Piotr Ulkowski, Wojciech Bulski and Barbara Gwiazdowska
Analiza niepewności pomiaru współczynnika wzorcowania dawkomierzy terapeutycznych i diagnostycznych, wzorcowanych w polskim Laboratorium Wtórnych Wzorców Dozymetrycznych
Celem pracy było opracowanie metody szacowania niepewności i wstępne określenie wartości szacowanych wielkości w przypadku wzorcowanych w LWWD dawkomierzy. Zastosowano metodę obliczania niepewności typu A i B wg międzynarodowej normy ISO. Uwzględniono stosowany w LWWD sposób wzorcowania dawkomierzy wg raportów Międzynarodowej Agencji Energii Atomowej. Przedstawione wyniki dotyczą wszystkich wzorcowanych w LWWD dawkomierzy. Zarówno szczegółowa analiza wszystkich etapów wzorcowania jak i otrzymane wyniki wskazują, że niepewność pomiarów jest w zakresie akceptowanym przez Polskie Centrum Akredytacji.
Wojciech Bulski, Piotr Ulkowski and Barbara Gwiazdowska
Analysis of Calibration Coefficients of Plane-Parallel Markus Type Ionization Chambers Calibrated in Co-60 and Electron Beams
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.
Wojciech Bulski, Piotr Ulkowski, Barbara Gwiazdowska and Joanna Rostkowska
An analysis of calibration coefficients measured in water and in air for Farmer-type cylindrical ionization chambers
A comparison of calibration coefficients of Farmer-type cylindrical ionization chambers calibrated in a Co-60 beam according to the IAEA Reports 277 and 398 formalisms.
91 calibration coefficients measured in air and in water.
The 398ND, W/277ND, W ratios were calculated, taking into account the wall material of the chambers. The correlation between 398ND, W and NK, ND, air277ND, W was made and the equations of the regression straight lines were determined.
Calibration coefficients, determined according to the IAEA Report 398, are higher by about 1% than those determined according to the IAEA Report 277, and they depend slightly on the wall material of the chamber.
The introduction of the IAEA dosimetry Report 398 resulted in an increase in the doses delivered to patients by about 1% as compared with the period of application of Report 277. The regression equations may assure increased accuracy in retrospective comparative calculation of doses according to both reports as compared with the correction based on mean percent values.
Piotr Ulkowski, Wojciech Bulski, Barbara Gwiazdowska and Joanna Rostkowska
Analysis of the calibration results of ionization chambers for orthovoltage and brachytherapy
An analysis of the exposure calibration coefficients for ionization chambers used for dose determination in orthovoltage radiotherapy and in brachytherapy in the Polish SSDL was performed. The coefficients were determined by calibrating the chambers in the X-ray beam of 235 kV or 290 kV and HVL of 2.5 mmCu or 4 mmCu respectively, and also in the Co-60 gamma beam. Calibration procedures followed IAEA recommendations in Report No. TECDOC 1274 and Technical Reports Series No. 277. The characteristics of the energy dependence of various types of chambers used in orthovoltage radiotherapy and in brachytherapy are presented. On the basis of the summarized data and on the energy dependence curves it was possible to conclude that: 1) in he case of calibration of the chambers for orthovoltage radiotherapy the energy dependence has to be established for the full range beam qualities; 2) in the case of calibration, of the investigated chambers, for brahytherapy with Cs-137 and Ir-192 sources, establishing of the exposure calibration factor for Co-60 is sufficient.