AbstractID: 8252 Title: Film dosimetry for high energy photon beams (greater than 4 to 6 MV) using lead
filters
Film dosimetry has been used for dose verification of photon beams such as intensity modulated radiation therapy. In the past,1-3 we
have shown accurate relative dosimetry of photon beams using lead sheets on both sides of the film(Figure 1). The good results were
obtained for low energies such as 4 to 6 MV only. Thus, we aim to extend this study to higher energies. We have sandwiched x-ray
films in solid water phantom with and without, respectively, lead sheets being placed at 5 mm distance from the film (i.e. a typical
setup). Using this setup, we have irradiated the film both in parallel with and perpendicularly to the photon beams of 10 and 18 MV,
respectively using various field sizes. We processed the films and after calibration the film measurement was compared with water
phantom measurement using ion chamber. Additional experiments were carried out using 4 MV photon beam for comparison. As
photon energies increased, the percent depth dose measured by bare film showed increasing agreement to ion chamber measurement.
The error in film dosimetry was reduced by using filters for both 10 and 18 MV photon beams. The known errors in film dosimetry in
off-axis ratios at distances from the central axis were not noticeable, unlike that at 4 MV. However, in outside penumra region, the
off axis ratio for 10 MV showed significant overresponse, which was suppressed by filtration.
1. Burch et al, Med. Phys. 24, 775-783, 1997; 2. Yeo et al, Med. Phys. 24, 1943-1953, 1997;3.Ju et al, Med. Phys. 29, 351-355, 2002