The purpose of this work is to study dose non-linearity in medical linear
accelerators used in conventional radiotherapy and intensity-modulated
radiation therapy. Open fields, as well as the enhanced dynamic wedge ones,
were used to collect data for 6 MV and 15 MV photon beams obtained from the
VARIAN linear accelerator. Beam stability was checked and confirmed for
different dose rates, energies, and application of enhanced dynamic wedge by
calculating the charge per monitor unit. Monitor unit error was calculated by
the two-exposure method for open and enhanced dynamic wedge beams of 6 MV and
15 MV photons. A significant monitor unit error with maximum values of
?2.05931 monitor unit and ?2.44787 monitor unit for open and enhanced dynamic
wedge beams, respectively, both energy and dose rate dependent, was observed
both in the open photon beam and enhanced dynamic wedge fields. However, it
exhibited certain irregular patterns at enhanced dynamic wedge angles. Dose
monitor unit error exists only because of the overshoot phenomena and
electronic delay in dose coincident and integrated circuits with a dependency
on the dose rate and photon energy. Monitor unit errors are independent of
the application of enhanced dynamic wedge. The existence of monitor unit
error demands that the dose non-linearity of the linear accelerator dosimetry
system be periodically tested, so as to avoid significant dosimetric errors.
Optimized point dose measurement for monitor unit verification in intensity modulated radiation therapy using 6 MV photons by three different methodologies with different detector-phantom combinations: A comparative study
