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Replacement correction factors for plane‐parallel ion chambers in electron beams
Author(s) -
Wang Lilie L. W.,
Rogers David W. O.
Publication year - 2010
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3276735
Subject(s) - monte carlo method , dosimetry , imaging phantom , physics , optics , electron , plane (geometry) , computational physics , atomic physics , nuclear physics , nuclear medicine , geometry , medicine , statistics , mathematics
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 replis 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 nonunityP replvalues. 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 replare 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 ofP replon 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 replvalues 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 ofP replatd refare not unity for some well‐guarded plane‐parallel chambers. The calculatedP replvalues 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 replvalues 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 ofP 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 aP replvalue of 1.00.