Portal dosimetry using x-ray film: an experimental and computational study

To evaluate the accuracy and precision of relative portal dosimetry using x-ray film, we compared the radiation doses measured by x-ray film and by an ion chamber at various portal planes that were 0 to 50 cm behind an 18 cm thick phantom. In addition, we calculated photon spectra at the measurement planes by using the Monte Carlo particle transport technique. The experiments showed that the film usually measured relative doses to within 5% of the measurements by the ion chamber and that the errors were associated with the changes in the spectra of photon energy fluence at portal planes. The relative magnitude of low-energy fluence in the photon fluence spectrum and its variance across the portal plane caused the film response to differ from the ion-chamber response especially with lead screen on top of film in the film cassette. Relative film dosimetry may be improved to accuracy of better than 2% by using solid water or other tissue equivalent cassettes.