AbstractID: 6952 Title: The Effect of Beam-Hold Dosimetry in Multiple Static Segment and Dynamic IMRT
In multiple static segment (MSS) and dynamic IMRT, the x-ray beam is triggered on and off
multiple times during the course of treatment delivery. In the MSS approach, the beam is
intentionally turned off between IMRT segments. In the dynamic approach, the beam is
temporarily turned off when an MLC leaf exceeds a positional tolerance. When the leaf moves
within tolerance, the beam is automatically turned back on. In order to measure the dosimetric
effects of beam holds, a parallel-plate ionization chamber was centered in a 10×10 cm field at
dmax. MSS and Dynamic IMRT sequences were used to test beam dosimetry where one MLC
leaf located underneath the primary collimators was moved. Test sequences were developed to
investigate different sources of dosimetric error in IMRT delivery. The first test evaluated the
ability of the linear accelerator to deliver five beam-on pulses of equal dose at dose rates ranging
from 80 to 400 MU/min. The second test evaluated dosimetric error as the number of MUs in a
single segment were decreased from 50 to 0.5 MUs for dose rates ranging from 200 to 600
MU/min. The third test evaluated the effect of increasing the frequency of beam-holds in
Dynamic IMRT delivery. The limiting factor in all three cases is the product of linac pulse
control feedback time (50 msec) and the dose rate. Results and conclusions for the three QA
tests will be presented.