Beam Quality Correction Factors kQ Research Articles

Monte Carlo calculated beam quality correction factors for high energy electron beams

Purpose:As the dosimetry protocol TRS 398 is being revised and the ICRU report 90 provides new recommendations for density correction as well as the mean ionization energies of water and graphite, updated beam quality correction factors kQ are calculated for reference dosimetry in electron beams and for independent validation of previously determined values. Methods:Monte Carlo simulations have been performed using EGSnrc to calculate the absorbed dose to water and the dose to the active volumes of ionization chambers SNC600c, SNC125c and SNC350p (all Sun Nuclear, A Mirion Medical Company, Melbourne, FL). Realistic clinical electron beam spectra were used to cover the entire energy range of therapeutic electron accelerators. The Monte Carlo simulations were validated by measurements on a clinical linear accelerator.With regards to the cylindrical chambers, the simulations were performed according to the setup recommendations of TRS 398 and AAPM TG 51, i.e. with and without consideration of a reference point shift by rcav/2. Results:kQ values as a function of the respective beam quality specifier R50 were fitted by recommended equations for electron beam dosimetry in the range of 5 MeV to 18 MeV. The fitting curves to the calculated values showed a root mean square deviation between 0.0016 and 0.0024. Conclusion:Electron beam quality correction factors kQ were calculated by Monte Carlo simulations for the cylindrical ionization chambers SNC600c and SNC125c as well as the plane parallel ionization chamber SNC350p to provide updated data for the TRS 398 and TG 51 dosimetry protocols.

Monte Carlo calculated ionization chamber correction factors in clinical proton beams – deriving uncertainties from published data

For the update of the IAEA TRS-398 Code of Practice (CoP), global ionization chamber factors (fQ) and beam quality correction factors (kQ) for air-filled ionization chambers in clinical proton beams have been calculated with different Monte Carlo codes. In this study, average Monte Carlo calculated fQ and kQ factors are provided and the uncertainty of these factors is estimated. Average fQ factors in monoenergetic proton beams with energies between 60 MeV and 250 MeV were derived from Monte Carlo calculated fQ factors published in the literature. Altogether, 195 fQ factors for six plane-parallel and three cylindrical ionization chambers calculated with penh, fluka and geant4 were incorporated. Additionally, a weighted standard deviation of fQ factors was calculated, where the same weight was assigned to each Monte Carlo code. From average fQ factors, kQ factors were derived and compared to the values from the IAEA TRS-398 CoP published in 2000 as well as to the values of the upcoming version. Average Monte Carlo calculated fQ factors are constant within 0.6% over the energy range investigated. In general, the different Monte Carlo codes agree within 1% for low energies and show larger differences up to 2% for high energies. As a result, the standard deviation of fQ factors increases with energy and is ∼0.3% for low energies and ∼0.8% for high energies. kQ factors derived from average Monte Carlo calculated fQ factors differ from the values presented in the IAEA TRS-398 CoP by up to 2.4%. The overall estimated uncertainty of Monte Carlo calculated kQ factors is ∼0.5%–1% smaller than the uncertainties estimated in IAEA TRS-398 CoP since the individual ionization chamber characteristics (e.g. fluence perturbations) are considered in detail in Monte Carlo calculations. The agreement between Monte Carlo calculated kQ factors and the values of the upcoming version of IAEA TRS-398 CoP is better with deviations smaller than 1%.

Monte Carlo calculated beam quality correction factors for two cylindrical ionization chambers in photon beams.

Although several studies provide data for reference dosimetry, the SNC600c and SNC125c ionization chambers (Sun Nuclear Corporation, Melbourne, FL) are in clinical use worldwide for which no beam quality correction factors kQ are available. The goal of this study was to calculate beam quality correction factors kQ for these ionization chambers according to dosimetry protocols TG-51, TRS 398 and DIN 6800-2. Monte Carlo simulations using EGSnrc have been performed to calculate the absorbed dose to water and the dose to air within the active volume of ionization chamber models. Both spectra and simulations of beam transport through linear accelerator head models were used as radiation sources for the Monte Carlo calculations. kQ values as a function of the respective beam quality specifier Q were fitted against recommended equations for photon beam dosimetry in the range of 4 MV to 25 MV. The fitting curves through the calculated values showed a root mean square deviation between 0.0010 and 0.0017. The investigated ionization chamber models (SNC600c, SNC125c) are not included in above mentioned dosimetry protocols, but are in clinical use worldwide. This study covered this knowledge gap and compared the calculated results with published kQ values for similar ionization chambers. Agreements with published data were observed in the 95% confidence interval, confirming the use of data for similar ionization chambers, when there are no kQ values available for a given ionization chamber.

