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

Abstract

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.

Total skin electron beam therapy using an inclinable couch on motorized table and a compensating filter

“…Thus, we corrected the value that was extrapolated from the h pl that was calculated at the reference depth (d ref ) of various energies in solid phantom by Araki. 10 The stopping power which is an important factor was also calculated from the equation of Burn et al 11 The correction factors of P TP , P pol , and P ion were previously measured. Percentage depth ionization (PDI) was converted to PDD by multiplying various correction factors, which are mentioned above, and stopping power for quality of ionization.…”

Monte Carlo calculations of correction factors for plane‐parallel ionization chambers in clinical electron dosimetry

“…Figure 6 shows the calculated P wall values at a reference depth as a function of R 50 for each chamber and the values are in good agreement with the values reported in previous papers. [6][7][8]15 The P wall values decrease from 1.019 to 1.008 for NACP-02, from 1.019 to 1.005 for Markus, and from 1.015 to 1.006 for Roos, in a range of 4 MeV ͑R 50 = 1.31 cm͒ to 18 MeV ͑R 50 = 7.6 cm͒. The variation in P wall with beam quality is approximately 1%.…”

“…The modeling of Monte Carlo simulations for an electron beam is described in previous papers. 14,15 Phase space data were taken below the applicator with a 15 ϫ 15 cm 2 field size for all electron energies. The dose distributions for electron beams in water were calculated with the EGSnrc/DOSXYZnrc code 16 using the phase space data as input.…”

Monte Carlo calculations of correction factors for plastic phantoms in clinical photon and electron beam dosimetry

“…The water-to-plastic ionization conversion factor for electron beams is known as the chamber-dependent fluence correction factor h pl . 6-9 h pl can be computed from the electron fluence correction factor pl w and the wall correction factor P wall of the ionization chamber used, 8,9 which accounts for the nonphantom equivalence of the chamber wall material. pl w is the ratio of the electron fluence in the water phantom to that in the plastic phantom and accounts for the difference in the electron fluence in the two phantoms at the same waterequivalent depth.…”

“…The electron spectra in the two phantom materials are also identical in shape at the two positions of water-equivalent depth. 7,9 So far there have been many studies on calculated pl w values and measured h pl values for various plastic phantoms in electron beam dosimetry, 6-16 and the measured h pl values for several plastic phantoms are summarized in the IAEA TRS-398 code of practice. However, there have been only few studies 9 on calculated h pl values that considered two wall correction factors, P wall,w and P wall,pl , for a combination of water or plastic phantoms and wall materials of planeparallel chambers because it is difficult to obtain wall correction factors experimentally, especially for plane-parallel chambers.…”