Exposure Calibration Factors Research Articles

Analysis of the calibration results of ionization chambers for orthovoltage and brachytherapy

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.

Stability of A-150 plastic ionisation chamber response over a 30-y period

At the Northern Illinois University Institute for Neutron Therapy at Fermilab, the clinical tissue-equivalent ionisation chamber response is measured every treatment day using a cesium source that was configured to match readings obtained at the National Bureau of Standards. Daily measurements are performed in air using the air-to-tissue dose conversion factors given in AAPM Report #7. The measured exposure calibration factors have been tabulated and graphed as a function of time from 1978 to present. For A-150 plastic ionisation chambers, these factors exhibit a sinusoidal variation with a period of approximately 1 y and amplitude of +/- 1%. This variation, attributable to the hygroscopic nature of A-150 plastic, is correlated with the relative humidity of the facility, and is greater than the humidity corrections for gas described in the literature. The data suggest that chamber calibration should be performed at least weekly to accommodate these variations.

112. The role of the secondary standard dosimetry laboratory in the quality assurance in radiotherapy in poland

In Poland, in 1937, a special measurement unit was established in the Radium Institute (former name of the Institute of Oncology). The continuation of this unit is the SSDL, which was established in 1966 at the Medical Physics Department (MPD) of the Institute of Oncology. In 1988, the Laboratory was approved as a member of the IAEA/WHO SSDL network, and is periodically audited by the IAEA. The SSDL plays an important role in maintaining proper dosimetry standards in radiotherapy in Poland. The main activities of the SSDL are: - Collecting data concerning the intrastructure of radiotherapy in Poland. This activity started in 1993 and is continued on the yearly basis. - Calibration of dosimeters at the national level; it started as early as 1937. The SSDL calibrates over 20 ionisation chambers each year. The Co-60 exposure calibration factors are established, and then the air-kerma and the absorbed dose calibration factors are calculated. The system linearity and energy characteristics are determined in the orthovoltage range. In 2002 the measurement facility for calibrating the mammography dosimeters was set-up. - External quality audits of dosimetry in radiotherapy centres in Poland. During the period 1991–2001, 151 audits, based on TLD dose inter-comparisons were performed in reference conditions. The deviations above 3.5% (detined as acceptance level) were observed in 18 cases (nearly 12%). Since 2002 the audits in more complex, non standard conditions started. - Preparation of protocols and recommendations on dosimetry in radiotherapy. The QC recommendations for Co-60 units and accelerators were issued (brachytherapy has been taken care of by the Centre of Oncology in Cracow). The QC- recommendations for simulators and CT-scanners are in preparation. Training of physicists and radiotherapists (including the participants from other East European countries) in order to adhere to the increasing complexity of modern radiotherapy procedures. Trainings are performed individually or in a form of courses. This activity follows the recommendations of the Euroatom Directive 97/43 on health protection of individuals against the dangers of ionizing radiation in relation to medical exposure.

Calculated absorbed-dose ratios, TG51/TG21, for most widely used cylindrical and parallel-plate ion chambers over a range of photon and electron energies.

