Department of Radiation Oncology, Division of Medical Physics, University of Iowa Hospitals and Clinics, Iowa City, IA 52242, USA.
J Appl Clin Med Phys. 2013 Jul 8;14(4):4211. doi: 10.1120/jacmp.v14i4.4211.
The clinical application of the flattening filter-free photon beam (FFF) has enjoyed greater use due to its advantage of reduced treatment time because of the increased dose rate. Its unique beam characteristics, along with the very high-dose rate, require a thorough knowledge of the capability and accuracy in FFF beam modeling, planning, and delivery. This work verifies the feasibility of modeling an equivalent quality unflattened photon beam (eqUF), and the dosimetric accuracy in eqUF beam planning and delivery. An eqUF beam with a beam quality equivalent to a conventional 6 MV photon beam with the filter in place (WF) was modeled for the Pinnacle3 TPS and the beam model quality was evaluated by gamma index test. Results showed that the eqUF beam modeling was similar to that of the WF beam, as the overall passing rate of the 2%/2 mm gamma index test was 99.5% in the eqUF beam model and 96% in the WF beam model. Hypofractionated IMRT plans were then generated with the same constraints using both WF and eqUF beams, and the similarity was evaluated by DVH comparison and generalized 3D gamma index test. The WF and eqUF plans showed no clinically significant differences in DVH comparison and, on average > 98% voxels passed the 3%/3 mm 3D gamma index test. Dosimetric accuracy in gated phantom delivery was verified by ion chamber and film measurements. All ion chamber measurements at the isocenter were within 1% of calculated values and film measurements passed the 3 mm/3% gamma index test with an overall passing rate > 95% in the high-dose and low-gradient region in both WF and eqUF cases. Treatment plan quality assurance (QA), using either measurement-based or independent calculation-based methods of ten clinically treated eqUF IMRT plans were analyzed. In both methods, the point dose differences were all within 2% difference. In the relative 2D dose distribution comparison, >95% points were within 3% dose difference or 3 mm DTA.
FFF 光子束的临床应用因其剂量率增加而缩短治疗时间的优势而得到更广泛的应用。其独特的束特性和非常高的剂量率需要对 FFF 束建模、计划和交付的能力和准确性有透彻的了解。这项工作验证了对等效平展光子束（eqUF）进行建模以及在 eqUF 束计划和交付中进行剂量学准确性的可行性。为 Pinnacle3 TPS 对等效于带有滤波器的传统 6 MV 光子束（WF）的质量的 eqUF 束进行建模，并通过伽马指数测试评估束模型质量。结果表明，eqUF 束建模类似于 WF 束，因为在 eqUF 束模型中 2%/2mm 伽马指数测试的整体通过率为 99.5%，在 WF 束模型中为 96%。然后使用 WF 和 eqUF 束生成相同约束的适形调强放疗计划，并通过剂量体积直方图比较和广义 3D 伽马指数测试评估相似性。WF 和 eqUF 计划在剂量体积直方图比较中没有明显的临床差异，平均有 >98%的体素通过 3%/3mm 3D 伽马指数测试。在门控体模交付中的剂量学准确性通过电离室和胶片测量进行验证。在等中心的所有电离室测量值都在计算值的 1%以内，在 WF 和 eqUF 情况下，在高剂量和低梯度区域，胶片测量值通过 3mm/3%伽马指数测试，整体通过率均大于 95%。使用基于测量或独立计算的方法对十个临床治疗的 eqUF 调强放疗计划进行了治疗计划质量保证（QA）分析。在这两种方法中，点剂量差异均在 2%差异以内。在相对 2D 剂量分布比较中，>95%的点在 3%剂量差异或 3mm DTA 以内。
