General ion recombination for ionization chambers used under irradiation conditions relevant for diagnostic radiology

Evaluation of ion chamber response for applications in electron FLASH radiotherapy

Ion chambers are required for calibration and reference dosimetry applications in radiation therapy (RT). However, exposure of ion chambers in ultra-high dose rate (UHDR) conditions pertinent to FLASH-RT leads to severe saturation and ion recombination, which limits their performance and usability. The purpose of this study was to comprehensively evaluate a set of commonly used commercially available ion chambers in RT, all with different design characteristics, and use this information to produce a prototype ion chamber with improved performance in UHDR conditions as a first step toward ion chambers specific for FLASH-RT. The Advanced Markus and Exradin A10, A26, and A20 ion chambers were evaluated. The chambers were placed in a water tank, at a depth of 2 cm, and exposed to an UHDR electron beam at different pulse repetition frequency (PRF), pulse width (PW), and pulse amplitude settings on an IntraOp Mobetron. Ion chamber responses were investigated for the various beam parameter settings to isolate their dependence on integrated dose, mean dose rate and instantaneous dose rate, dose-per-pulse (DPP), and their design features such as chamber type, bias voltage, and collection volume. Furthermore, a thin parallel-plate (TPP) prototype ion chamber with reduced collector plate separation and volume was constructed and equally evaluated as the other chambers. The charge collection efficiency of the investigated ion chambers decreased with increasing DPP, whereas the mean dose rate did not affect the response of the chambers (± 1%). The dependence of the chamber response on DPP was found to be solely related to the total dose within the pulse, and not on mean dose rate, PW, or instantaneous dose rate within the ranges investigated. The polarity correction factor (Ppol ) values of the TPP prototype, A10, and Advanced Markus chambers were found to be independent of DPP and dose rate (± 2%), while the A20 and A26 chambers yielded significantly larger variations and dependencies under the same conditions. Ion chamber performance evaluated under different irradiation conditions of an UHDR electron beam revealed a strong dependence on DPP and a negligible dependence on the mean and instantaneous dose rates. These results suggest that modifications to ion chambers design to improve their usability in UHDR beamlines should focus on minimizing DPP effects, with emphasis on optimizing the electric field strength, through the construction of smaller electrode separation and larger bias voltages. This was confirmed through the production and evaluation of a prototype ion chamber specifically designed with these characteristics.

Performance evaluation of diagnostic radiology dosimeters in clinical and calibration x-ray beams

Diagnostic radiology dosimeters should comply with International Electrotechnical Commission (IEC) 61674 standard in order to perform measurements with sufficient accuracy and reliability. The calibration of a dosimeter is performed under, and pertains to, reference conditions. However, in most cases, dosimeters are used for clinical measurements under non-reference conditions. The performance, in terms of accuracy of dose measurements, of six commercial diagnostic radiology dosimeters was tested at reference calibration and at clinical non-reference conditions. The results showed that all dosimeters being tested exhibited limits of variation within the +/-5% IEC limits. Depending on the detector's physical and operational properties, the dosimeters' energy dependence of response values varied from -4.7% to +4.2%. To address this variation of response, calibration at three radiation qualities (RQR 3, RQR 5, and RQR 9), at least, is recommended. Different irradiation conditions such as air kerma rate, x-ray tube design, x-ray system, and dosimeter operational modes affect the dosimeters' response by less than 3%. A dosimeter that complies with IEC standards and operates according to its specifications could be used at typical clinical irradiation conditions taking into account only corrections for the energy dependence of response. In this case, the error in dose accuracy is expected to be less than 3%.

Câmara de ionização de placas paralelas para radiação-X de radiografia convencional e mamografia

OBJETIVO: Desenvolver uma câmara de ionização de placas paralelas de dupla face para determinação de kerma no ar e taxa de kerma no ar em campos de radiação-X utilizados em radiografia convencional e mamografia. MATERIAIS E MÉTODOS: A câmara desenvolvida tem janelas de entrada de poliéster aluminizado, elétrodos internos e anéis de guarda de alumínio em uma face (face A) e de grafite na outra (face G). Neste trabalho foram testadas as características operacionais de linearidade, dependência angular e energética de resposta. RESULTADOS: A linearidade de resposta foi de 0,86% para a face A e de 0,92% para a face G. Para ângulos de incidência da radiação de 0° a ± 5°, a variação da resposta relativa foi menor que 0,8% para ambas as faces da câmara. A dependência energética de resposta foi de 0,8% para a face A nas qualidades de raios-X para diagnóstico convencional e de 2,4% para a face G da câmara nas qualidades de raios-X para mamografia. CONCLUSÃO: Esta câmara pode ser utilizada rotineiramente na determinação de kerma no ar e taxa de kerma no ar em feixes de raios-X utilizados em radiografia convencional e mamografia.

Measurement of the leakage radiation from linear accelerators in the backward direction for 4, 6, 10, 15, and 18 MV x-ray energies

The x-ray leakage from the housing of a therapy x-ray source is regulated to be <0.1% of the useful beam exposure at 1 m from the source. It is to be expected that the machine leakage in the backward direction would be less because the gantry and stand contain significant amounts of additional metal to attenuate the x rays. A reduction in head leakage in this direction will have a direct effect on the thickness of the shielding wall behind the linear accelerator. However, no reports have been published to date on measurements in this area. The x-ray leakage in the backward direction has been measured from linacs having energies of 4, 6, 10, 15, and 18 MV using a 100 cm ionization chamber and Al2O3 dosimeters. The leakage was measured at nine different positions over the rear wall using a 3 x 3 matrix with a 1-m separation between adjacent horizontal and vertical points with either the leftmost or rightmost column aligned with the target and isocenter. In general, the leakage is less than the canonical value, but the exact value depends on energy, gantry angle, and measurement position. There is significantly greater attenuation directly behind the gantry stand for all energies. Leakage at 10 MV for some positions exceeded 0.1%. Additionally, neutron leakage measurements were made for 10, 15, and 18 MV x-ray beams using track-etch detectors. The average neutron leakage was less than 0.1% except for 18 MV, where neutron leakage was more than 0.1% of the useful beam at some positions.