Evaluating the Impact of Ionization Chamber-Specific Beam Quality Correction Factor in Dosimetry of Filtered and Unfiltered Photon Beams.

Aim:The response of ionization chamber changes when used at beam quality Q which is different from beam quality Qo (usually 60Co) that was used at the time of its calibration. Hence, one needs to apply beam quality correction factor (kQ, Qo) during dosimetric measurements. However, kQ, Qo data are unavailable for novel ion chambers in the literature. Moreover, most of such data do not differentiate between filtered (flat) and unfiltered (unflat) beams. In addition, literature-based data do not differentiate among different pieces of the ion chambers of the same make and model. Hence, the purpose of our study was to determine the ion chamber-specific experimental values of kQ, Qo and to evaluate their impact in dosimetry.Materials and Methods:In this work, the value of kQ, Qo were measured for six ionization chambers of three different types in 6, 10, and 15 MV filtered (with flattening filter [WFF]) as well as 6 and 10 MV unfiltered (flattening filter free [FFF]) photon beams. The measured values of kQ, Qo were compared with Monte Carlo-calculated values available in the literature. The uncertainties in measurement of kQ, Qo values were also evaluated.Results:For 6 MV FFF beam, the measured value of kQ, Qo was found to be consistently lower than 6 MV WFF beam for all Sun Nuclear Corporation ion chambers, while it was higher as per the theoretical data. The inter-chamber variation in kQ, Qo values was observed for the same model of the ion chambers. The maximum difference between absolute dose values on using the theoretical and experimental kQ, Qo values was up to 3.23%.Conclusion:The measured absolute dose values by the ion chamber of a given make and model were found different due to the use of its theoretical and experimental kQ, Qo values. Furthermore, the variation in response of different pieces of ion chambers of the same make and model cannot be accounted for theoretically, and hence, the use of theoretical kQ, Qo data may introduce an inherent error in the estimation of absorbed dose to water. This necessitates the use of measured value of kQ, Qo for each ionization chamber.

Cema‐based formalism for the determination of absorbed dose for high‐energy photon beams

Determination of absorbed dose is well established in many dosimetry protocols and considered to be highly reliable using ionization chambers under reference conditions. If dosimetry is performed under other conditions or using other detectors, however, open questions still remain. Such questions frequently refer to appropriate correction factors. A converted energy per mass (cema)-based approach to formulate such correction factors offers a good understanding of the specific response of a detector for dosimetry under various measuring conditions and thus an estimate of pros and cons of its application. Determination of absorbed dose requires the knowledge of the beam quality correction factor kQ,Qo , where Q denotes the quality of a user beam and Qo is the quality of the radiation used for calibration. In modern Monte Carlo (MC)-based methods, kQ,Qo is directly derived from the MC-calculated dose conversion factor, which is the ratio between the absorbed dose at a point of interest in water and the mean absorbed dose in the sensitive volume of an ion chamber. In this work, absorbed dose is approximated by the fundamental quantity cema. This approximation allows the dose conversion factor to be substituted by the cema conversion factor. Subsequently, this factor is decomposed into a product of cema ratios. They are identified as the stopping power ratio water to the material in the sensitive detector volume, and as the correction factor for the fluence perturbation of the secondary charged particles in the detector cavity caused by the presence of the detector. This correction factor is further decomposed with respect to the perturbation caused by the detector cavity and that caused by external detector properties. The cema-based formalism was subsequently tested by MC calculations of the spectral fluence of the secondary charged particles (electrons and positrons) under various conditions. MC calculations demonstrate that considerable fluence perturbation may occur particularly under non-reference conditions. Cema-based correction factors to be applied in a 6-MV beam were obtained for a number of ionization chambers and for three solid-state detectors. Feasibility was shown at field sizes of 4×4 and 0.5cm ×0.5cm. Values of the cema ratios resulting from the decomposition of the dose conversion factor can be well correlated with detector response. Under the small field conditions, the internal fluence correction factor of ionization chambers is considerably dependent on volume averaging and thus on the shape and size of the cavity volume. The cema approach is particularly useful at non-reference conditions including when solid-state detectors are used. Perturbation correction factors can be expressed and evaluated by cema ratios in a comprehensive manner. The cema approach can serve to understand the specific response of a detector for dosimetry to be dependent on (a) radiation quality, (b) detector properties, and (c) electron fluence changes caused by the detector. This understanding may also help to decide which detector is best suited for a specific measurement situation.

