Replacement correction factors for plane-parallel ion chambers in electron beams

Journal Article · Mon Feb 15 04:00:00 EST 2010 · Medical Physics

DOI:https://doi.org/10.1118/1.3276735· OSTI ID:22098500

Wang, Lilie L. W.; Rogers, David W. O. ^[1]

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

Purpose: Plane-parallel chambers are recommended by dosimetry protocols for measurements in (especially low-energy) electron beams. In dosimetry protocols, the replacement correction factor P{sub repl} is assumed unity for ''well-guarded'' plane-parallel chambers in electron beams when the front face of the cavity is the effective point of measurement. There is experimental evidence that ion chambers which are not well-guarded (e.g., Markus) have nonunity P{sub repl} values. Monte Carlo simulations are employed in this study to investigate the replacement correction factors for plane-parallel chambers in electron beams. Methods: Using previously established Monte Carlo calculation methods, the values of P{sub repl} are calculated with high statistical precision for the cavities of a variety of plane-parallel chambers in a water phantom irradiated by various electron beams. The dependences of the values of P{sub repl} on the beam quality, phantom depth, as well as the guard ring width are studied. Results: In the dose fall-off region for low-energy beams, the P{sub repl} values are very sensitive to depth. It is found that this is mainly due to the gradient effect, which originates from the fact that the effective point of measurement for many plane-parallel chambers should not be at the front face of the cavity but rather shifted toward the center of the cavity by a fraction of a millimeter. Using the front face of the cavity as the effective point of measurement, the calculated values of P{sub repl} at d{sub ref} are not unity for some well-guarded plane-parallel chambers. The calculated P{sub repl} values for the Roos chamber are close to 1 for all electron beams. The calculation results for the Markus chamber are in good agreement with the measured values. Conclusions: The appropriate selection of the effective point of measurement for plane-parallel chambers in electron beams is an important issue. If the effective point of measurement is correctly accounted for, the P{sub repl} values would be almost independent of depth. Both the guard ring width and the ratio of the collecting volume diameter to the cavity thickness can influence the values of P{sub repl}. For a diameter to thickness ratio of 5 (e.g., NACP02 chamber), the guard width has to be 6 mm for the chamber to be considered as well-guarded, i.e., have a P{sub repl} value of 1.00.

• OSTI ID: 22098500
• Journal Information: Medical Physics, Journal Name: Medical Physics Journal Issue: 2 Vol. 37; ISSN 0094-2405; ISSN MPHYA6
• Country of Publication: United States
• Language: English

Related Subjects

61 RADIATION PROTECTION AND DOSIMETRY
62 RADIOLOGY AND NUCLEAR MEDICINE
ACCURACY
BIOLOGICAL EFFECTS
COMPUTERIZED SIMULATION
DOSIMETRY
ELECTRON BEAMS
IONIZATION CHAMBERS
IONIZING RADIATIONS
MONTE CARLO METHOD
PHANTOMS
RADIATION DOSES