title: Characterization of air temperature in modern ion chambers due to phantom geometry and ambient temperature changes.
abstract: OBJECTIVE
  Temperature and pressure corrections are necessary to account for the varying mass of air in the sensitive volume of a vented ionization chamber (IC) when performing absolute dose measurements. Locations commonly used to measure the presumed IC air temperature may not accurately represent the chamber cavity air temperature, and phantoms undergoing temperature changes further compound the problem. Prior studies have characterized thermal equilibrium in separate phantoms for Farmer chambers alone. However, the purpose of this study was to characterize the cavity air temperature dependence on changes in the ambient temperature and phantom geometry configuration for a wider and more modern variety of chambers to determine if previously published wait times apply to these chambers as well.
  
  
  METHODS
  Thermal conduction properties were experimentally investigated by modifying a PTW 0.3 cm(3) Semiflex IC with a thermocouple replacing the central electrode. Air cavity temperature versus time was recorded in three phantom geometries characteristic of common absolute dose measurements. The phantoms were (15 ± 1) °C before measurement with an IC at the treatment vault temperature of (21 ± 1) °C. Simulations were conducted to provide a theoretical basis for the measurements and to simulate temperature response of a PTW PinPoint® and Farmer chamber. The simulation methods were first validated by comparison with measured Semiflex chamber thermal response curves before extension to the other chambers.
  
  
  RESULTS
  Two thermal equilibria curves were recorded on different time scales. IC temperature initially dropped to the colder phantom temperature but subsequently increased as the phantom itself equilibrated with the warmer room temperature. In a large phantom of dimensions (25.5 × 25.5 × 23.4) cm(3), 3 min was required before the IC temperature reached within 0.5 °C of its equilibrium within the phantom. Similarly, wait times of 2 min were needed for 7.5 and 2 cm slab phantoms.
  
  
  CONCLUSIONS
  Recording of temperature in the phantom was deemed far more accurate than measurement in ambient air due to the air cavity thermally equilibrating with phantom temperature instead of the vented ambient air. Wait times of 3 and 2 min are needed for a cube and 7.5 cm slab phantom, respectively, to achieve 0.2% dosimetric accuracy (temperature accuracy of 0.5 °C). Chamber volume alone did not determine wait times, as a 0.3 cm(3) IC required a longer wait time than a Farmer chamber, suggesting wall thickness as an important variable as well.
authors.affiliation: Department of Radiation Oncology, School of Medicine, Cancer Therapy and Research Center, The University of Texas Health Science Center San Antonio, San Antonio, Texas 78229.
authors.forename: Daniel L
authors.initials: DL
authors.lastname: Saenz
authors.affiliation: Department of Radiation Oncology, School of Medicine, Cancer Therapy and Research Center, The University of Texas Health Science Center San Antonio, San Antonio, Texas 78229.
authors.forename: Neil
authors.initials: N
authors.lastname: Kirby
authors.affiliation: Department of Radiation Oncology, School of Medicine, Cancer Therapy and Research Center, The University of Texas Health Science Center San Antonio, San Antonio, Texas 78229.
authors.forename: Alonso N
authors.initials: AN
authors.lastname: Gutiérrez
mesh_terms: D000388:Air; D003198:Computer Simulation; D004867:Equipment Design; D008962:Models, Theoretical; D019047:Phantoms, Imaging; D055618:Radiation Equipment and Supplies; D013696:Temperature