OC-0196 Determination of the beam-quality correction factors kQ for the PTW PinPoint 3D 31022 chamber

OC-0196 Determination of the beam-quality correction factors kQ for the PTW PinPoint 3D 31022 chamber

Dosimetric impacts of beam-hardening filter removal for the CyberKnife system.

Equipment refurbishment was performed to remove the beam-hardening filter (BHF) from the CyberKnife system (CK). This study aimed to confirm the change in the beam characteristics between the conventional CK (present-BHF CK) and CK after the BHF was removed (absent-BHF CK) and evaluate the impact of BHF removal on the beam quality correction factors kQ. The experimental measurements of the beam characteristics of the present- and absent-BHF CKs were compared. The CKs were modeled using Monte Carlo simulations (MCs). The energy fluence spectra were calculated using MCs. Finally, kQ were estimated by combining the MC results and analytic calculations based on the TRS-398 and TRS-483 approaches. All gamma values for percent depth doses and beam profiles between each CK were less than 0.5 following the 3%/1 mm criteria. The percentage differences for tissue-phantom ratios at depths of 20 and 10cm and percentage depth doses at 10cm between each CK were-1.20% and-0.97%, respectively. The MC results demonstrated that the photon energy fluence spectrum of the absent-BHF CK was softer than that of the present-BHF CK. The kQ values for the absent-BHF CK were in agreement within 0.02% with those for the present-BHF CK. The photon energy fluence spectrum was softened by the removal of BHF. However, no remarkable impact was observed for the measured beam characteristics and kQ. Therefore, the previous findings of the kQ values for the present-BHF CK can be directly used for the absent-BHF CK.

Monte Carlo calculation of perturbation correction factors for air-filled ionization chambers in clinical proton beams using TOPAS/GEANT

IntroductionCurrent dosimetry protocols for clinical protons using air-filled ionization chambers assume that the perturbation correction factor is equal to unity for all ionization chambers and proton energies. Since previous Monte Carlo based studies suggest that perturbation correction factors might be significantly different from unity this study aims to determine perturbation correction factors for six plane-parallel and four cylindrical ionization chambers in proton beams at clinical energies. Materials and methodsThe dose deposited in the air cavity of the ionization chambers was calculated with the help of the Monte Carlo code TOPAS/Geant4 while specific constructive details of the chambers were removed step by step. By comparing these dose values the individual perturbation correction factors pcel, pstem, psleeve, pwall, pcav⋅pdis as well as the total perturbation correction factor pQ were derived for typical clinical proton energies between 80 and 250MeV. ResultsThe total perturbation correction factor pQ was smaller than unity for almost every ionization chamber and proton energy and in some cases significantly different from unity (deviation larger than 1%). The maximum deviation from unity was 2.0% for cylindrical and 1.5% for plane-parallel ionization chambers. Especially the factor pwall was found to differ significantly from unity. It was shown that this is due to the fact that secondary particles, especially alpha particles and fragments, are scattered from the chamber wall into the air cavity resulting in an overresponse of the chamber. ConclusionPerturbation correction factors for ionization chambers in proton beams were calculated using Monte Carlo simulations. In contrast to the assumption of current dosimetry protocols the total perturbation correction factor pQ can be significantly different from unity. Hence, beam quality correction factors kQ,Q0 that are calculated with the help of perturbation correction factors that are assumed to be unity come with a corresponding additional uncertainty.

