Consistency of the treatment fields position accuracy
is confirmed through X-rays (called "port films") taken at pre-established
intervals during treatment. Many centers utilize a digital portal imaging
system instead of conventional film. The portal image is taken with a very
short exposure shot of the MV X-ray beam used for treatment.
The portal images taken at
treatment time are compared to X-ray images taken as part of the planning
process. This side by side comparison of the two images insures that the
anatomy to be treated matches the tissue selected during the planning process.