Measurement of scatter factors for 4, 6, 10, and 23 MV x rays at scattering angles between 30 degrees and 135 degrees

NCRP Report No. 49, published in 1976, describes how to calculate the shielding for the medical use of x rays and gamma rays for energies up to 10 MV, including primary, scattered, and leakage radiation. However, in that report, data for scattered radiation for linear accelerators exist only for 6 MV, and leakage radiation is assumed, incorrectly, to be equivalent to primary radiation. Since the publication of that report, linear accelerators with energies up to 25 MV have been widely used in the radiation therapy community. Thus, there is a need to measure additional data for all energies in the range 4-25 MV. In this study, measurements were made of the "a" factor for 4, 6, 10, and 23 MV x rays at scattering angles between 30 degrees and 135 degrees. The results show that the 6 and 10 MV "a" factor data are consistent with published data, and the 23 MV data are also consistent with recently published data at 18 and 25 MV. The data show that, in general, the "a" factor decreases with energy; the exception is that 23 MV data show a sharp increase at low scattering angles, much greater than at other energies.

Evaluation of physical performance of a scintillation dosemeter for patient dosimetry in diagnostic radiology

The physical performance of the patient scintillation dosemeter Skin Dose Monitor (SDM) was evaluated for use in diagnostic radiology. The SDM response was found to be linear, with output air kerma and output air kerma rate having a reproducibility in time lower than +/- 2.4%) (one standard deviation). A calibration protocol taking into account the more significant parameters, such as radiation quality dependence and the relative sensitivity of SDM detectors of the same batch, can be applied so that the maximum overall uncertainty is +/- 18% at the 95% confidence level. The SDM detectors did not show any loss of sensitivity during the 5-month period of evaluation. SDM performance was evaluated against thermoluminescent dosemeters (TLDs) (GR200A) by monitoring chest X-rays in 18 adult patients. The difference in entrance surface dose (ESD) values between the SDM and TLDs was less than 10%, but a lack of accuracy in ESD values of less than 0.3 mGy was observed. The main benefit of the SDM device compared with TLDs is the real-time read-out of dose combined with a better flexibility and rapidity of use for approximately the same cost per measurement. The SDM device is a good candidate for regular measurement of patient doses in diagnostic imaging departments as required under implementation of the European Council Medical Exposure Directive of 30 June 1997.

Measurement of the collection efficiency of a large volume spherical ionization chamber in megavoltage therapy beams

The collection efficiency of a 5.7 cm diameter spherical ionization chamber has been measured in 4 MV and 10 MV x-ray beams at various distances from the source. This chamber was found to have a substantial inefficiency due to its large volume and the high dose rate and pulsed nature of the therapy beams. It was also found that the efficiency depended on the dose rate of the machine because the inter-pulse separation time of the linac is significantly less than the ion transit-time for this chamber. Thus, ionization from more than one beam pulse is collected by the chamber at the same time. The efficiency was determined using three techniques (i) the two-voltage technique, (ii) the voltage extrapolation technique and (iii) a method originally devised for determining the collection efficiency of large volume ionization chambers in diagnostic radiology. The results show that methods (ii) and (iii) agree well, but that the two-voltage technique predicts a much lower efficiency. At about 4 m from the source, the collection efficiency for this chamber varied between 98% and 97% for dose rates between 50 and 250 cGy/min for 4 MV and between 97% and 90% for dose rates between 100 and 600 cGy/min for 10 MV. At isocenter, the comparable figures were 78% and 56% respectively for 4 MV and 65% and 38% respectively for 10 MV.

Ion recombination and polarity effect of ionization chambers in kilovoltage x-ray exposure measurements

Exposure measurements with ionization chambers are dependent on the correction factors related to the beam energy (ke), temperature and pressure (ktp), ionization recombination (Pion), and polarity (kpol) effects. In this work, six different chambers commonly used in diagnostic radiology were investigated for the Pion and kpol at various exposure rates by changing the tube voltage, beam current, exposure time, and distance. A special triaxial connector was used to connect chambers to an electrometer capable of measuring positive and negative polarity and 150 V and 300 V electrode potentials to measure the kpol and Pion, respectively. A mammography unit (24-35 kVp) and a diagnostic x-ray unit (60-125 kVp) were used. Results indicate that the magnitude of the Pion is linearly dependent on kVp for large volume (> 150 cm3) chambers and independent for small volume (< or = 150 cm3) chambers. In general, Pion is higher at higher exposures (increasing kVp, mAs, and decreasing distance); however, kpol is independent of exposure rate and kVp, but strongly depends on the sensitive volume of an ion chamber. Pion and kpol vary between 1-48% and 1-16%, respectively, among various chambers and exposure conditions. Chambers with larger volumes have higher values of Pion and kpol. The desired accuracy of +/- 5% in exposure measurements might not be feasible unless both the polarity and recombination effects are known and accounted accurately.