Monte Carlo calculation of beam quality correction factors for PTW cylindrical ionization chambers in photon beams

The beam quality correction factor kQ for megavoltage photon beams has been calculated for eight PTW (Freiburg, Germany) ionization chambers (Farmer chambers PTW30010, PTW30011, PTW30012, and PTW30013, Semiflex 3D chambers PTW31021, PTW31010, and PTW31013, and the PinPoint 3D chamber PTW31016). Simulations performed on the widely used NE-2571 ionization chamber have been used to benchmark the results. The Monte Carlo code PENELOPE/penEasy was used to calculate the absorbed dose to a point in water and the absorbed dose to the active air volume of the chambers for photon beams in the range 4 to 24 MV. Of the nine ionization chambers analysed, only five are included in the current version of the International Code of Practice for dosimetry based on standards of absorbed dose to water (IAEA TRS 398). The values reported in this work agree with those in the literature within the uncertainty estimates and are to be included in the average values of the data obtained by different working groups for the forthcoming update of TRS 398.

Beam Monitor Calibration for Radiobiological Experiments With Scanned High Energy Heavy Ion Beams at FAIR

Precise and reliable monitoring of the particle rate is of great importance at accelerator facilities worldwide. In this article we describe the standard beam monitor calibration currently employed at the multi-purpose experimental sites Cave A and Cave M at GSI, where intense highly energetic ion beams are routinely used for a wide variety of experiments. An absolute dose-to-water measurement is performed with an air-filled ionization chamber and transferred into a calibration per primary particle. This is necessary for the raster scanning system used to enable the irradiation of extended fields, required for biophysical experiments in the research fields of particle therapy or space radiation protection. The main focus of this work is to understand through Monte Carlo simulations whether the currently used dosimetry procedure is valid for all the ion species and energies that are provided at GSI Cave A by the SIS18 synchrotron and that will be provided by the SIS100 at FAIR. With this aim the detailed geometry of the 30013 Farmer ionization chamber currently used in the cave was implemented in the transport code FLUKA and the beam quality correction factor kQ for different energies and ion species was calculated. Further details about the robustness of the calibration are investigated as well, e.g. appropriate irradiation depth of biological samples. Evidence is presented that for ions above 1 GeV/u the kQ factor decreases due to the density effect, which modifies the water-to-air stopping power ratio at relativistic energies. These findings are of particular importance for future biophysics experiments with ion beams from the SIS100 in the framework of the FAIR project. For energies in the regime of several GeV/u the constant kQ value as used in common practice should be replaced with the energy-dependent correction factor provided in this work.

Reference dose determination in 60Co and high-energy radiotherapy photon beams by using Farmer-type cylindrical ionization chambers – an experimental investigation

Ionization chamber dosimetry is predominantly used for determination of the absorbed dose to water in 60Co and high-energy radiotherapy photon beams. The most widespread ionization chambers employed for absolute or reference dose determinations in reference conditions are the Farmer-type cylindrical ionization chambers. The Farmer-type ionization chambers have a variety of constructions and materials and their responses vary in the radiation beam. Clinical accelerators, in addition to conventional photon beams with flattening-filter, can also deliver flattening-filter-free (FFF) photon beams. The responses of five different Farmer-type cylindrical ionization chambers were experimentally examined with reference to absorbed dose determination in reference conditions when using the International Atomic Energy Agency (IAEA) - American Association of Physicists in Medicine (AAPM) Technical Reports Series no. 483 (TRS-483) and the IAEA TRS-398 dosimetry protocol in the present investigation. The irradiations were performed using 60Co and megavoltage photon beams with 6 MV, 15 MV, 6 MV FFF and 10 MV FFF nominal photon energies. The chamber calibrations were performed at different Secondary Standard Dosimetry Laboratories and are traceable to primary standards at different Primary Standard Dosimetry Laboratories. The chambers were also cross-calibrated at our laboratory using 60Co γ-beam. The variation found in the data regarding the reference dose determination using the various Farmer-type chambers in the photon beams employed was about 1% at maximum. Thus, the selection of the ionization chamber in reference dose determinations may affect the outcomes. The differences in the absorbed dose values were similar in the conventional as well as in the FFF photon beams. For the FFF photon beams the absorbed dose computations were performed using the IAEA-AAPM TRS-483 dosimetry protocol. Two of the ionization chambers used had identical construction but different central electrodes, i.e. graphite versus aluminium. The results obtained using these two chambers show that, in the photon beams examined, the employed correction for the central electrode (pcel) regarding these two chambers is associated with an inaccuracy which is larger than the calculated uncertainty for this correction. The outcomes found in the present experimental investigation using the various ionization chambers also indicate possible inaccuracy in the employed beam quality correction factors (kQ) and imply the need for a revision of these factors.

