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dc.contributor.authorKupfer, Tom-
dc.contributor.authorLehmann, Joerg-
dc.contributor.authorButler, Duncan J-
dc.contributor.authorRamanathan, Ganesan-
dc.contributor.authorBailey, Tracy E-
dc.contributor.authorFranich, Rick D-
dc.identifier.citationJournal of Applied Clinical Medical Physics 2017; 18(6): 206-217en_US
dc.description.abstractPURPOSE: This study investigates a large-area plane-parallel ionization chamber (LAC) for measurements of dose-area product in water (DAPw ) in megavoltage (MV) photon fields. METHODS: Uniformity of electrode separation of the LAC (PTW34070 Bragg Peak Chamber, sensitive volume diameter: 8.16 cm) was measured using high-resolution microCT. Signal dependence on angle α of beam incidence for square 6 MV fields of side length s = 20 cm and 1 cm was measured in air. Polarity and recombination effects were characterized in 6, 10, and 18 MV photons fields. To assess the lateral setup tolerance, scanned LAC profiles of a 1 × 1 cm2 field were acquired. A 6 MV calibration coefficient, ND,w,LAC , was determined in a field collimated by a 5 cm diameter stereotactic cone with known DAPw . Additional calibrations in 10 × 10 cm2 fields at 6, 10, and 18 MV were performed. RESULTS: Electrode separation is uniform and agrees with specifications. Volume-averaging leads to a signal increase proportional to ~1/cos(α) in small fields. Correction factors for polarity and recombination range between 0.9986 to 0.9996 and 1.0007 to 1.0024, respectively. Off-axis displacement by up to 0.5 cm did not change the measured signal in a 1 × 1 cm2 field. ND,w,LAC was 163.7 mGy cm-2 nC-1 and differs by +3.0% from the coefficient derived in the 10 × 10 cm2 6 MV field. Response in 10 and 18 MV fields increased by 1.0% and 2.7% compared to 6 MV. CONCLUSIONS: The LAC requires only small correction factors for DAPw measurements and shows little energy dependence. Lateral setup errors of 0.5 cm are tolerated in 1 × 1 cm2 fields, but beam incidence must be kept as close to normal as possible. Calibration in 10 × 10 fields is not recommended because of the LAC's over-response. The accuracy of relative point-dose measurements in the field's periphery is an important limiting factor for the accuracy of DAPw measurements.en_US
dc.subjectdose-area producten_US
dc.subjectlarge-area chamberen_US
dc.subjectlinear acceleratorsen_US
dc.subjectquality assuranceen_US
dc.subjectsmall field dosimetryen_US
dc.titleCommissioning of a PTW 34070 large-area plane-parallel ionization chamber for small field megavoltage photon dosimetryen_US
dc.typeJournal Articleen_US
dc.identifier.journaltitleJournal of Applied Clinical Medical Physicsen_US
dc.identifier.affiliationSchool of Science, RMIT University, Melbourne, Victoria, Australiaen_US
dc.identifier.affiliationRadiation Oncology Centre, Austin Health, Heidelberg, Victoria, Australiaen_US
dc.identifier.affiliationFaculty of Science, The University of Sydney, Sydney, NSW, Australiaen_US
dc.identifier.affiliationDepartment of Radiation Oncology, Calvary Mater Newcastle, Waratah, NSW, Australiaen_US
dc.identifier.affiliationAustralian Radiation Protection and Nuclear Safety Agency, Yallambie, Victoria, Australiaen_US
dc.type.austinJournal Articleen_US
item.fulltextNo Fulltext-
item.openairetypeJournal Article-
item.openairecristype Oncology-
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