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

Abstract

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.

Differences in electron beam dosimetry using two commercial ionization chambers and the TG‐21 protocol: Another reason to switch to TG‐51

“…Therefore, conversion to the TG‐51 protocol should result in the same magnitude of change for the parallel plate system as would be expected from previously published values 20 – 25 for the Farmer‐type chamber system (NEL 2571 and PTW N23333 chambers), i.e., approximately $0.5-1.2\%$ depending on the energy. However, the TG‐51 protocol explicitly prohibits the use of parallel plate chambers and plastic media for the calibration of photon beams.…”

“…In addition to the uncertainty in the $P_{\text{repl}}$ and $P_{\text{wall}}$ values, another source of uncertainty that may assist in explaining the electron dose differences is the electron beam water to polystyrene fluence correction, ${\varphi }_{\text{poly}}^w$. Ding et al 25 reported that there are large variations in the fluence correction factors published in the literature for polystyrene. There even exist differences in the fluence correction factors between the TG‐21 and TG‐25 protocols 26 .…”

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

“…Corrections are required for differences in stopping power and scattering power. Ding et al 5,6 introduced a depthscaling or range-scaling factor, c pl , and an electron fluence correction factor, pl w , and a chamber-dependent fluence correction factor, h pl , respectively, for the correction factors. c pl converts depth in plastic to the equivalent depth in water so that the electron spectra are identical in shape at two positions in two materials.…”

“…Similarly, h pl corrects the electrometer reading in the plastic phantom to that in the water phantom and accounts for the difference in the reading in the two phantoms at the same water-equivalent depth. h pl depends on the wall correction factor, P wall , of the ionization chamber used, 6,7 which accounts for the nonphantom equivalence of the chamber wall material. h pl is equal to pl w when P wall in water and plastic is the same.…”

“…So far there have been many studies on calculated pl w values and measured h pl values for various plastic phantoms, 6,7,[9][10][11][12][13][14][15] …”