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

Abstract

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).

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

“…The R gas/med water quantity of Lu and Chin 13 can be shown to be equal, in principle, to our quantity (L/) air water p w . Their val- ues are for monoenergetic electron beams and they have used an effective density which differs from what we have used.…”

“…We follow this approach and derive a very similar equation in the next section but without introducing the Bragg-Gray stopping-power ratios and the need for the approximation. Lu and Chin 13 have introduced another approach in which they use Monte Carlo techniques to calculate a single factor, R gas/med water to replace the factors (L/) a p (S/) p w p w in Eq. ͑5͒.…”