Monte Carlo study of correction factors for the use of plastic phantoms in clinical electron dosimetry
- Department of Radiological Technology, Kumamoto University School of Health Sciences, 4-24-1, Kuhonji, Kumamoto, 862-0976 (Japan)
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 R{sub 50}<4 g/cm{sup 2}. In electron dosimetry with plastic, a depth-scaling factor, c{sub pl}, and a chamber-dependent fluence correction factor, h{sub 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{sub pl} values for plastic materials were calculated as a function of a half-value depth of maximum ionization, I{sub 50}, in plastic. The c{sub 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 I{sub 50} in plastic. Since h{sub pl} values depend on the wall correction factor, P{sub wall}, of the chamber used, they are evaluated using a pure electron fluence correction factor, {phi}{sub pl}{sup w}, and P{sub wall}{sup w} and P{sub wall}{sup pl} for a combination of water or plastic phantoms and plane-parallel ionization chambers (NACP-02, Markus and Roos). The {phi}{sub pl}{sup w} and P{sub wall} (P{sub wall}{sup w} and P{sub wall}{sup pl}) values were calculated as a function of the water-equivalent depth in plastic materials and at a reference depth as a function of R{sub 50} in water, respectively. The {phi}{sub pl}{sup 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{sub wall}{sup 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{sub wall}{sup pl} values of RMI-457 and WE-211 were close to unity for all the energy beams. Finally, calculated h{sub 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{sub pl} values for the Markus chamber agreed within their combined uncertainty with the measured data.
- OSTI ID:
- 21032848
- Journal Information:
- Medical Physics, Journal Name: Medical Physics Journal Issue: 11 Vol. 34; ISSN 0094-2405; ISSN MPHYA6
- Country of Publication:
- United States
- Language:
- English
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