Portal Film Dosimetry

 
The control of patient positioning is necessary during teletherapy treatments. The positioning errors can be recognized and corrected before the treatment by comparing the radiographs taken at the simulator with the DRR’s (Digitally Reconstructed Radiograph) and the portal image as well.
A radiation therapy department is usually using various portal imaging systems including film. The port film is an official document which certifies the accuracy of treatment delivery. The quality of portal images is reduced by several conditions:
1. too high external photon energy (≥ 10 MV)
2. too large patient diameter (≥ 20 cm)
3. poor quality of radiographic technique
In these cases it is very helpful if the simulator film with patient anatomy is available.
Disadvantages of port film:
1. The film developing is time-consuming
2. The portal film set-up before the treatment is also time-consuming
3. Energy dependence problems (≥ 6 MV)
4. Darkroom and processing facilities and storage required

For the portal film imaging it is necessary to use a special portal film and cassette system, which provides exceptional definition of bony structures for more confident verification of treatment fields, for exposure only a few monitor units (MU) are needed. The portal film system needs a special holder device for verification, because the beam has to be perpendicular with the film surface (Figure 1).