Monte Carlo calculation of beam quality correction factors in proton beams using TOPAS/GEANT4

To provide Monte Carlo calculated beam quality correction factors (kQ) for monoenergetic proton beams using , a toolkit based on the Monte Carlo code .Monte Carlo simulations of six plane-parallel and four cylindrical ionization chambers were carried out. The latest ICRU 90 recommendations on key data for ionizing-radiation dosimetry were used to calculate the electronic stopping powers and to select the mean energy necessary to create an ion pair in air (). factors were calculated for a 60Co spectrum at a depth of 5 g cm−2. f Q factors and ratios as well as kQ factors were calculated at the entrance region of monoenergetic proton beams with energies between 60 MeV and 250 MeV.Additionally, perturbation correction factors for the Exradin A1SL ionization chamber at an energy of 250 MeV were calculated. factors agreed within 0.7% or better, f Q factors within 1.7% or better and ratios within 2.2% or better with Monte Carlo calculated values provided in the literature. Furthermore, kQ factors calculated in this work were found to agree within 1% or better with experimentally determined kQ factors provided in the literature, with only two exceptions with deviations of 1.4% and 2.4%.The total perturbation correction factor for the Exradin A1SL chamber was 0.969(7) and hence significantly different than unity in contrast to the assumption from the IAEA TRS-398 code of practice (CoP). can be used to calculate kQ factors in clinical proton beams. kQ factors for six plane-parallel and four cylindrical ionization chambers were calculated and provided for the upcoming update of the IAEA TRS-398 CoP.

Impact of new ICRU 90 key data on stopping-power ratios and beam quality correction factors for carbon ion beams

The recent update of key dosimetric data by the International Commission on Radiation Units and Measurements (ICRU) makes several changes to the computation of beam quality correction factors kQ with regard to, for example, the mean excitation energies, I, which enter the stopping power computation for water and air, the computation procedure itself, the average energy expended in the production of an ion pair in air, W/e, as well as chamber-specific factors for cobalt-60. With the new recommendations an accurate assessment of the water-to-air stopping-power ratio, , in reference conditions is necessary to update the dosimetry protocols for carbon ion beams. The ICRU 90 key data were considered for computation of for carbon ion beams using Monte Carlo transport simulations for a number of reference conditions, namely monoenergetic carbon ion beams with a range in water from 3 to 30 cm and spread-out Bragg peaks (SOBPs) of different widths and depths in water. New recommendations for are presented, namely 1.1247 for the reference condition of depth 1 g cm−2 for monoenergetic carbon ion beams and 1.1273 at the center of physically optimized SOBPs. The recommendation of a constant value (1.126) represents the stopping-power ratio within a 0.3% variation of for all reference conditions considered. The impact of these new values and the updated key data on kQ for carbon ion beams was evaluated in a second step. Changes and the difference from experimental data were found to be non-significant, but larger discrepancies to measurements were observed for plane-parallel ionization chambers. The combined uncertainty for kQ in carbon ion beams decreased to 2.4%. In future, it could be further lowered by using chamber-specific Monte Carlo transport simulations, for which the implementation of ICRU 90 key data as done in this study is a prerequisite.

Experimental and Monte-Carlo characterization of the novel compact ionization chamber PTW 31023 for reference and relative dosimetry in high energy photon beams

