C Cojocaru

Extraction of depth-dependent perturbation factors for parallel-plate chambers in electron beams using a plastic scintillation detector

PURPOSE This work presents the experimental extraction of the overall perturbation factor PQ in megavoltage electron beams for NACP-02 and Roos parallel-plate ionization chambers using a plastic scintillation detector (PSD). METHODS The authors used a single scanning PSD mounted on a high-precision scanning tank to measure depth-dose curves in 6, 12, and 18 MeV clinical electron beams. The authors also measured depth-dose curves using the NACP-02 and PTW Roos chambers. RESULTS The authors found that the perturbation factors for the NACP-02 and Roos chambers increased substantially with depth, especially for low-energy electron beams. The experimental results were in good agreement with the results of Monte Carlo simulations reported by other investigators. The authors also found that using an effective point of measurement (EPOM) placed inside the air cavity reduced the variation of perturbation factors with depth and that the optimal EPOM appears to be energy dependent. CONCLUSIONS A PSD can be used to experimentally extract perturbation factors for ionization chambers. The dosimetry protocol recommendations indicating that the point of measurement be placed on the inside face of the front window appear to be incorrect for parallel-plate chambers and result in errors in the R50 of approximately 0.4 mm at 6 MeV, 1.0 mm at 12 MeV, and 1.2 mm at 18 MeV.

Measurement of multiple scattering of 13 and 20 MeV electrons by thin foils

To model the transport of electrons through material requires knowledge of how the electrons lose energy and scatter. Theoretical models are used to describe electron energy loss and scatter and these models are supported by a limited amount of measured data. The purpose of this work was to obtain additional data that can be used to test models of electron scattering. Measurements were carried out using 13 and 20MeV pencil beams of electrons produced by the National Research Council of Canada research accelerator. The electron fluence was measured at several angular positions from 0° to 9° for scattering foils of different thicknesses and with atomic numbers ranging from 4 to 79. The angle, θ1∕e, at which the fluence has decreased to 1∕e of its value on the central axis was used to characterize the distributions. Measured values of θ1∕e ranged from 1.5° to 8° with a typical uncertainty of about 1%. Distributions calculated using the EGSnrc Monte Carlo code were compared to the measured distributions. In general, the calculated distributions are narrower than the measured ones. Typically, the difference between the measured and calculated values of θ1∕e is about 1.5%, with the maximum difference being 4%. The measured and calculated distributions are related through a simple scaling of the angle, indicating that they have the same shape. No significant trends with atomic number were observed.

SU‐FF‐T‐443: Measurement of Lateral Dose Distributions Using GafChromic EBT Films and PTW Starcheck 2‐D Array

Purpose: One‐dimensional in‐air scans have been proposed for accurate Monte Carlo commissioning of clinically linacbeams. The purpose of this work was to extend the measurement of such electron and photon dose distributions to two dimensions, in order to confirm the circular symmetry assumption of the 1‐D case and investigate differences previously reported between ion chamber scans and EGSnrc calculations. Method and Materials: Two systems were investigated ‐ GafChromic EBT film and the PTW Stacheck device ‐ and dose profiles were obtained for electron and photonbeams from a Vickers research accelerator. A range of scattering foils (electron beams) and target materials (photonbeams ) were investigated for a 20 MeV beam. Prior to the investigation, both systems were accurately commissioned in beams from an Elekta Precise clinical linac. All results were compared with previously published 1‐D ion chamber and Monte Carlo data. Results: The EBT film measurements confirmed the circular symmetry of both electron and photonbeams for all scatter foils and targets. Electron relative dose profiles measured using film were consistently narrower than Monte Carlo simulations, while ion chamber scans were broader than simulation. For X‐rays, relative dose profiles measured using film generally agree better with Monte Carlo than ion chamber profiles. X‐ray profiles measured by the Starcheck also show better agreement with Monte Carlo than 1‐D ion chamber scan. In comparing the two 2‐D systems, EBT film shows a lower uncertainty and smoother uniformity response than the Starcheck. Absolute X‐ray dose measurements using film and Starcheck showed significant target‐dependent differences of up to 5%. Conclusion: Both EBT film and Starcheck can be used to obtain relative dose profiles with uncertainties at the 2% level. The systematic difference between absolute dose measurement with the two systems suggests that the Starcheck is more sensitive to changes in X‐ray spectra.

