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.
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