A direct approach for the determination of absorbed dose from electron beams using non-water phantoms

Non-water solid phantoms are often used in the determination of absorbed dose to water for electron beams. Protocols have been established and widely accepted. In these procedures, several assumptions in addition to the Spencer-Attix conditions are required, and several correction factors are needed. A direct approach, in which the conversion is carried out in a single step using a modified Spencer-Attix formula, is studied in this paper. The approach is consistent with the protocols for water phantom, and the conversion factors can be calculated using Monte Carlo simulation. The behavior of the conversion factors is described by comparing the results from the AAPM protocol and experiment data for three electron energies (6, 12, and 16 MeV). This study demonstrates that for beam calibration at dmax, the results from the new approach agree with those from the protocol with a maximum discrepancy of 1% for PMMA and 1.3% for polystyrene. For the depth dose measurement from near the surface to R80, the agreement is within 1.5% for PMMA, 2.5% for polystyrene, and 2.8% for electron solid water. It also demonstrates that for electron solid water, the new approach provides better agreement with experiment data for the beam calibration at d(max).