Electron beam water calorimetry measurements to obtain beam quality conversion factors

Purpose: To provide results of water calorimetry and ion chamber measurements in high‐energy electron beams carried out at the National Research Council Canada (NRC). There are three main aspects to this work: (a) investigation of the behavior of ionization chambers in electron beams of different energies with focus on long‐term stability, (b) water calorimetry measurements to determine absorbed dose to water in high‐energy beams for direct calibration of ion chambers, and (c) using measurements of chamber response relative to reference ion chambers, determination of beam quality conversion factors, kQ, for several ion chamber types. Methods: Measurements are made in electron beams with energies between 8 MeV and 22 MeV from the NRC Elekta Precise clinical linear accelerator. Ion chamber measurements are made as a function of depth for cylindrical and plane‐parallel ion chambers over a period of five years to investigate the stability of ion chamber response and for indirect calibration. Water calorimetry measurements are made in 18 MeV and 22 MeV beams. An insulated enclosure with fine temperature control is used to maintain a constant temperature (drifts less than 0.1 mK/min) of the calorimeter phantom at 4°C to minimize effects from convection. Two vessels of different designs are used with calibrated thermistor probes to measure radiation induced temperature rise. The vessels are filled with high‐purity water and saturated with H2 or N2 gas to minimize the effect of radiochemical reactions on the measured temperature rise. A set of secondary standard ion chambers are calibrated directly against the calorimeter. Finally, several other ion chambers are calibrated in the NRC 60Co reference field and then cross‐calibrated against the secondary standard chambers in electron beams to realize kQ factors. Results: The long‐term stability of the cylindrical ion chambers in electron beams is better (always Symbol%) than plane‐parallel chambers (0.2% to 0.4%). Calorimetry measurements made at 22 MeV with two different vessel geometries are consistent within 0.2% after correction for the vessel perturbation. Measurements of absorbed dose calibration coefficients for the same secondary standard chamber separated in time by 10 yr are within 0.2%. Drifts in linac output that would affect the transfer of the standard are mitigated to the 0.1% level by performing daily ion chamber normalization measurements. Calibration coefficients for secondary standard ion chambers can be achieved with uncertainties less than 0.4% (k = 1) in high‐energy electron beams. The additional uncertainty in deriving calibration coefficients for well‐behaved chambers indirectly against the secondary standard reference chambers is negligible. The kQ factors measured here differ by up to 1.3% compared to those in TG‐51, an important change for reference dosimetry measurements. Symbol. No Caption available. Conclusions: The measurements made here of kQ factors for eight plane‐parallel and six cylindrical ion chambers will impact future updates of reference dosimetry protocols by providing some of the highest quality measurements of this crucial dosimetric parameter.

The Fricke dosimeter as an absorbed dose to water primary standard for Ir-192 brachytherapy

The aim of this project was to develop an absorbed dose to water primary standard for Ir-192 brachytherapy based on the Fricke dosimeter. To achieve this within the framework of the existing TG-43 protocol, a determination of the absorbed dose to water at the reference position, D(r0,θ0), was undertaken. Prior to this investigation, the radiation chemical yield of the ferric ions (G-value) at the Ir-192 equivalent photon energy (0.380 MeV) was established by interpolating between G-values obtained for Co-60 and 250 kV x-rays.An irradiation geometry was developed with a cylindrical holder to contain the Fricke solution and allow irradiations in a water phantom to be conducted using a standard Nucletron microSelectron V2 HDR Ir-192 afterloader. Once the geometry and holder were optimized, the dose obtained with the Fricke system was compared to the standard method used in North America, based on air-kerma strength.Initial investigations focused on reproducible positioning of the ring-shaped holder for the Fricke solution with respect to the Ir-192 source and obtaining an acceptable type A uncertainty in the optical density measurements required to yield the absorbed dose. Source positioning was found to be reproducible to better than 0.3 mm, and a careful cleaning and control procedure reduced the variation in optical density reading due to contamination of the Fricke solution by the PMMA holder. It was found that fewer than 10 irradiations were required to yield a type A standard uncertainty of less than 0.5%.Correction factors to take account of the non-water components of the geometry and the volume averaging effect of the Fricke solution volume were obtained from Monte Carlo calculations. A sensitivity analysis showed that the dependence on the input data used (e.g. interaction cross-sections) was small with a type B uncertainty for these corrections estimated to be 0.2%.The combined standard uncertainty in the determination of absorbed dose to water at the reference position for TG-43 (1 cm from the source on the transverse axis, in a water phantom) was estimated to be 0.8% with the dominant uncertainty coming from the determination of the G-value. A comparison with absorbed dose to water obtained using the product of air-kerma strength and the dose rate constant gave agreement within 1.5% for three different Ir-192 sources, which is within the combined standard uncertainties of the two methods.