IntroductionThe aim of the present work is to perform dosimetric characterization of a novel vented PinPoint ionization chamber (PTW 31023, PTW-Freiburg, Germany). This chamber replaces the previous model (PTW 31014), where the diameter of the central electrode has been increased from 0.3 to 0.6mm and the guard ring has been redesigned. Correction factors for reference and non-reference measurement conditions were examined. Materials and methodsMeasurements and calculations of the correction factors were performed according to the DIN 6800-2. The shifts of the effective point of measurement (EPOM) from the chamber's reference point were determined by comparison of the measured PDD with the reference curve obtained with a Roos chamber. Its lateral dose response functions, which act according to a mathematical convolution model as the convolution kernel transforming the dose profile D(x) to the measured signal M(x), have been approximated by Gaussian functions with standard deviation σ. Additionally, the saturation correction factors kS have been determined using different dose-per-pulse (DPP) values. The polarity effect correction factors kP were measured for field sizes from 5cm×5cm to 40cm×40cm. The influence of the diameter of the central electrode and the new guard ring on the beam quality correction factors kQ was studied by Monte-Carlo simulations. The non-reference condition correction factors kNR have been computed for 6MV photo beam by varying the field size and measurement depth. Comparisons on these aspects have been made to the previous model. ResultsThe shifts of the EPOM from the reference point, Δz, are found to be −0.55 (6MV) and −0.56 (10MV) in the radial orientation and −0.97mm (6MV) and −0.91mm (10MV) in the axial orientation. All values of Δz have an uncertainty of 0.1mm. The σ values are 0.80mm (axial), 0.75mm (radial lateral) and 1.76mm (radial longitudinal) for 6MV photon beam and are 0.85mm (axial), 0.75mm (radial lateral) and 1.82mm (radial longitudinal) for 15MV photon beam. All σ values have an uncertainty of 0.05mm. The correction factor kS was found to be 1.0034±0.0009 for the PTW 31014 chamber and 1.0024±0.0007 for the PTW 31023 chamber at the highest DPP (0.827mGy) investigated in this study. Under reference conditions, the polarity effect correction factor kP of the PTW 31014 chamber is 1.0094 and 1.0116 for 6 and 10MV respectively, while the kP of the PTW 31023 chamber is 1.0005 and 1.0013 for 6 and 10MV respectively, all values have an uncertainty of 0.002. The kP of the new chamber also exhibits a weaker field size dependence. The kQ values of the PTW 31023 chamber are closer to unity than those of the PTW 31014 chamber due to the thicker central electrode and the new guard ring design. The kNR values of the PTW 31023 chamber for 6MV photon beam deviate by not more than 1% from unity for the conditions investigated. DiscussionsCorrection factors associated with the new chamber required to perform reference and relative dose measurements have been determined according to the DIN-protocol. The correction factor kS of the new chamber is 0.1% smaller than that of the PTW 31014 at the highest DPP investigated. Under reference conditions, the correction factor kP of the PTW 31023 chamber is approximately 1% smaller than that of the PTW 31014 chamber for both energies used. The dosimetric characteristics of the new chamber investigated in this work have been demonstrated to fulfil the requirements of the TG-51 addendum for reference-class dosimeters at reference conditions.

OA025] Response of ionization chambers in the presence of magnetic fields

Purpose The next generation of medical electron linear accelerators will integrate Magnetic Resonance Tomography (MRI). Therefore it will be possible to take direct images of the moving tumor during radiotherapy treatment. However serious consideration must be given to the strong magnetic field, which has an impact on the trajectories of the produced secondary electrons, because of the Lorentz force. This force affects both the dose distribution in water and the dose response of used detectors. For an accurate patient dosimetry these effects must be taken into account. Methods Monte Carlo methods correctly describe the radiation transport in different media, even in presence of magnetic fields and are therefore the gold standard for the evaluation of the impact of magnetic fields on clinical dosimetry. In this present study, the beam quality correction factors Kq and the relative response in magnetic fields of different ion chambers (PTW-31013, PTW-31021, EXRADIN-1ASL, NE2571) were investigated with Monte Carlo simulations, using the code EGSnrc. The chambers were modelled in detail according to the information given by the manufacturer and placed in a water phantom (30 × 30 × 30 cm3). The chambers were irradiated under reference conditions, following the recommendations of present dosimetry protocols, such as IAEA TRS-398: i.e. the field size at the phantom surface was 10 × 10cm2; the focus-surface-distance 100 cm and the depth of the chamber’s reference point was 10 cm. The source of the photons were several spectra of clinical medical accelerators applying nominal energies between 4 and 24 MV-X. The magnetic field was applied in different directions relative to the beam axis (z-direction) and the chamber’s symmetry axis and was varied between 0 and 3T. Results All chambers successfully passed the Fano test, the response of all chambers varied up to about 8% depending on the magnetic field strength and the field directions. Conclusions The response of the chambers with the magnetic fields, has a complex dependence on chamber radius, magnetic field strength, the orientation between radiation beams, chamber axis and magnetic field, and lastly the energy spectrum. Those variables consequently played a major role in the increase or decrease of correction factor, Kq.

