Diagnosing atmospheric communication of a sealed monitor chamber.

Autor: McCaw TJ; Department of Human Oncology, University of Wisconsin, Madison, WI, USA., Barraclough BA; Department of Human Oncology, University of Wisconsin, Madison, WI, USA., Belanger M; Department of Human Oncology, University of Wisconsin, Madison, WI, USA., Besemer A; Department of Radiation Oncology, University of Nebraska Medical Center, Omaha, NE, USA., Dunkerley DAP; Department of Radiation Oncology, University of Iowa, Iowa City, IA, USA., Labby ZE; Department of Human Oncology, University of Wisconsin, Madison, WI, USA.
Jazyk: angličtina
Zdroj: Journal of applied clinical medical physics [J Appl Clin Med Phys] 2020 Aug; Vol. 21 (8), pp. 309-314. Date of Electronic Publication: 2020 Jul 10.
DOI: 10.1002/acm2.12975
Abstrakt: Daily output variations of up to ±2% were observed for a protracted time on a Varian TrueBeam® STx; these output variations were hypothesized to be the result of atmospheric communication of the sealed monitor chamber. Daily changes in output relative to baseline, measured with an ionization chamber array (DQA3) and the amorphous silicon flat panel detector (IDU) on the TrueBeam®, were compared with daily temperature-pressure corrections (P TP ) determined from sensors within the DQA3. Output measurements were performed using a Farmer® ionization chamber over a 5-hour period, during which there was controlled variation in the monitor chamber temperature. The root mean square difference between percentage output change from baseline measured with the DQA3 and IDU was 0.50% over all measurements. Over a 7-month retrospective review of daily changes in output and P TP , weak correlation (R 2  = 0.30) was observed between output and P TP for the first 5 months; for the final 2 months, daily output changes were linearly correlated with changes in P TP , with a slope of 0.84 (R 2  = 0.89). Ionization measurements corrected for ambient temperature and pressure during controlled heating and cooling of the monitor chamber differed from expected values for a sealed monitor chamber by up to 4.6%, but were consistent with expectation for an air-communicating monitor chamber within uncertainty (1.3%, k = 2). Following replacement of the depressurized monitor chamber, there has been no correlation between daily percentage change in output and P TP (R 2  = 0.09). The utility of control charts is demonstrated for earlier identification of changes in the sensitivity of a sealed monitor chamber.
(© 2020 The Authors. Journal of Applied Clinical Medical Physics published by Wiley Periodicals, Inc. on behalf of American Association of Physicists in Medicine.)
Databáze: MEDLINE
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