Sci-Sat AM: Radiation Dosimetry and Practical Therapy Solutions - 09: Stability of a water calorimetry system as a primary standard for absorbed dose to water

Purpose: To investigate the stability of a water calorimetry system as a primary standard for absorbed dose to water using measurements performed in cobalt-60 and high-energy linac photon beams over a span of more than a decade. Methods: Calorimetry measures adsorbed dose directly by recording the amount of heat created when ionizing radiation passes through matter. The radiation-induced temperature rise was measured using two thermistors calibrated against the NRC temperature primary standard, using an AC bridge with lock-in amplifier for precise measurement. The calorimeter system was operated under thermal equilibrium at 4 °C (to eliminate convection) with drifts in water temperature less than 0.1 mK/min. Seven water vessels of various designs were used to make repeated measurements over the course of 17 years. Results: The standard uncertainty achieved for a set of ten calorimeter measurements (4 Gy delivered) was generally well below 0.15 % while the variation between multiple sets for a given vessel was consistent with this value. The long-term stability of the system combined with inter-vessel variations indicated that there was good control of the radiochemistry (chemical heat defect). Conclusions: The measurements performed over a period of several years showed that the combined water calorimeters showed stability at +/− 0.25 % level. Thus, rather than relying on a particular vessel as an artifact one can realize the Gray through the more generalized method of combining a glass vessel, high-purity water and thermistor probes. This provides increased robustness in the dissemination of absorbed dose to Canadian users.

SU-F-19A-02: Comparison of Absorbed Dose to Water Standards for HDR Ir-192 Brachytherapy Between the LCR, Brazil and NRC, Canada.

PURPOSE To compare absorbed dose to water standards for HDR brachytherapy dosimetry developed by the Radiological Science Laboratory of Rio de Janeiro State University (LCR) and the National Research Council, Canada (NRC). METHODS The two institutions have separately developed absorbed dose standards based on the Fricke dosimetry system. There are important differences between the two standards, including: preparation and read-out of the Fricke solution, irradiation geometry of the Fricke holder in relation to the Ir-192 source, and determination of the G-value to be used at Ir-192 energies. All measurements for both standards were made directly at the NRC laboratory (i.e., no transfer instrument was used) using a single Ir-192 source (microSelectron v2). In addition, the NRC group has established a self-consistent method to determine the G-value for Ir-192, based on an interpolation between G-values obtained at Co-60 and 250kVp X-rays, and this measurement was repeated using the LCR Fricke solution to investigate possible systematic uncertainties. RESULTS G-values for Co-60 and 250 kVp x-rays, obtained using the LCR Fricke system, agreed with the NRC values within 0.5 % and 1 % respectively, indicating that the general assumption of universal G-values is appropriate in this case. The standard uncertainty in the determination of G for Ir-192 is estimated to be 0.6 %. For the comparison of absorbed dose measurements at the reference point for Ir-192 (1 cm depth in water, perpendicular to the seed long-axis), the ratio Dw(NRC)/Dw(LCR) was found to be 1.011 with a combined standard uncertainty of 1.7 %, k=1. CONCLUSION The agreement in the absorbed dose to water values for the LCR and NRC systems is very encouraging. Combined with the lower uncertainty in this approach compared to the present air-kerma approach, these results reaffirm the use of Fricke solution as a potential primary standard for HDR Ir-192 brachytherapy.

