ELECTRON FLUENCE CORRECTION FACTORS FOR CONVERSION OF DOSE IN PLASTICTO DOSE IN WATER

In radiation dosimetry protocols, plastic is allowed as a phantom mate rial for the determination of absorbed dose to water in electron beams . The electron fluence correction factor is needed in conversion of do se measured in plastic to dose in water. There are large discrepancies among recommended values as well as measured values of electron fluen ce correction factors when polystyrene is used as a phantom material. Using the Monte Carlo technique, we have calculated electron fluence c orrection factors for incident clinical beam energies between 5 and 50 MeV as a function of depth for clear polystyrene, white polystyrene a nd PMMA phantom materials and compared the results with those recommen ded in protocols as well as experimental values from published data. I n the Monte Carlo calculations, clinical beams are simulated using the EGS4 user-code BEAM for a variety of medical accelerators. The study shows that our calculated fluence correction factor, phi(p)(w), is a f unction of depth and incident beam energy <(E)over bar (o)> with littl e dependence on other aspects of beam quality. However the phi(p)(w) v alues at d(max) are indirectly influenced by the beam p quality since they vary with depth and d(max) also varies with the beam quality. Cal culated phi(p)(w) values at d(max) are in a range of 1.005-1.045 for a clear polystyrene phantom, 1.005-1.038 for a white polystyrene phanto m and 0.996-1.016 for a PMMA phantom. Our values of phi(p)(w) are abou t 1-2% higher than those determined according to the AAPM TG-25 protoc ol at d(max) for clear or white polystyrene. Our calculated values of phi(p)(w) also explain some of the variations of measured data because of its depth dependence. A simple formula is derived which gives the electron fluence correction factor phi(p)(w) as a function of R(50) at d(max) or at the depth of 0.6R(50)-0.1 for any clinical electron beam with energy between 5 and 25 MeV for three plastics: clear polystyren e, white polystyrene and PMMA. The study also makes a careful distinct ion between phi(p)(w) and the corresponding IAEA Code of Practice quan tity, h(m).