An investigation of photon energy spectrum and its effect on film dosimetry in dynamic treatment fields

The purposes of this work are to investigate whether there is a significant change in photon energy spectrum within a water phantom for dynamic or intensity-modulated fields and its effect on the film dosimetry. We developed a Monte Carlo program using the BEAM code to simulate the enhanced dynamic wedge used on Varian linear accelerators. The photon energy spectrum at various locations within a large water phantom was computed based on the phase space data from the Monte Carlo simulation. The dose distributions in the dynamic fields were also measured using films (Kodak XV) and compared with the ion chamber measurement. The Monte Carlo results showed that for the dynamic wedges, the relative contribution from soft photons to which the film is most sensitive increased with depth, and decreased with the jaw move-in direction (field-size effect). The degree of the spectral variation across the field was the least at the dmax. Compared to the ion chamber measurement, the film data showed significant underestimation at the wedge toe region (10% at 12.5 cm depth). This corresponded to the hardening of the energy spectrum along this direction and agreed well with the results from the Monte Carlo simulation. Therefore, we recommended that films be used at a depth of no more than dmax for verifying fluence or dose patterns of intensity-modulated or dynamic fields. Films are also adequate for use in small dynamic fields where the effect of soft-photon scatter is minimal