Intensity modulated radiation therapy (IMRT)
is a highly accurate technique that is usually implemented in either dynamic or step-and-shoot fashion with many
segments each having low monitor units (MUs). The
present study evaluated the effects of beam startup characteristics on the
dose delivery accuracy for each segment at low MUs for step-and-shoot IMRT with
an Elekta Precise accelerator at the highest dose rates. We used a
two-dimensional semi-conductor detector for the dose measurements. The field
size of each segment was assumed to be 20 ×20 cm2 and each segment was set to deliver 1 - 10 MUs. Our results show a variation in dose
delivery accuracy between segments for the same IMRT beam, which can be
attributed to the beam startup characteristics. This variability is attributed
to the changes in the transient changes in the temperatures of the electron gun
filament and the magnetron. That is, the transient increase in
the temperature of the filament leads to increasing doses with time and that of
the magnetron leads to decreasing doses with time during the first few MUs.
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