User manual

59
In the example Case the gamma test passing rate is >99.6% for 3% dose tolerance at 3
mm. Passing rates >99% for the 3%/3mm criteria are common with film evaluation, but
with passing rates that high, there is little sensitivity in using the passing rate to search
for a more optimal fit
while using the coarse
3%/3mm evaluation
criteria.
For instance, by using
the fiducial marks on
the film to register the
dose measurements
with the Treatment
Plan, human error in
scribing those marks
affects the comparison of the dose distributions. This can be investigated by manually
moving the distributions relative to one another while searching for an optimum fit. In
doing this it is strongly is recommended to use the more stringent test values of 2%
tolerance at 2 mm. Even small changes <0.5 mm in the relative positions of the
distributions can have a large effect on the gamma passing rate. For the example Case
the gamma passing rate for 2%/2 mm is 95.11% .
Optimizatiing the dose distribution comparison involves fine movement of the
Projected Dose Map in the x and y directions and rotationally relative to the treatment
plan. The tools to do this are activated by the icon to the left of the Image Window as
previously described.
The maximum dose in the Treatment Plan is shown to right of the xy translation button
and on the far right of the panel are the x and y translations and the combined xy
translation. Right click on any of the values to set the units.