Assessment of Monitor Units and Gamma Pass Rate for 6 MV and Flattening Filter Free (FFF) Beams in Volumetric Modulated Arc Therapy (VMAT)

Background: In linear accelerators, the treatment field’s uniform intensity is achieved by including a flattening filter in the beam. However, to produce more conformal dose distributions, contemporary radiotherapy practice now frequently uses fluence and aperture modifying techniques, such as volumetric modulated arc therapy. In these circumstances, the flattening filter in the beam manufacturing process is no longer required. It is therefore necessary to compare the monitor units of 6 MV and flattening filter free plans and how it influences the gamma pass rates to determine which is best for treating cervical cancer with pelvic lymph node metastasis. Methods: VMAT plans for fifteen patients with cervical cancer with pathological pelvic lymph node metastasis were included in this study. Each patient had two VMAT plans using conventional 6 MV beam with flattening filter and one with flattening filter free beam (FFF). The VMAT plans were made using two arcs, and then recalculated to give the planned dose distribution to the detectors in a Delta4 phantom. The VMAT plans were irradiated on the Delta4 phantom using an Elekta linear accelerator (6 MV). Results: The mean monitor unit for the 6 MV plans was 506.3 MU and a standard deviation of 48.6 while that of the FFF plans had a mean MU of 701.5 with a standard deviation of 87.6. The total monitor units (MUs) for the FFF plans were significantly greater than the 6 MV plans (p = 6.1 × 10-5). Conclusion: Flattening filter free (FFF) plans require more numbers of monitor units in comparison to conventional 6 MV filtered beams for external radiation of cervical cancer with pelvic lymph nodes involvement.

A preliminary study of a new high-resolution detector matrix applied to three-dimensional dose verification in intensity-modulated radiotherapy

Objective:To test the basic dosimetry characteristics of a new high-resolution matrix and to perform a preliminary study on the three-dimensional(3D)dose verification of intensity-modulated treatment(IMRT).Methods:The dosimetry characteristics of the new matrix were investigated,including repeatability,dose-rate response,and dose linearity.Twenty cases of nasopharyngeal carcinoma(NPC)and 20 cases with lung cancer were randomly selected for IMRT plans,and the novel matrix was employed for 3D dose verification.The measured results were evaluated using the gamma passing rate(GPR)and dose volume histogram(DVH).The action limit(AL)and tolerance limit(TL)of the target volume and each organ at risk(OAR)were calculated with reference to the American Association of Physicists in Medicine(AAPM)TG218 report.Results:The matrix performed well for all dosimetry characteristic tests,with a deviation of<1%.The average GPRs of the body were(99.32±0.32)%,(98.36±0.59)%,and(96.27±1.20)%for NPC,and(99.17±0.74)%,(98.09±1.33)%,and(95.83±2.22)%for lung cancer at the gamma standards of 3%/3 mm,3%/2 mm,and 2%/2 mm.The average GPRs difference between the head-neck and thorax-abdomen plans were<1%for the same gamma standard.For both the target volumes and OARs,the average GPRs were>90%under the relatively strict standard of 2%/2 mm.The DVH showed that the measurement results of D_(98) and D_(95) for the target volumes were slightly lower and D_(2) were higher than those of treatment planning system(TPS)(P<0.01).In addition,with the same standard,there may be significant differences in the values of AL and TL between different structures for target volumes and OARs,especially small-volume OARs such as the chiasma and optic nerve-L.Conclusions:The new matrix showed good dosimetry characteristics and can be effectively applied to the treatment planning dose verification of the head-neck and lung cancer.Further research is needed to establish how to analyze the GPR and DVH of the target volume and OARs,and to determine more precise dose verification standards combined with the parameters of AL and TL to better guide 3D dose verification in clinic.