Analysis of Calibration Coefficients of Plane-Parallel Markus Type Ionization Chambers Calibrated in Co-60 and Electron Beams
Languages of publication
The Polish Secondary Standard Dosimetry Laboratory is part of the IAEA/WHO network of such laboratories. The SSDLs are usually not equipped with accelerators generating high energy electron beams for calibration of dosimeters. The access to medical accelerators is seriously limited due to the heavy patient load. Therefore attempts are made to use Co-60 beams for calibration of plane parallel chambers and calculate the calibration coefficients for other radiation quality, the high energy electron beam.The Markus ionization chambers, most frequently used in Poland, were analyzed in this study. The material was composed of 36 plane parallel chambers, from 20 radiotherapy centers in Poland, calibrated at the Polish SSDL during the period of 2003-2006. Before actual calibration, a number of chamber parameters were tested: long term stability, dark current, chamber sensitivity, non-linearity of dosimeter readings. Each chamber was calibrated in two different radiation beams: a) Varian 2300 accelerator 22 MeV electron beams, beam output 1.2 cGy/MU at 300 MU/min; b) Co-60 Theratron 780/403 unit with a Cobalt-60 source of 155700 GBq (4208 Ci) activity as of 6.01.2006. A reference dosimeter Keithley Instruments Inc. 6517-A with cylindrical ionization chambers Nuclear Enterprises Technology Limited type 2571 was used as the reference standard. The methods of IAEA Code of Practice for Dosimetry TRS 398 were adopted. The long term stability was analyzed on basis of calibration coefficients of 23 Markus chambers calibrated several times during the period 1994-2002.Very small differences in calibration coefficients were detected between the two calibration methods used. They ranged between -0.3 to +0.5%, the mean value being 0.1%. A very good long term stability of calibration coefficients of Markus chambers, related to the mean value over the 7 year period, ranging between -0.5 to +0.3%, was recorded.Very small differences in the results for the two calibration methods, confirmed by small standard deviations observed, indicate that these two calibration methods in the case of Markus-type plane parallel chambers may be used alternatively.
1 - 1 - 2007
30 - 12 - 2008
- Almond PR, Attix RH, Humphries LJ, Kubo H, Nath R, Goetsch S, Rogers DWO. The calibration and use of plane parallel ionisation chambers for dosimetry of electron beams: An extension of the 1983 AAPM protocol report of AAPM Radiation Therapy Committee Task Group No 39. Med Phys 1994; 21(8): 1251-1260.
- IAEA. The use of plane parallel ionisation chambers in high energy electron and photon beams. An International Code of Practice for Dosimetry. Technical Reports Series. Vienna, 1997; No TRS 381.
- Dąbrowski R. Dozymetria z zastosowaniem komór jonizacyjnych. In: Pawlicki G, Pałko T, Golnik N, Gwiazdowska B, Królicki L, redaktorzy. Fizyka medyczna. Warszawa: Akademicka Oficyna Wydawnicza Exit; 2002. p. 209-262.
- Gerbi BJ, Khan FM. Measurement of dose in build-up region using fixed - separation plane-parallel ionisation chambers. Med Phys 1990; 17(1): 17-26.[Crossref]
- Mellenberg D. Determination of build-up region over-response corrections for Markus-type chamber. Med Phys 1990; 17(6): 1041-1044.[PubMed][Crossref]
- Ross CK, Shortt KR. The effect of waterproofing sleeves on ionisation chamber response. Phys Med Biol 1992; 37: 1403-1411.
- IAEA. Absorbed Dose Determination in External Beam Radiotherapy. An International Code of Practice for Dosimetry Based on Standards of Absorbed Dose to Water. Technical Reports Series. Vienna, 2000; No TRS 398.
- Morawska M. Testy kontroli jakości dawkomierzy z komorami płaskimi. Nowotwory 2000; 50(3): 294-302.
- Boag JW, Currant J. Current collection and ionic recombination in small cylindrical ionisation chambers exposed to pulsed radiation. Brit J Radiol 1980; 53: 471-478.
- IAEA. Absorbed dose determination in photon and electron beams. An international code of practice. Technical Reports Series, second edition, Vienna, 1997; No TRS 277.
Publication order reference