TH‐C‐BRB‐07: Development of a Wall‐Less Fricke Dosimetry System for Megavoltage Photon and Electron Beams

Purpose: To develop a dosimetry system based on the Fricke (ferrous sulphate) dosimeter that can be used to determine absolute dose in regions of large dose gradients in megavoltage photon and electron beams.Method and Materials: Building on the techniques first used at the Swiss national standards laboratory METAS we have developed a system where Fricke solution can be irradiated in polyethylene bags rather than the usual glass vials. This approach means that there is no wall correction and the dosimeter size and shape can be tailored to each application. As a first test the Fricke system was used to determine the absorbed dose absolutely in low energy electron beams. The Fricke response was compared with the NRC primary standard water calorimeter and calibrated ionization chambers using the TG‐51 protocol. Measurements were made in 4 8 12 18 and 22 MeV beams and the delivered dose was in the range of 7 Gy to 50 Gy. Results: The accuracy and precision of the Fricke dosimetry is very dependent on solution preparation and contaminants which affect the readout. It was found that the contaminating effect of the polyethylene bag on the readout signal was small and could be accurately corrected by using unirradiated controls. The standard uncertainty in the sensitivity coefficient of the Fricke system was found to be 0.2% and the standard uncertainty in the determination of absorbed dose to water was estimated to be 0.6 %. Conclusion: The Fricke dosimeter system remains relevant to megavoltage dosimetry offering accuracy comparable with or better than other integrating systems. The dosimeter is insensitive to dose homogeneities and can be used to determine mean dose to arbitrary volumes. It therefore has applications in reference dosimetry for (TG‐51) non‐compliant beams as well as dose verification in IMRT.

Validating Fricke dosimetry for the measurement of absorbed dose to water for HDR 192Ir brachytherapy: a comparison between primary standards of the LCR, Brazil, and the NRC, Canada

Two Fricke-based absorbed dose to water standards for HDR Ir-192 dosimetry, developed independently by the LCR in Brazil and the NRC in Canada have been compared. The agreement in the determination of the dose rate from a HDR Ir-192 source at 1 cm in a water phantom was found to be within the k  =  1 combined measurement uncertainties of the two standards: D NRC/D LCR  =  1.011, standard uncertainty  =  2.2%. The dose-based standards also agreed within the uncertainties with the manufacturers stated dose rate value, which is traceable to a national standard of air kerma. A number of possible influence quantities were investigated, including the specific method for producing the ferrous-sulphate Fricke solution, the geometry of the holder, and the Monte Carlo code used to determine correction factors. The comparison highlighted the lack of data on the determination of G(Fe3+) in this energy range and the possibilities for further development of the holders used to contain the Fricke solution. The comparison also confirmed the suitability of Fricke dosimetry for Ir-192 primary standard dose rate determinations at therapy dose levels.

TU‐C‐108‐10: Development of An Absorbed Dose to Water Primary Standard for HDR Ir‐192 Brachytherapy Based On the Fricke Dosimetry System

Purpose: To develop a Fricke based absorbed dose standard for HDR Ir‐192 brachytherapy that will give a significantly lower uncertainty on the dose to water at 1 cm compared to methods based on air‐kerma standards. Methods: A ring shaped PMMA Fricke holder was developed to allow measurements to be conducted in a water phantom with the Fricke volume centred at 1 cm from the source. The seed was positioned at the ring centre to allow for a uniform irradiation geometry. The radiation chemical yield of Ferric ions, or G‐value, was determined at 250KVp X‐ray and at Co‐60 and a linear interpolation of effective photon energies allowed the determination of the G‐value at Ir‐192. Using this interpolated G‐value the radiation induced change in the Fricke solutions optical density could then be related to the absorbed dose to water at 1 cm. Results: The G‐value experiments yielded values for 250 KVp X‐ray and Co‐60 of 1.56 umol/J and 1.61 umol/J respectively. The uncertainty in the G‐value for Ir‐192 photon energies is estimated to be around ±1%. The absorbed dose to water using the Fricke system was calculated and compared with the value obtained via an air‐kerma measurement and TG‐43 agreement was obtained within the combined uncertainties. An uncertainty analysis of the Fricke system indicates an overall uncertainty of 1.2% to 1.5% (for dose at 1 cm) is achievable. Conclusion: This work demonstrates the ability to reduce the standard uncertainty for Ir‐192 HDR brachytherapy sources by using a Fricke chemical dosimeter that measures the absorbed dose to water directly and allows measurements to be conducted in the quantity of interest. The overall uncertainty is on par, or better than current water calorimetry based methods.