Calculation of the replacement correction factors for ion chambers in megavoltage beams by Monte Carlo simulation

Journal Article · Thu May 15 04:00:00 EDT 2008 · Medical Physics

DOI:https://doi.org/10.1118/1.2898139· OSTI ID:21120681

Wang, L L. W.; Rogers, D W. O. ^[1]

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1. Ottawa Carleton Institute of Physics, Carleton University Campus, Ottawa K1S 5B6 (Canada)

This article describes four methods of calculating the replacement correction factor, P{sub repl} (or the product p{sub cav}p{sub dis} in the IAEA's notation), for a plane-parallel chamber in both electron and photon beams, and for a Farmer chamber in photon beams, by using the EGSnrc Monte Carlo code. The accuracy of underlying assumptions and relative merits of each technique are assessed. With careful selection of parameters it appears that all four methods give reasonable answers although the direct methods are more intellectually satisfying and more accurate in some cases. The direct methods are shown to have an accuracy of 0.1% when appropriate calculation parameters are selected. The depth dependence of P{sub repl} for the NACP02 plane-parallel chamber has been calculated in both 6 and 18 MeV electron beams. At the reference depth (0.6R{sub 50}-0.1 cm) P{sub repl} is 0.9964 for the 6 MeV beam and 1.0005 for the 18 MeV beam for this well-guarded chamber; at the depth of maximum dose for the 18 MeV beam, P{sub repl} is 1.0010. P{sub repl} is also calculated for the NACP02 chamber and a Farmer chamber (diameter 6 mm) at a depth of 5 cm in a {sup 60}Co photon beam, giving values of 1.0063 and 0.9964, respectively. For the Farmer chamber, P{sub repl} is about half a percent higher than the value (0.992) recommended by the AAPM dosimetry protocol. It is found that the dosimetry protocols may have adopted an incorrect value of P{sub repl} for cylindrical chambers in photon beams. The nonunity values of P{sub repl} for plane-parallel chambers in lower energy electron beams imply a variety of values used in dosimetry protocols must be reassessed.

• OSTI ID: 21120681
• Journal Information: Medical Physics, Journal Name: Medical Physics Journal Issue: 5 Vol. 35; ISSN 0094-2405; ISSN MPHYA6
• Country of Publication: United States
• Language: English

Related Subjects

61 RADIATION PROTECTION AND DOSIMETRY
ACCURACY
COBALT 60
COMPUTERIZED SIMULATION
CORRECTIONS
DOSIMETRY
ELECTRON BEAMS
IONIZATION CHAMBERS
MONTE CARLO METHOD
PHOTON BEAMS