User manual

77
B. Collimator Star Shot
1. Tape a sheet of film at the center of one of the plastic slabs with the edges
of the film and the slab roughly parallel to one another.
2. Place the slab under the collimator with the film on top and the long edge
of the film roughly parallel to the longitudinal axis and the center of the film
roughly at isocenter.
3. Using the laser lines and light field markers adjust the couch height and
slab/film position so the film is level and at isocenter. Using a fine-tipped
marker scribe marks near the edges of the film to indicate the position of
the longitudinal and transverse cross hairs.
4. Close the collimator to produce a slit beam about 2-4 mm wide parallel to
the longitudinal rotation axis. Note: The beam width will influence the
analysis in the following ways. As the slit becomes narrower more monitor
units may be required to obtain the same darkening of the film. Widening
the slit may reduce the accuracy and precision of the analysis because the
lines will be less sharp. Note: For the collimator star shot no build-up slab
is used on top of the film. In the absence of the build-up material the effect
of scattered radiation will be reduced and exposed lines will be sharper.
The lack of a build-up layer will also reduce the dose/MU by a factor >2
depending on the beam energy.
5. Set the gantry and collimator at zero degrees and using a 6MV photon
beam (or other beam at the discretion of the physicist) expose the film with
300 MU. Then rotate the collimator in increments of 30° up to 150° and
expose the film with 300 MU at each position. Note that each exposure
should be the same with an ideal dose to the film of 100200 cGy each
time. Adjustment of MU should be made to achieve this aim.
6. Remove the film from the slab and proceed to Scanning and Star shot
Image Analysis, Step D1. Note: The couch star shot should look similar to
Figure 2. As no build-up layer the dose/MU is lower than for the gantry
star shot and the darkening on the film is less.