output factors for certain linear accelerators as well as to test the beam model. The Pinnacle treatment planning system model uses output factors as a function of jaw setting. Consequently, plans using the Elekta unit, which conforms the jaws to the segments, are sensitive to small field measurement accuracy. On the other hand, for the Varian unit, jaws are fixed and segments are modeled as blocked fields hence, the impact of small field output factors on IMRT monitor unit calculation is not evaluable by this method.

摘要

目的

评估测量的小射野输出因子的变化如何影响调强治疗计划中的剂量。

方法

使用飞利浦Pinnacle治疗计划系统创建调强放疗计划。这些计划被优化用于治疗一个直径2 cm、长度2 cm的圆柱形靶区。2×2和3×3 cm²射野大小的输出因子在基线测量值的基础上分别以±5%、±10%和±20%的增量进行改变，并输入到计划系统中。在每次改变输出因子后，在治疗计划系统中对治疗单元重新进行调试，并重新优化治疗计划。所有计划均照射到固体水模体上，并使用电离室进行剂量测量。计算测量剂量与计算剂量之间的百分比差异。分别对一台医科达Synergy直线加速器和一台瓦里安2300CD直线加速器进行了评估。

结果

对于医科达直线加速器，降低输出因子导致测量剂量比计算剂量高，降低5%时高0.8%，降低10%时高3.6%，降低20%时高8.7%。增加输出因子导致剂量降低，增加5%时降低2.9%，增加10%时降低5.4%，增加20%时降低8.3%。对于瓦里安直线加速器，输出因子增加或降低时均未观察到变化。

结论

小射野输出因子的测量精度很重要，尤其是在治疗计划如调强放疗那样由小射野组成时。这里提出的方法可用于验证某些直线加速器测量的小射野输出因子的准确性，以及测试射束模型。Pinnacle治疗计划系统模型将输出因子用作准直器设置的函数。因此，使用医科达直线加速器（其准直器与射野匹配）的计划对小射野测量精度敏感。另一方面，对于瓦里安直线加速器，准直器是固定的，射野被建模为遮挡射野，因此，该方法无法评估小射野输出因子对调强放疗监测单位计算的影响。