Impact of new ICRU Report 90 recommendations on calculated correction factors for reference dosimetry

In 2016 the ICRU published a new report dealing with key data for ionizing radiation dosimetry (ICRU Report 90). New recommendations have been made for the mean excitation energies I for air, graphite and liquid water as well as for the graphite density to use when evaluating the density effect. In addition, the ICRU Report 90 discusses renormalized photoelectric cross sections, but refuses to give a recommendation on the use of renormalization factors. However, the Consultative Committee for Ionizing Radiation recommends to use renormalized photoeffect cross sections. Goal of the present work is to evaluate the impact of these new recommendations on clinical reference dosimetry for high energy photon and electron beams.The beam quality correction factor kQ was calculated by Monte Carlo simulations for compact and parallel plate ionization chambers. In case of photons seven phase space files from clinical accelerators and twelve spectra taken from literature from 4 MV to 24 MV and additionally a 60Co source were applied. As electron source thirteen electron spectra available in literature were used in the range of 4 MeV–21 MeV.The new ICRU recommendations have a small impact on Monte Carlo calculated kQ values for the chosen ionization chambers in the range of 0.1%–0.35% only—the difference increases for higher photon energies.The impact of the ICRU Report 90 recommendations on Monte Carlo calculated stopping power ratios sw,a, perturbation factors p and beam quality correction factors kQ was investigated and confirmed a decrese of sw,a by a fraction of a percent for photon and electron beams. This study indicates that the impact of the new ICRU recommendation is within 0.35%. The determined deviations should be taken into account, when widely published Monte Carlo calculated values are examined.

Comparative investigation of beam quality index Q on flattening filter and flattening filter free 6 and 15 MV photon beams of an medical linear accelerator

Abstract An Elekta Synergy LINAC was used to investigate the beam quality index Q and the beam quality correction factor kQ, which is used for absolute dosimetry, in off-axis photon fields. It was found that the beam quality index Q of the photon energies with flattening filter decreases with increasing distance to the central axis, at 6 MV by 4.27% and at 15 MV by 3.98% inside a range of 15 cm off-axis. The beam quality index for flattening filter free photon fields also decreases with increasing distance to the central axis. In this case Q changed only by 1.01% inside the above range.

Consistency in quality correction factors for ionization chamber dosimetry in scanned proton beam therapy.

The IAEA TRS-398 code of practice details the reference conditions for reference dosimetry of proton beams using ionization chambers and the required beam quality correction factors (kQ ). Pencil beam scanning (PBS) systems cannot approximate reference conditions using a single spot. However, dose distributions requested in TRS-398 can be reproduced with PBS using a combination of spots. This study aims to demonstrate, using Monte Carlo (MC) simulations, that kQ factors computed/measured for broad beams can be used with scanned beams for similar reference dose distributions with no additional significant uncertainty. We consider the Alfonso formalism13 usually employed for nonstandard photon beams. To approach reference conditions similar as IAEA TRS-398 and the associated dose distributions, PBS must combine many pencil beams with range or energy modulation and shaping techniques that differ from those used in passive systems (broad beams). In order to evaluate the impact of these differences on kQ factors, ionization chamber responses are computed with MC (Geant4 9.6) in three different proton beams, with their corresponding quality factors (Q), producing a 10 × 10 cm2 field with a flat dose distribution for (a) a dedicated scanned pencil beam (Qpbs ), (b) a hypothetical proton source (Qhyp ), and (c) a double-scattering beam (Qds ). The tested ionization chamber cavities are a 2 × 2 × 0.2 mm³ air cavity, a Roos-type ionization chamber, and a Farmer-type ionization chamber. Ranges of Qpbs , Qhyp , and Qds are consistent within 0.4 mm. Flatnesses of dose distributions are better than 0.5%. Calculated kQpbs,Qhypfpbs,fref is 0.999 ± 0.002 for the air cavity and the Farmer-type ionization chamber and 1.001 ± 0.002 for the Roos-type ionization chamber. The quality correction factors kQpbs,Qdsfpbs,fref is 0.999 ± 0.002 for the Farmer-type and Roos-type ionization chambers and 1.001 ± 0.001 for the Roos-type ionization chamber. The Alfonso formalism was applied to scanned proton beams. In our MC simulations, neither the difference in the beam profiles (scanned beam vs hypothetical beam) nor the different incident beam energies influenced significantly the beam correction factors. This suggests that ionization chamber quality correction factors in scanned or broad proton beams are indistinguishable within the calculation uncertainties provided dose distributions achieved by both modalities are similar and compliant with the TRS-398 reference conditions.

