title: Polarity effect on surface dose measurement for an attix parallel plate ionisation chamber.
abstract: The effects of chamber polarity have been investigated for the measurement of 6MV and 18MV x-ray surface dose using a parallel plate ionization chamber. Results have shown that a significant difference in measured ionization is recorded between polarities at 6MV and 18MV at the phantom surface. A polarity ratio ranging from 1.062 to 1.005 is seen for 6MV x-rays at the phantom surface for field sizes 5 cm x 5 cm to 40 cm x 40 cm when comparing positive to negative polarity. These ratio's range from 1.024 to 1.004 for 18MV x-rays with the same field sizes. When these charge reading are compared to the Dmax readings of the same polarity it is found that these polarity effects are minimal for the calculation of percentage dose results with variations being less than 1% of maximum.
authors.affiliation: City University of Hong Kong, Dept. of Physics and Materials Science, Kowloon Tong, Hong Kong. butsonm@iahs.nsw.gov.au
authors.forename: M J
authors.initials: MJ
authors.lastname: Butson
authors.forename: P K N
authors.initials: PK
authors.lastname: Yu
authors.forename: T
authors.initials: T
authors.lastname: Cheung
mesh_terms: D019544:Equipment Failure Analysis; D011786:Quality Control; D011874:Radiometry / Q000295:instrumentation* / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation*; D011882:Radiotherapy, High-Energy / Q000295:instrumentation*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity