Beam Quality Conversion Factors Research Articles

Beam quality conversion factor of BeO-OSLD for high-energy photon beams

Optically stimulated luminescence (OSL) dosimeters are used in radiotherapy. The beam quality conversion factor () of dosimeters is essential for correcting energy dependence. However, the of an OSL dosimeter using BeO (BeO-OSLD) has not yet been reported for high-energy photon beams. In this study, the of BeO-OSLD was investigated for high-energy photon beams using experiments and Monte Carlo calculations. In the experiments, was evaluated as the ratio of calibration coefficients determined using high-energy photon beams and 60Co γ-rays. Ten OSL dosimeters were used to reduce the statistical uncertainty of the determined values, and the same dosimeters were repeatedly used to obtain the data set of high-energy photon beams and 60Co γ-rays. Pre-irradiation with 10 Gy and bleaching were performed on fresh dosimeters before dosimetry to reduce the dependence of the detector response on the accumulated dose. In Monte Carlo calculations, both the absorbed dose to water and that of the BeO dosimeter for high-energy photon beams and 60Co γ-rays were calculated, and then was determined using the ratio of these values. The determined values in high-energy photon beam were 0.998 ± 0.011 and 1.021 ± 0.005 for the experiment and the calculation, respectively. These results showed that the variation in for 6 to 25 MV was negligible in practical dosimetry. This result indicates that BeO can be used to evaluate doses for different beam qualities without correcting for energy dependence.

Modified calibration protocols in electron beam dosimetry: comparison with IAEA TRS-398 and AAPM TG-51

This study aimed to compare absolute calibration outputs based on the protocols of the International Atomic Energy Agency (IAEA) Technical Report Series (TRS)-398, the American Association of Physicists in Medicine (AAPM) Task Group (TG)-51, and modified calibration approach. The electron beam output calibration followed the IAEA TRS-398 and AAPM TG-51 protocols, both of which required cylindrical chambers and parallel plates. However, the use of cylindrical chambers is not recommended at low energies because of the large fluence-correction factor. TG-51 recommended cross-calibration of the parallel-plate chamber against the cylindrical chamber in a high-energy electron beam. In 2020, an electron beam dosimetry modification was introduced that used a cylindrical ionisation chamber at all energies and updated the data for beam quality conversion factors. This modification provided a lower deviation than that reported in AAPM TG-51. Thus, the modified calibration based on TRS-398 was applied in the present study, which yielded results below the permissible tolerance. The beam calibration at 6, 8, 10, 12, and 15 MeV energies was carried out for two Elekta linear accelerators.. Electron beam dosimetry followed the AAPM TG-51 and TRS-398 protocols, and modified calibration were performed to measure the dose at the maximum depth expressed in dose/monitor units (cGy/MU). Charge-reading measurements were measured using ionisation chambers PTW 30013, IBA CC13, and Exradin A11. The average absorbed dose ratios were 1.004 and 1.009 using the modified calibration and TRS-398 and modified calibration and TG-51, respectively. Therefore, based on IAEA TRS-398, the results were below the tolerance limit (±2%).

Comparison of derived correction factors for effects of ion recombination and photon beam quality index following TG-51 and TRS-398 dosimetry protocols

In this study, ion recombination correction factor (kS) and beam quality conversion factor (kQ,Q0) values were extracted following the recommendations of the TRS-398 and TG-51 dosimetry protocols for widely used cylindrical ionization chambers for high energy photon beam dosimetry to quantify the agreement between the instructions for these two protocols for absolute dosimetry inside water. Four different types of cylindrical ionization chambers comprising Farmer (TM30013), Semiflex 0.125 cm3 (TM31010), Semiflex 0.3 cm3 (TM31013), and PinPoint (TM31016) were considered, and kS and kQ,Q0 values were determined at photon energies of 6 MV and 15 MV. The maximum difference between the measured kS values according to the instructions in the TRS-398 and TG-51 protocols was 0.03%. The kS data measured with both protocols agreed well with those measured by using the Jaffe-plot approach, where the maximum difference was about 0.33%. The observed differences between the kQ,Q0 factors measured by using the TRS-398 and TG-51 dosimetry protocols at photon energies of 6 MV and 15 MV were 0.37% and 0.55%, respectively. The kQ,Q0 values measured using the TG-51 dosimetry protocols were slightly closer to those measured by a reference ionization chamber dosimeter. We conclude that the maximum differences were about 0.4% and 0.6% in the absorbed dose measurements according to the TRS-398 and TG-51 instructions at photon energies of 6 MV and 15 MV, respectively. The type of ionization chamber employed also affected the differences, where the maximum and minimum dose differences were found using the Farmer and PinPoint chambers, respectively.

Development and application of absorbed dose primary standard for 60Co and high-energy photon beams using water calorimetry

Several new glass vessels, thermistor probes, and water phantoms have been designed and built at the National Institute of Metrology (China) to upgrade and develop the existing water calorimeter. The increased plane-parallel vessels have shorter thermal stability times and shallower positioning depths (∼1.3 cm) than the previous cylindrical vessels, which makes them suitable for electron beams. The sensitivity of the new probes is 10% greater than the previous ones. In this study, detailed experimental and theoretical investigations of various factors affecting the new water calorimeter are performed. The system uncertainty of the water calorimeter is reduced and the robustness of the determination of the absorbed-dose-to-water D w is improved using a variety of geometric detector vessels and two kinds of high-purity water systems saturated with high-purity gases. This new calorimeter is employed as a primary standard for determining the D w, has achieved a combined standard uncertainty of 0.24% for a 60Co beam, 0.27% for 6 MV and 10 MV photon beams and 0.30% for a 25 MV photon beam. The beam quality conversion factors kQ of ten cylindrical and three plane-parallel ionization chambers are measured using the new calorimeter to improve the reference dosimetry accuracy of high-energy clinical photon beams.

Quantification of the role of lead foil in flattening filter free beam reference dosimetry

To quantify the potential error in outputs for flattening filter free (FFF) beams associated with use of a lead foil in beam quality determination per the addendum protocol for TG-51, we examined differences in measurements of the beam quality conversion factor kQ when using or not using lead foil. Two FFF beams, a 6 MV FFF and a 10 MV FFF, were calibrated on eight Varian TrueBeams and two Elekta Versa HD linear accelerators (linacs) according to the TG-51 addendum protocol by using Farmer ionization chambers [TN 30013 (PTW) and SNC600c (Sun Nuclear)] with traceable absorbed dose-to-water calibrations. In determining kQ , the percentage depth-dose at 10cm [PDD(10)] was measured with 10×10 cm2 field size at 100cm source-to-surface distance (SSD). PDD(10) values were measured either with a 1 mm lead foil positioned in the path of the beam [%dd(10)Pb ] or with omission of a lead foil [%dd(10)]. The %dd(10)x values were then calculated and the kQ factors determined by using the empirical fit equation in the TG-51 addendum for the PTW 30013 chambers. A similar equation was used to calculate kQ for the SNC600c chamber, with the fitting parameters taken from a very recent Monte Carlo study. The differences in kQ factors were compared for with lead foil vs. without lead foil. Differences in %dd(10)x with lead foil and with omission of lead foil were 0.9±0.2% for the 6 MV FFF beam and 0.6±0.1% for the 10 MV FFF beam. Differences in kQ values with lead foil and with omission of lead foil were -0.1±0.02% for the 6 MV FFF and -0.1±0.01% for the 10 MV FFF beams. With evaluation of the lead foil role in determination of the kQ factor for FFF beams. Our results suggest that the omission of lead foil introduces approximately 0.1% of error for reference dosimetry of FFF beams on both TrueBeam and Versa platforms.

Absorbed-dose-to-water measurement using alanine in ultra-high-pulse-dose-rate electron beams

Objective. The aim of the presented study is to evaluate the dose response of the PTB’s secondary standard system, which is based on alanine and electron spin resonance (ESR) spectroscopy measurement, in ultra-high-pulse-dose-rate (UHPDR) electron beams. Approach. The alanine dosimeter system was evaluated in the PTB’s UHPDR electron beams (20 MeV) in a range of 0.15–6.2 Gy per pulse. The relationship between the obtained absorbed dose to water per pulse and the in-beamline charge measurement of the electron pulses acquired using an integrating current transformer (ICT) was evaluated. Monte Carlo simulations were used to determine the beam quality conversion and correction factors required to perform alanine dosimetry. Main results. The beam quality conversion factor from the reference quality 60Co to 20 MeV obtained by Monte Carlo simulation, 1.010(1), was found to be within the standard uncertainty of the consensus value, 1.014(5). The dose-to-water relative standard uncertainty was determined to be 0.68% in PTB’s UHPDR electron beams. Significance. In this investigation, the dose-response of the PTB’s alanine dosimeter system was evaluated in a range of dose per pulse between 0.15 Gy and 6.2 Gy and no evidence of dose-response dependency of the PTB’s secondary standard system based on alanine was observed. The alanine/ESR system was shown to be a precise dosimetry system for evaluating absorbed dose to water in UHPDR electron beams.

AAPM WGTG51 Report 374: Guidance for TG-51 reference dosimetry.

Practical guidelines that are not explicit in the TG-51 protocol and its Addendum for photon beam dosimetry are presented for the implementation of the TG-51 protocol for reference dosimetry of external high-energy photon and electron beams. These guidelines pertain to:(i)measurement of depth-ionization curves required to obtain beam quality specifiers for the selection of beam quality conversion factors, (ii)considerations for the dosimetry system and specifications of a reference-class ionization chamber, (iii)commissioning a dosimetry system and frequency of measurements, (iv)positioning/aligning the water tank and ionization chamber for depth ionization and reference dose measurements, (v)requirements for ancillary equipment needed to measure charge (triaxial cables and electrometers) and to correct for environmental conditions, and (vi)translation from dose at the reference depth to that at the depth required by the treatment planning system. Procedures are identified to achieve the most accurate results (errors up to 8% have been observed) and, where applicable, a commonly used simplified procedure is described and the impact on reference dosimetry measurements is discussed so that the medical physicist can be informed on where to allocateresources.

Determination of an ionization chamber response using quality index for kilovoltage x-ray beams

This study aims to investigate the energy response of a Farmer-type ionization chamber for kilovoltage x-ray beams. The chamber response in free air and at depths of 1 to 15 cm in a water phantom was calculated using the Monte Carlo method. The response in the phantom was evaluated using a beam quality conversion factor, kQx, for x-ray beams Qx to a 60Co beam. The response was expressed as a function of the effective energy or a half-value layer (HVL, mm Al) for x-ray beams with given quality index (QI) values of 0.4, 0.5, and 0.6 and for monoenergetic photon beams. QI is defined as the ratio of the effective energy to the tube voltage. The response for 25 fluence spectra of x-ray beams was calculated for the beam qualities with HVL of 1.39-10.4 mm Al and tube voltages of 50-137.6 kVp, as classified by QI. The chamber response calculated in free air for the given QI values agreed with 1% of that provided by secondary standard laboratories. The kQx values depended on depth at the lower energy levels. The depth dependence was less than 1% for an HVL (mm Al) of more than 6 mm. The dose to water obtained from kQx and a 60Co absorbed dose-to-water calibration coefficient ND,wC60o was within 1.5% at depths of 1 to 15 cm with that determined by the AAPM TG-61 for HVL of 2.9 to 10.8 mm Al. This method can reduce the uncertainty of doses in the phantom without any correction using PQ,cham and [(μ‾en/ρ)airw]w, and can directly determine the absorbed dose to water from the ionization chamber reading for medium-energy x-ray beams, including kilovoltage x-ray CT and image-guided cone beam CT in radiotherapy.

[Radiotherapy Dosimeter Calibration for Absorbed Dose to Water Using Medical Accelerators].

A calibration service using a medical accelerator has been launched to calibrate a radiotherapy dosimeter in terms of an absorbed dose to water. The radiotherapy dosimeter calibrated by the calibration service can measure the absorbed dose to water without a beam quality conversion factor. In this paper, an overview of the calibration service for a high-energy photon beam and a high-energy electron beam was described, as well as methods of absorbed dose measurement and cross-calibration using the calibrated radiotherapy dosimeter. And the development status of a dose standard for a particle beam was reported.

A modified formalism for electron beam reference dosimetry to improve the accuracy of linac output calibration.

To present and demonstrate the accuracy of a modified formalism for electron beam reference dosimetry using updated Monte Carlo calculated beam quality conversion factors. The proposed, simplified formalism allows the use of cylindrical ionization chambers in all electron beams (even those with low beam energies) and does not require a measured gradient correction factor. Data from a previous publication are used for beam quality conversion factors. The formalism is tested and compared to the present formalism in the AAPM TG-51 protocol with measurements made in Elekta Precise electron beams with energies between 4MeV and 22MeV and with fields shaped with a 10×10cm2 clinical applicator as well as a 20×20cm2 clinical applicator for the 18MeV and 22MeV beams. A set of six ionization chambers are used for measurements (two cylindical reference-class chambers, two scanning-type chambers and two parallel-plate chambers). Dose per monitor unit is derived using the data and formalism provided in the TG-51 protocol and with the proposed formalism and data and compared to that obtained using ionization chambers calibrated directly against primary standards for absorbed dose in electron beams. The standard deviation of results using different chambers when TG-51 is followed strictly is on the order of 0.4% when parallel-plate chambers are cross-calibrated against cylindrical chambers. However, if parallel-plate chambers are directly calibrated in a cobalt-60 beam, the difference between results for these chambers is up to 2.2%. Using the proposed formalism and either directly calibrated or cross-calibrated parallel-plate chambers gives a standard deviation using different chambers of 0.4%. The difference between results that use TG-51 and the primary standard measurements are on the order of 0.6% with a maximum difference in the 4MeV beam of 2.8%. Comparing the results obtained with the proposed formalism and the primary standard measurements are on the order of 0.4% with a maximum difference of 1.0% in the 4MeV beam. The proposed formalism and the use of updated data for beam quality conversion factors improves the consistency of results obtained with different chamber types and improves the accuracy of reference dosimetry measurements. Moreover, it is simpler than the present formalism and will be straightforward to implement clinically.

Effect of ICRU report 90 recommendations on Monte Carlo calculated kQ for ionization chambers listed in the Addendum to AAPM’s TG‐51 protocol

The ICRU has published new recommendations for ionizing radiation dosimetry. In this work, the effect of recommendations on the water-to-air and graphite-to-air restricted mass electronic stopping power ratios (sw, air and sg, air ) and the individual perturbation correction factors Pi was calculated. The effect on the beam quality conversion factors kQ for reference dosimetry of high-energy photon beams was estimated for all ionization chambers listed in the Addendum to AAPM's TG-51 protocol. The sw, air , sg, air , individual Pi, and kQ were calculated using EGSnrc Monte Carlo code system and key data of both ICRU report 37 and ICRU report 90. First, the Pi and kQ were calculated using precise models of eight ionization chambers: NE2571 (Nuclear Enterprise), 30013, 31010, 31021 (PTW), Exradin A12, A12S, A1SL (Standard imaging), and FC-65P (IBA). In this simulation, the radiation sources were one 60 Co beam and ten photon beams with nominal energy between 4MV and 25MV. Then, the change in kQ for ionization chambers listed in the Addendum to AAPM's TG-51 protocol was calculated by changing the specification of the simple-model of ionization chamber. The simple-models were made with only cylindrical component modules. In this simulation, the radiation sources of 60 Co beam and 24MV photon beam were used. The significant changes (p<0.05) were observed for sw, air , sg, air , the wall correction factor Pwall , and the waterproofing sleeve correction factor Psleeve . The decrease in sw, air varied from -0.57% for a 60 Co beam to -0.36% for the highest beam quality. The decrease in sg, air varied from -0.72% to -1.12% in the same range. The changes in Pwall and Psleeve were up to 0.41% and 0.14% and those maximum changes were observed for the 60 Co beam. All changes in the central electrode correction factor Pcel , the stem correction factor Pstem , and the replacement correction factor Prepl were from -0.02% to 0.12%. Those changes were statistically insignificant (p=0.07 or more) and were independent of photon energy. The change in kQ was mainly characterized by the change in sw, air , Pwall , and Psleeve . The relationship between the change in kQ and the beam quality index was linear approximately. The changes in kQ of the simple-models were agreed with those of the precise-models within 0.08%. The effects of ICRU-90 recommendations on kQ for the ionization chambers listed in the Addendum to AAPM's TG-51 protocol were from -0.15% to 0.30%. To remove the known systematic effect on the clinical reference dosimetry, the kQ based on ICRU-37 should be updated to the kQ based on ICRU-90.

Monte Carlo determination of kQ and kQmsr values for the exradin A26 ionisation chamber for the Varian TrueBeam

We have calculated conversion factors, kQ for the A26 micro ionisation chamber along with machine specific reference beam quality factors, kQmsr, for a number of field sizes and beam qualities for the Varian TrueBeam accelerator. The A12 ionisation chamber was simulated alongside the A26, so as to validate against known literature values. Both ionisation chambers were modelled from manufacturer data sheets and schematics. The egs_chamber Monte Carlo user code was used to simulate each absorbed dose relevant to the beam quality conversion factors kQ and kQmsr. Tabulated spectra for beam energies of 4 through 25 MV were used in the kQ calculations for both investigated chambers. Varian TrueBeam phase space files for 6 MV flattened as well as 6 and 10 MV unflattened beams were used in the simulations of the A26 chamber in field sizes from 2  ×  2 cm square to 20  ×  20 cm square in order to determine kQmsr values. The PDD(10)x  values of the tabulated spectra were found to be within variation between studies, with an average deviance of 0.4% from one prior study. The simulated A12 kQ values matched the accepted literature values with an average variation of  <0.1%. The A26 kQ values match the manufacturer provided values to within 0.5%. For all investigated field sizes the kQmsr values are within 0.006 of unity. There is no published data for this chamber for a direct comparison, but there is similarity between these results and results from other chambers regularly used in similar circumstances. Furthermore, the agreement of the simulated kQ values to knowns, and the agreement of the PDD(10)x  factors would suggest the correctness and accuracy of the study.

Ionization chamber dosimetry based on 60Co absorbed dose to water calibration for diagnostic kilovoltage x-ray beams

The purpose of this study was to develop a new ionization chamber dosimetry formalism for diagnostic kilovoltage x-ray beams based on a 60Co absorbed dose-to-water calibration coefficient . To validate the new chamber dosimetry, 39 fluence spectra of kilovoltage x-rays were calculated, using a SpekCalc program, for Al half-value-layer (Al-HVL) values of 1.4–8.5 mm and tube voltages of 50–137.6 kVp. We used these spectra to calculate the beam quality conversion factor for kilovoltage x-ray beams Qx to a 60Co beam for a PTW 30013 Farmer® ionization chamber in a reference geometrical setup (depth  =  1 cm in water, field size  =  20  ×  20 cm2). The absorbed dose to water for kilovoltage x-rays was obtained using and . Meanwhile, the water surface dose based on the air kerma calibration coefficient NK was obtained using a Monte Carlo-calculated correction factor CF (corresponding to in AAPM TG-61) for the 39 x-ray fluence spectra. The absorbed dose based on was validated by comparing the water surface dose for the 39 x-ray fluence spectra with the water surface dose based on NK. The water surface dose based on was obtained using Monte Carlo-calculated percentage depth doses. For Al-HVL ranging from 1.4 to 8.5 mm, ranged from 0.884 to 0.956 and CF ranged from 1.264 to 1.663. The water surface dose based on for the x-ray fluence spectra was 1.0% higher on average than that based on NK, except for Al-HVLs of 1.4 and 1.5 mm and 1% was within their uncertainty ranges. Therefore, it is possible to use the absorbed dose-to-water value based on instead of on NK. With the new chamber dosimetry, the chamber reading can be easily converted to the absorbed dose using and the calculated .

[P190] Investigation of beam quality correction factors for flattening filter free photon beams

Purpose It has previously been shown that the use of a dual beam quality specifier consisting of two tissue-phantom ratios, TPR20/10 and TPR10/5, provides an accurate prediction of stopping power ratios (sw,air) for flattening filter free (FFF) photon beams with different filtration, using a bi-linear relationship between the two TPRs and sw,air. For the current project, the intention is to evaluate this dual beam quality specifier on kQ factors used in radiotherapy reference dosimetry, calculated using Monte Carlo modelling. Methods Two different ionisation chambers were used in the simulations, NE2571 and FC65-G. Calculations of kQ factors, TPR20/10 and TPR10/5 were performed using EGSnrc, and the chamber geometry was acquired from technical specifications from the manufacturer and from previously published articles. A total of 18 photon beams (10 with flattening filter (FF) and 8 flattening filter free) were used. Results For the FFF beams, the calculated beam quality correction factors were found to be 0.1–0.3% lower than for the beams with a flattening filter at the same TPR20/10. Using data from TRS398 overestimated kQ by up to 1% for FFF beams but also up to 0.9% for FF beam for the FC65-G chamber. The corresponding numbers for the NE2571 were 0.6% for both FF and FFF beams. Using the dual beam quality metric fitted to the calculations, kQ factors could be predicted within 0.12% for all beams. Conclusions Beam quality conversion factors for NE2571 and FC65-G farmer type ionisation chambers have been calculated, for beams with and without a flattening filter. Differences between beams with and without a flattening filter were found to be 0.1–0.3%. An overestimation of kQ factors for conventional photon beams of up to 0.9% was found using TRS398. Using a dual beam quality measure, consisting of both TPR20/10 and TPR10/5, KQ factors could be predicted to within 0.12%. Although the difference between FF and FFF beams is small, it is a systematic difference which can be accounted for.

On the impact of ICRU report 90 recommendations on kQ factors for high-energy photon beams.

To assess the impact of the ICRU report 90 recommendations on the beam-quality conversion factor, kQ , used for clinical reference dosimetry of megavoltage linac photon beams. The absorbed dose to water and the absorbed dose to the air in ionization chambers representative of those typically used for linac photon reference dosimetry are calculated at the reference depth in a water phantom using Monte Carlo simulations. Depth-dose calculations in water are also performed to investigate changes in beam quality specifiers. The calculations are performed in a cobalt-60 beam and MV photon beams with nominal energy between 6MV and 25MV using the EGSnrc simulation toolkit. Inputs to the calculations use stopping-power data for graphite and water from the original ICRU-37 report and the new proposed values from the recently published ICRU-90 report. Calculated kQ factors are compared using the two different recommendations for key dosimetry data and measured kQ factors. Less than about 0.1% effects from ICRU-90 recommendations on the beam quality specifiers, the photon component of the percentage depth-dose at 10cm, %dd(10)x , and the tissue-phantom ratio at 20cm and 10cm, TPR1020, are observed. Although using different recommendations for key dosimetric data impact water-to-air stopping-power ratios and ion chamber perturbation corrections by up to 0.54% and 0.40%, respectively, we observe little difference (≤0.14%) in calculated kQ factors. This is contradictory to the predictions in ICRU-90 that suggest differences up to 0.5% in high-energy photon beams. A slightly better agreement with experimental values is obtained when using ICRU-90 recommendations. Users of the addendum to the TG-51 protocol for reference dosimetry of high-energy photon beams, which recommends Monte Carlo calculated kQ factors, can rest assured that the recommendations of ICRU report 90 on basic data have little impact on this central dosimetric parameter.

TG-51 reference dosimetry for the Halcyon™: A clinical experience.

Halcyon™ is a single‐energy (6 MV‐FFF), bore‐enclosed linear accelerator. Patient setup is performed by first aligning to external lasers mounted to the front of the bore, and then loading to isocenter through pre‐defined couch shifts. There is no light field, optical distance indicator or front pointer mechanism, so positioning is verified through MV imaging with kV imaging scheduled to become available in the future. TG‐51 reference dosimetry was successfully performed for Halcyon™ in this imaging‐based setup paradigm. The beam quality conversion factor, k Q, was determined by measuring %dd(10)x three ways: (a) using a Farmer chamber with lead filtering, (b) using a Farmer chamber without lead filtering, and (c) using a PinPoint chamber without lead filtering. Values of k Q were determined to be 0.995, 0.996, and 0.996 by each measurement technique, respectively. Halcyon™'s 6 MV‐FFF beam was found to be broader than other FFF beams produced by Varian accelerators, and profile measurements at d max showed the beam to vary less than 0.5% over the dimensions of our Farmer chamber's active volume. Reference dosimetry can be performed for the Halcyon™ accelerator simply, without specialized equipment or lead filtering with minimal dosimetric impact. This simplicity will prove advantageous in clinics with limited resources or physics support.

Determination of a beam quality conversion factor from 60Co to 192Ir

Radiation dosimetry is an integral part of the radiation therapy process. The ultimate goal of radiation dosimetry is to determine the dose delivered to the tumor and to the normal tissues in a patient undergoing radiotherapy. The importance of HDR 192Ir brachytherapy as the main mode of treatment calls for an accurate dosimetry standard. A dosimetry standard for the direct measurement of absolute dose to water in 192Ir sources is not available.The AAPM TG-43, along with its update, constitutes the accepted protocol for dose to water determination based on an air kerma strength Sk,air measurement. The air kerma strength of the radioactive source is converted to dose to water via dose rate constant Λ (a calculated absolute quantify) and several relative correction factors accounting for scatter attenuation and anisotropy of the dose distribution among other effects.In this work, we proposed an absorbed dose conversion by determination of a beam quality factor (kQ) with an ionization chamber. Ideally, the kQ should be measured directly for each chamber at the same quality as the user beam. However, this is not achievable in most standards laboratories. When no experimental data are available, or it is difficult to measure kQ directly to realistic beams, in many cases the correction factors can be calculated theoretically with Monte Carlo method. The geometric design and material details of the source was provided by the manufacturer and was used to define the Monte Carlo geometry. To validate the source geometry, a few dosimetry parameters had to be calculated according to the AAPM TG-43U1 formalism. The dosimetry studies included the calculation of the air kerma strength Sk, collision kerma in water along the transverse axis with an unbounded phantom, dose rate constant and radial dose function. The Monte Carlo code system that was used was EGSnrc with a new cavity code, which is a part of EGS++ that allows calculating the radial dose function around the source. The XCOM photon cross-section library was used in subsequent simulations. Consequently, the photoelectric effect, pair production, Rayleigh scattering and bound Compton scattering were included in the simulation. Variance reduction techniques were also used in the simulations to speed up the calculation. The obtained radial dose function and dose rate constant of the Gammamed Plus 192Ir source in this work were compared with those obtained by Ballester et al. and Taylor &Rogers et al.A value of 1.109 ± 0.003 for the dose to the chamber to dose to water ratio was determined for PTW 30013 ionization chamber. The agreement between the EGSnrc calculated and TRS-398 reported result is encouragingly similar. The beam quality correction factor, kQ, was obtained at 2 cm. This ratio corresponds to the beam quality conversion factor from 60Co to 192Ir. With this kQ it is possible measure the absorbed dose to water at any point for HDR 192Ir source.

Monte Carlo study of ionization chamber magnetic field correction factors as a function of angle and beam quality.

To use EGSnrc Monte Carlo simulations for magnetic field dosimetry to determine optimal measurement orientations, calculate beam quality conversion factors for 32 cylindrical and three parallel-plate (PP) ion chambers, evaluate the beam quality and angular dependence of these factors, and examine the magnetic field effects on %dd(10)x and TPR1020. Beam quality conversion factors, kQmag, and magnetic field conversion factors, kB =kQmag/kQ , are calculated as a function of chamber rotation for six cylindrical ionization chamber in either a 60 Co beam with a 0.35T magnetic field or a 7MV beam with a 1.5T field, both magnetic fields are perpendicular to the photon beam. The chambers' sensitive air volumes are varied by either using the entire geometric volume or excluding the air volume associated with the first 1mm away from the stem. The kB and kQmag factors are evaluated using four clinical photon spectra. The variation in %dd(10)x and TPR1020 as a function of magnetic field for six photon spectra are studied using DOSXYZnrc. When the magnetic field is perpendicular to the photon beam, orienting the chamber parallel with the magnetic field reduces the magnetic field effect on chamber response (i.e., dose to air per water dose) and variations due to the unknown sensitive volume are essentially eliminated. Calculated kB factors are within 1% of unity for the majority of cylindrical chambers, although larger kB values are associated with chambers with high-Z electrodes. PP chambers have kB corrections as large as 8.9% and have a larger angular sensitivity compared to cylindrical chambers. Values of kB for cylindrical ion chambers are independent of beam quality, except for chambers with high-Z electrodes. For %dd(10)x values between 63.3% and 73.8%, kB varies by at most (0.26±0.15)% when the magnetic field is perpendicular to the photon beam and parallel to the chamber. Differences in %dd(10)x , between no magnetic field and with a 1.5T field perpendicular to the photon beam are (0.04±0.10)%, (1.89±0.10)%, and (6.20±0.10)% for a 60 Co, 7, and 25MV photon beam, respectively, while TPR1020 shows less than (0.36±0.10)% change. Applying the ICRU-90 recommendations for stopping powers instead of ICRU-37 is found to change kQ (and hence kB ) by less than 0.1%. Orienting the chamber parallel to the magnetic field when the field is perpendicular to the photon beam will minimize the effect of the magnetic field on chamber response, and eliminate the problem of the unknown sensitive volume. Values of kB and kQmag can bring ion chamber dosimetry in magnetic fields in-line with the TG-51 protocol. PP chamber are sensitive to the magnetic field and variation in chamber response due to small angular changes makes them unlikely candidates for clinical reference dosimetry in magnetic fields. The stability in TPR1020, as a function of magnetic fields and beam qualities, makes it the best beam quality specifier in magnetic fields.

Characteristics of 2.5 MV beam and imaging dose to patients

PurposeThis work provides the beam characteristics and evaluates the imaging dose to patients for a 2.5 MV portal imaging beam. Method and materialsThe Monte Carlo technique has been used to simulate the 2.5 MV imaging beam. Beam characteristics have been analyzed including the energy spectra and the fluence distributions as a function of position away from the beam central axis. The accuracy of a simulated beam was validated through comparisons between the Monte Carlo calculated and measured dose distributions in a water phantom. The simulated 2.5 MV beam was also used to obtain the absorbed-dose beam quality conversion factor, kQ, for absorbed dose calibration. The simulated beams were then used to evaluate the imaging dose to patients compared with that from a conventional therapeutic 6 MV beam. ResultsThe mean energies of photons and electrons in the 2.5 MV beam are 0.48 MeV and 0.37 MeV respectively. The photon fluence decreases at 20 cm away from the central axis by only up to 30% for this flattening-filter free beam. The values of %dd curves at depth = 10 cm are 53% and 63% for 10 × 10 cm2 and 40 × 40 cm2 fields respectively. Portal imaging doses (D50 of the DVHs) to the eyes, heart and bladder from representative pairs of 2.5 MV (or 6 MV) setup images are 1.8 cGy (3.5 cGy), 1.1 cGy (2.5 cGy) and 1.0 cGy (2.4 cGy) for head, thorax and pelvis image acquisitions respectively. ConclusionWe provide dosimetric data, as well as estimates of organ imaging doses, for this 2.5 MV beam. When clinical default imaging protocols are used, the imaging dose from the 2.5 MV beam is about 50% of that from a 6 MV beam. The information can be used to select image procedures and to estimate organ dose from imaging procedures.

Electron beam water calorimetry measurements to obtain beam quality conversion factors.

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. Measurements are made in electron beams with energies between 8MeV and 22MeV 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 18MeV and 22MeV beams. An insulated enclosure with fine temperature control is used to maintain a constant temperature (drifts less than 0.1mK/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 60 Co reference field and then cross-calibrated against the secondary standard chambers in electron beams to realize kQ factors. The long-term stability of the cylindrical ion chambers in electron beams is better (always <0.15%) than plane-parallel chambers (0.2% to 0.4%). Calorimetry measurements made at 22MeV 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 10yr 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. 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.