Electron fluence correction factors for conversion of dose in plastic to dose in water.

In radiation dosimetry protocols, plastic is allowed as a phantom material for the determination of absorbed dose to water in electron beams. The electron fluence correction factor is needed in conversion of dose measured in plastic to dose in water. There are large discrepancies among recommended values as well as measured values of electron fluence correction factors when polystyrene is used as a phantom material. Using the Monte Carlo technique, we have calculated electron fluence correction factors for incident clinical beam energies between 5 and 50 MeV as a function of depth for clear polystyrene, white polystyrene and PMMA phantom materials and compared the results with those recommended in protocols as well as experimental values from published data. In 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 pw, is a function of depth and incident beam energy Eo with little dependence on other aspects of beam quality. However the phi pw values at dmax are indirectly influenced by the beam quality since they vary with depth and dmax also varies with the beam quality. Calculated phi pw values at dmax are in a range of 1.005-1.045 for a clear polystyrene phantom, 1.005-1.038 for a white polystyrene phantom and 0.996-1.016 for a PMMA phantom. Our values of phi pw are about 1-2% higher than those determined according to the AAPM TG-25 protocol at dmax for clear or white polystyrene. Our calculated values of phi pw 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 pw as a function of R50 at dmax or at the depth of 0.6R50-0.1 for any clinical electron beam with energy between 5 and 25 MeV for three plastics: clear polystyrene, white polystyrene and PMMA. The study also makes a careful distinction between phi pw and the corresponding IAEA Code of Practice quantity, hm.