A FILTRATION METHOD FOR IMPROVING FILM DOSIMETRY IN PHOTON RADIATION-THERAPY

Successful radiotherapy requires accurate dosimetry for treatment veri fication. Existing dosimeters such as ion chambers, TLD, and diodes ha ve drawbacks such as relatively long measurement time and poor spatial resolution. These disadvantages become serious problems for dynamic-w edged beams. Thus the clinical use of dynamic wedges requires an impro ved dosimetry method. X-ray film may serve this purpose. However, x-ra y film is not clinically accepted as a dosimeter for photon beams, bec ause it overresponds to photons with energies below about 400 keV. Thi s paper presents and develops a method which was initially proposed by Burch to improve the dose response of x-ray film in a phantom. The me thod is based on placing high-atomic number foils next to the film. Th e foils are used as filters to preferentially remove low-energy photon s. The optimal film and filter configuration in a phantom was determin ed using a mathematical scheme derived in this study and a Monte Carlo technique (ITS code). The optimal configuration thus determined is as follows: the filter-to-film distance of 6 mm and the filter thickness of 0.15 mm for percent depth-dose measurement; the distance of I cm a nd the thickness of 0.25 mm for off-axis (dose) ratio measurement. The configuration was then tested with photon beams from a 4 MV linac. Th e test result indicates that the in-phantom dose distribution based on the optimal configuration agrees well with those measured by ion cham bers. (C) 1997 American Association of Physicists in Medicine.