Task Group 51 (TG51), of the Radiation Therapy Committee of the American Association of Physicists in Medicine (AAPM), has developed a calibration protocol for high-energy photon and electron therapy beams based on absorbed dose standards. This protocol is intended to replace the air-kerma based protocol developed by an earlier AAPM task group (TG21). Conversion to the newer protocol introduces a change in the determined absorbed dose. In this work, the change in dose is expressed as the ratio of the doses (TG51/TG21) based on the two protocols. Dose is compared at the TG-51 reference depths of 10 cm for photons and d(ref) for electrons. Dose ratios are presented for a variety of ion chambers over a range of photon and electron energies. The TG51/TG21 dose ratios presented here are based on the dosimetry factors provided by the two protocols and the chamber-specific absorbed dose and exposure calibration factors (N60Co(D,w) and Nx) provided by the Accredited Dosimetry Calibration Laboratory (ADCL) at The University of Texas, M. D. Anderson Cancer Center (MDACC). As such, the values presented here represent the expected discrepancies between the two protocols due only to changes in the dosimetry parameters and the differences in chamber-specific dose and air-kerma standards. These values are independent of factors such as measurement uncertainties, setup errors, and inconsistencies arising from the mix of different phantoms and ion chambers for the two protocols. Therefore, these ratios may serve as a guide for institutions performing measurements for the switch from TG21-to-TG51 based calibration. Any significant deviation in the ratio obtained from measurements versus those presented here should prompt a review to identify possible errors and inconsistencies. For all cylindrical chambers included here, the TG51/TG21 dose ratios are the same within +/-0.6%, irrespective of the make and model of the chamber, for each photon and electron beam included. Photon beams show the TG51/TG21 dose ratios decreasing with energy, whereas electrons exhibit the opposite trend. The dose ratio for photons is near 1.00 at 18 mV increasing to near 1.01 at 4 mV while the dose ratio for electrons is near 1.02 at 20 MeV decreasing only 0.5% to near 1.015 at 6 MeV. For parallel-plate chambers, the situation is complicated by the two possible methods of obtaining calibration factors: through an ADCL or through a cross-comparison with a cylindrical chamber in a high-energy electron beam. For some chambers, the two methods lead to significantly different calibration factors, which in turn lead to significantly different TG51/TG21 results for the same chamber. Data show that if both N60Co(D,w) and Nx are obtained from the same source, namely an ADCL or a cross comparison, the TG51/TG21 results for parallel-plate chambers are similar to those for cylindrical chambers. However, an inconsistent set of calibration factors, i.e., using N60Co(D,w) x k(ecal) from an ADCL but Ngas from a cross comparison or vice versa, can introduce an additional uncertainty up to 2.5% in the TG51/TG21 dose ratios.

Dosimetry of pulsed clinical proton beams by a small ionization chamber.

Response of a micro volume (0.01 ml) ionization chamber has been studied with pulsed proton beams which are used for clinical purposes and has been compared with those of some JARP ionization chambers (0.6 ml). All chambers used had been calibrated by standard 60Co beams at the Electrotechnical Laboratory (ETL) and exposure calibration factors, Nx, were obtained on advance. Two methods are used to compensate the general recombination which occurs during pulsed beam irradiations: theoretical correction by a Boag's formulation and a modified two-voltage technique. An evaluation of absolute absorbed dose-to-water is performed on the basis of the protocol provided by ICRU report 59. The results imply that, to a first approximation, both chambers indicate the almost same result within 2% when unknown chamber-dependent parameters of the micro chamber are tentatively assumed to be identical to those of the JARP chamber for the calibration with 60Co beams. The about 1.5% discrepancy observed in the response of both chambers is not discussible due to presumably 1-2% uncertainty of the protocol of ICRU report 59 which does not include any chamber-dependent corrections for the perturbation effects in proton beams.

The advantages of absorbed-dose calibration factors.

A formalism for clinical external beam dosimetry based on use of ion chamber absorbed-dose calibration factors is outlined in the context and notation of the AAPM TG-21 protocol. It is shown that basing clinical dosimetry on absorbed-dose calibration factors ND leads to considerable simplification and reduced uncertainty in dose measurement. In keeping with a protocol which is used in Germany, a quantity kQ is defined which relates an absorbed-dose calibration factor in a beam of quality Q0 to that in a beam of quality Q. For 38 cylindrical ion chambers, two sets of values are presented for ND/NX and Ngas/ND and for kQ for photon beams with beam quality specified by the TPR20(10) ratio. One set is based on TG-21's protocol to allow the new formalism to be used while maintaining equivalence to the TG-21 protocol. To demonstrate the magnitude of the overall error in the TG-21 protocol, the other set uses corrected versions of the TG-21 equations and the more consistent physical data of the IAEA Code of Practice. Comparisons are made to procedures based on air-kerma or exposure calibration factors and it is shown that accuracy and simplicity are gained by avoiding the determination of Ngas from NX. It is also shown that the kQ approach simplifies the use of plastic phantoms in photon beams since kQ values change by less than 0.6% compared to those in water although an overall correction factor of 0.973 is needed to go from absorbed dose in water calibration factors to those in PMMA or polystyrene. Values of kQ calculated using the IAEA Code of Practice are presented but are shown to be anomalous because of the way the effective point of measurement changes for 60Co beams. In photon beams the major difference between the IAEA Code of Practice and the corrected AAPM TG-21 protocol is shown to be the Prepl correction factor. Calculated kQ curves and three parameter equations for them are presented for each wall material and are shown to represent accurately the kQ curve for all ion chambers in this study with a wall of that specified material and a thickness less than 0.25 g/cm2. Values of kQ can be measured using the primary standards for absorbed dose in photon beams.

