Monte Carlo study of correction factors for the use of plastic phantoms in clinical electron dosimetry.

In some recent dosimetry protocols, plastic is allowed as a phantom material for the determination of an absorbed dose to water in electron beams, especially for low energy with beam qualities R50 < 4 g/cm2. In electron dosimetry with plastic, a depth-scaling factor, cpl, and a chamber-dependent fluence correction factor, h(pl), are needed to convert the dose measured at a water-equivalent reference depth in plastic to a dose at a reference depth in water. The purpose of this study is to calculate correction factors for the use of plastic phantoms for clinical electron dosimetry using the EGSnrc Monte Carlo code system. RMI-457 and WE-211 were investigated as phantom materials. First the c(pl) values for plastic materials were calculated as a function of a half-value depth of maximum ionization, I50, in plastic. The c(pl) values for RMI-457 and WE-211 varied from 0.992 to 1.002 and from 0.971 to 0.979, respectively, in a range of nominal energies from 4 MeV to 18 MeV, and varied slightly as a function of I50 in plastic. Since h(pl) values depend on the wall correction factor, P(wall), of the chamber used, they are evaluated using a pure electron fluence correction factor, phi(pl)w, and P(wall)w and P(wall)pl, for a combination of water or plastic phantoms and plane-parallel ionization chambers (NACP-02, Markus and Roos). The phi(pl)w and P(wall) (P(wall)w and P(wall)pl) values were calculated as a function of the water-equivalent depth in plastic materials and at a reference depth as a function of R50 in water, respectively. The phi(pl)w values varied from 1.024 at 4 MeV to 1.013 at 18 MeV for RMI-457, and from 1.025 to 1.016 for WE-211. P(wall)w values for plane-parallel chambers showed values in the order of 1.5% to 2% larger than unity at 4 MeV, consistent with earlier results. The P(wall)pl values of RMI-457 and WE-211 were close to unity for all the energy beams. Finally, calculated h(pl) values of RMI-457 ranged from 1.009 to 1.005, from 1.010 to 1.003 and from 1.011 to 1.007 for NACP-02, Markus and Roos chambers, respectively, in the range of 4 MeV to 18 MeV, and the values of WE-211 were 1.010 to 1.004, 1.010 to 1.004 and 1.012 to 1.008, respectively. The calculated h(pl), values for the Markus chamber agreed within their combined uncertainty with the measured data.