User manual

75
strip for another use. The best way to tear EBT3+ is to grip the edge of the film
as close as possible on either side of the perforation. Push down on one side
and pull up on the other to start a tear and split the film into two pieces.
A. Gantry Star Shot
1. Tape a sheet of film at the center of the large face of one of the plastic
slabs with the edges of the film and the slab roughly parallel to one
another.
2. Place the second slab over the film and stand the two slabs on end on the
couch and under the gantry. If desired the slabs could be fixed together
with some adhesive tape, but it is not essential.
3. Using the laser lines and light field marker maneuver the slabs/film so the
film is in the transverse plane. The horizontal joint at the top of the slab
should be aligned using the light field marker. Make sure this horizontal
joint is parallel to the transverse field line and that the slabs are placed so
the longitudinal field line is close to the center of the slabs.
4. Adjust the couch height so that the vertical center of the film is close to
isocenter and use a pencil or fine-tipped marker to scribe lines on the
edges of the slab to which the film is taped The marks will later be
transferred to the film to indicate the position of isocenter. Marks on the
slabs can be removed later with eraser or an alcohol wipe.
5. Close the collimator to produce a slit beam about 2-4 mm wide parallel to
the gantry 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.
6. Set the gantry at zero degrees and use a 6MV photon beam (of other
beam at the discretion of the Physicist) to expose the film with 150 MU.
Then change the gantry angle in increments of 40° up to 160° expose the