Thermal Equilibration in the Cavity Volume of a Farmer Ion Chamber for Routine Dosimetry

Yuichi Kato^1,3, Hiraku Fuse², Kazuya Shinoda^1,3, Katsumi Miyamoto³, Tatsuya Fujisaki²

¹Graduate School of Radiological Sciences, Ibaraki Prefectural University of Health Sciences, Ami Town, Japan.
²Department of Radiological Sciences, Ibaraki Prefectural University of Health Sciences, Ami Town, Japan.
³Department of Radiological Technology, Tsukuba Medical Center Hospital, Tsukuba City, Japan.

DOI: 10.4236/ijmpcero.2015.44032                

Abstract

A Farmer ion chamber with an air cavity volume is the most widely used dosimeter for accurate dose determinations in radiotherapy. The quantity of ionization in the cavity volume occurred a given radiation dose has to be corrected to the cavity air temperature according to a dosimetry protocol because the mass of air in the cavity volume is subject to atmospheric variations. In the present study, we aim to measure the thermal equilibration time in the cavity volume of a Farmer ion chamber for the routine dosimetry. The Farmer ion chamber’s electrode was replaced by a thin thermocouple and coated by the PMMA for a waterproofing so that the measurement of the temperature in the cavity performed in water. As a result of the measurement, A Farmer ion chamber in thermal equilibrium with waterproofing equilibrates rapidly, followed by an exponential fall-off. In water, equilibration to less than 10% of the initial temperature difference required only a few minutes. Thermal equilibrium time is hardly affected by the room temperature change.

Keywords

Farmer Ion Chamber, Thermal Equilibrium Time, Temperature Characteristic, Routine Dosimetry

Conflicts of Interest

The authors declare no conflicts of interest.

References

[1]	Japan Society of Medical Physics (2012) Standard Dosimetry of Absorbed Dose to Water in External Beam Radiotherapy 12. Tsusho-Sangyo Kenkyu-Sha, Tokyo.
[2]	International Atomic Energy Agency (2000) Technical Reports Series No. 398, Absorbed Dose Determination in External Beam Radiotherapy. International Atomic Energy Agency, Vienna.
[3]	Almond, P.R., Biggs, P.J., Coursey, B.M., Hanson, W.F., Saiful Huq, M., Nath, R. and Rogers, D.W.O. (1999) AAPM Task Group No. 51, Protocol for Clinical Reference Dosimetry of High-Energy Photon and Electron Beams. Medical Physics, 26, 1847-1870. http://dx.doi.org/10.1118/1.598691
[4]	Hiraoka, T., Kawashima, K., Hoshino, K. and Matsuzawa, H. (1982) Temperature and Pressure Characteristics of JAPM Dosimeter and Long-Term Stability of Several Barometers. Nippon Acta Radiologica, 42, 1137-1146. (In Japanese)
[5]	Kubo, H. (1983) Ionization Chamber Response to a Sudden Change of Ambient Temperature in Air, Water and a Polystyrene Phantom. Medical Physics, 10, 676-679. http://dx.doi.org/10.1118/1.595363
[6]	van der Giessen, P.H. (1986) About the Rate of Temperature Changes in a Thimble Chamber. Radiation Oncology, 7, 287-291. http://dx.doi.org/10.1016/S0167-8140(86)80040-8
[7]	Mayo, C.S. and Gottschalk, B. (1992) Temperature Coefficient of Open Thimble Chambers. Physics in Medicine and Biology, 37, 289-291. http://dx.doi.org/10.1088/0031-9155/37/1/022
[8]	Tailor, R.C., Chu, C., Followill, D.S. and Hanson, W.F. (1998) Equilibration of Air Temperature Inside the Thimble of a Farmer-Type Ion Chamber. Medical Physics, 25, 496-502. http://dx.doi.org/10.1118/1.598226