Monte Carlo-based investigations on the impact of removing the flattening filter on beam quality specifiers for photon beam dosimetry.

The impact of removing the flattening filter in clinical electron accelerators on the relationship between dosimetric quantities such as beam quality specifiers and the mean photon and electron energies of the photon radiation field was investigated by Monte Carlo simulations. The purpose of this work was to determine the uncertainties when using the well-known beam quality specifiers or energy-based beam specifiers as predictors of dosimetric photon field properties when removing the flattening filter. Monte Carlo simulations applying eight different linear accelerator head models with and without flattening filter were performed in order to generate realistic radiation sources and calculate field properties such as restricted mass collision stopping power ratios (L¯/ρ)airwater, mean photon and secondary electron energies. To study the impact of removing the flattening filter on the beam quality correction factors kQ , this factor for detailed ionization chamber models was calculated by Monte Carlo simulations. Stopping power ratios (L¯/ρ)airwater and kQ values for different ionization chambers as a function of TPR1020 and %dd(10)x were calculated. Moreover, mean photon energies in air and at the point of measurement in water as well as mean secondary electron energies at the point of measurement were calculated. The results revealed that removing the flattening filter led to a change within 0.3% in the relationship between %dd(10)x and (L¯/ρ)airwater, whereby the relationship between TPR1020 and (L¯/ρ)airwater changed up to 0.8% for high energy photon beams. However, TPR1020 was a good predictor of (L¯/ρ)airwater for both types of linear accelerator with energies < 10MeV with a maximal deviation between both types of accelerators of 0.23%. According to the results, the mean photon energy below the linear accelerators head as well as at the point of measurement may not be suitable as a predictor of (L¯/ρ)airwater and kQ to merge the dosimetry of both linear accelerator types. It was possible to derive (L¯/ρ)airwater using the mean secondary electron energy at the point of measurement as a predictor with an accuracy of 0.17%. A bias between kQ for linear accelerators with and without flattening filter within 1.1% and 1.6% was observed for TPR1020 and %dd(10)x respectively. The results of this study have shown that removing the flattening filter led to a change in the relationship between the well-known beam quality specifiers and dosimetric quantities at the point of measurement, namely (L¯/ρ)airwater, mean photon and electron energy. Furthermore, the results show that a beam profile correction is important for dose measurements with large ionization chambers in flattening filter free beams.

Characterization of a synthetic single crystal diamond detector for dosimetry in spatially fractionated synchrotron x‐ray fields

Modern radiotherapy modalities often use small or nonstandard fields to ensure highly localized and precise dose delivery, challenging conventional clinical dosimetry protocols. The emergence of preclinical spatially fractionated synchrotron radiotherapies with high dose-rate, sub-millimetric parallel kilovoltage x-ray beams, has pushed clinical dosimetry to its limit. A commercially available synthetic single crystal diamond detector designed for small field dosimetry has been characterized to assess its potential as a dosimeter for synchrotron microbeam and minibeam radiotherapy. Experiments were carried out using a synthetic diamond detector on the imaging and medical beamline (IMBL) at the Australian Synchrotron. The energy dependence of the detector was characterized by cross-referencing with a calibrated ionization chamber in monoenergetic beams in the energy range 30-120 keV. The dose-rate dependence was measured in the range 1-700 Gy/s. Dosimetric quantities were measured in filtered white beams, with a weighted mean energy of 95 keV, in broadbeam and spatially fractionated geometries, and compared to reference dosimeters. The detector exhibits an energy dependence; however, beam quality correction factors (kQ) have been measured for energies in the range 30-120 keV. The kQ factor for the weighted mean energy of the IMBL radiotherapy spectrum, 95 keV, is 1.05 ± 0.09. The detector response is independent of dose-rate in the range 1-700 Gy/s. The percentage depth dose curves measured by the diamond detector were compared to ionization chambers and agreed to within 2%. Profile measurements of microbeam and minibeam arrays were performed. The beams are well resolved and the full width at halfmaximum agrees with the nominal width of the beams. The peak to valley dose ratio (PVDR) calculated from the profiles at various depths in water agrees within experimental error with PVDR calculations from Gafchromic film data. The synthetic diamond detector is now well characterized and will be used to develop an experimental dosimetry protocol for spatially fractionated synchrotron radiotherapy.