Equations for Ngas and Nair in terms of Nx and Nk.

Task Group 21 of the American Association of Physicists in Medicine defined the cavity-gas calibration factor Ngas for ionization chambers in a protocol for radiotherapy dosimetry published in this journal [Med. Phys. 10, 741 (1983)]. Later, Schulz et al., [Med. Phys. 13, 755 (1986)] published a letter of clarification that proposed a revised equation for Ngas in which the effect of atmospheric humidity was specifically dealt with. The present report points out errors in both of those presentations of the Ngas equation, and derives formulations for cases where the standardization laboratory does or does not correct for ambient humidity during chamber calibration. In the case where humidity is corrected for, the resulting quantity is more properly called "Nair." The Ngas equation is stated in terms of the air kerma calibration factor Nk as well as the more familiar exposure calibration factor Nx.

The effect of differences in data base on the determination of absorbed dose in high‐energy photon beams using the American Association of Physicists in Medicine protocol

Exposure rates were adjusted at the National Institute of Standards and Technology (NIST) on January 1, 1986 to take into account more recent values for some physical parameters, mainly in electron stopping power ratios. Exposure calibration factors for 60Co gamma rays Nx will therefore be lowered by 1.1%. Consequently, absorbed dose determinations in high-energy photon beams will be reduced by the same amount if the values for these physical parameters remain unchanged in the American Association of Physicists in Medicine (AAPM) protocol. If the same data base as used at NIST is applied in the AAPM protocol, then Ngas/Nx values, water-air stopping power ratios, and Pwall values will be different. The overall change in absorbed dose determinations using a consistent set of data will be a reduction of 0.8% for 60Co gamma rays and 1.5% for a 20-MV x-ray beam compared to the values before January 1, 1986. Since the net effect is small when different sets of data are applied, the new NIST exposure calibration factors may be used in combination with the AAPM protocol without significant error.

Variations in response to radiation of a nylon-walled ionization chamber induced by humidity changes.

The volume of nylon-walled ionization chambers varies with relative air humidity due to the hygroscopic properties of nylon. A 6% increase in exposure calibration factor of a commonly employed 0.6 cm3 nylon-walled ionization chamber was observed when the relative air humidity decreased from 98% to 11%. The increase as well as the decrease in volume shows an initial fast change during the first day followed by an exponential change with half-lives of about 1.2 days and 2.4 days, for water uptake and water loss, respectively.

A proposal concerning the absorbed dose conversion factor

New definitions of the absorbed dose conversion factors Clambda and CE are proposed. The absorbed dose in water is given by the product of absorbed dose conversion factor, exposure calibration factor, ionisation chamber reading, cap displacement correction factor and perturbation correction factor. At exposure calibration the material of the build-up cap must be the same as that of the chamber wall. An ionisation chamber of which the wall material is water-equivalent or air-equivalent may be used. In the latter case the wall must be thin. For these two cases absorbed dose conversion factors are introduced and it is recommended that either of the two sets should be adopted. Furthermore, if the chamber wall is neither water- nor air-equivalent, the factor by which these currently defined values should be multiplied is also given: again the wall must be thin.