Current Dosimetry Protocols Research Articles

MR-Linac Dosimetry: Current Approaches and Challenges

The use of MR-integrated linacs (MR-linac) in clinical radiation oncology applications are becoming more prevalent. However, given the novel and radically different designs of these systems from conventional linacs, current radiation dosimetry protocols for high energy photons are no longer appropriate for use in modern MR-linac systems: First, given the presence of the magnetic field and the linac design, traditional reference conditions defined by previous high energy photon dosimetry protocols cannot be met; Second, the presence of the strong magnetic field can affect the performance of conventional equipment used for dosimetry. In this manuscript, we describe some of the challenges faced in radiation dosimetry in external MR-guided radiotherapy delivery systems, summarize some of the publications in this area, and finally discuss the early work by the American Association of physicists in Medicine (AAPM) Task Group 351 which is mandated with producing a protocol for reference dosimetry in MR-linac units.

Monte Carlo-calculated beam quality and perturbation correction factors validated against experiments for Farmer and Markus type ionization chambers in therapeutic carbon-ion beams

Objective. In current dosimetry protocols, the estimated uncertainty of the measured absorbed dose to water D w in carbon-ion beams is approximately 3%. This large uncertainty is mainly contributed by the standard uncertainty of the beam quality correction factor k Q. In this study, the k Q values in four cylindrical chambers and two plane-parallel chambers were calculated using Monte Carlo (MC) simulations in the plateau region. The chamber-specific perturbation correction factor P of each chamber was also determined through MC simulations. Approach. k Q for each chamber was calculated using MC code Geant4. The simulated k Q ratios in subjected chambers and reference chambers were validated through comparisons against our measured values. In the measurements in Heavy-Ion Medical Accelerator in Chiba, k Q ratios were obtained from D w values of 60Co, 290- and 400 MeV u−1 carbon-ion beams that were measured with the subjected ionization chamber and the reference chamber. In the simulations, f Q (the product of the water-to-air stopping power ratio and P) was acquired from D w and the absorbed dose to air calculated in the sensitive volume of each chamber. k Q values were then calculated from the simulated f Q and the literature-extracted W air and compared with previous publications. Main results. The calculated k Q ratios in the subjected chambers to the reference chamber agreed well with the measured k Q ratios. The k Q uncertainty was reduced from the current recommendation of approximately 3% to 1.7%. The P values were close to unity in the cylindrical chambers and nearly 1% above unity in the plane-parallel chambers. Significance. The k Q values of carbon-ion beams were accurately calculated in MC simulations and the k Q ratios were validated through ionization chamber measurements. The results indicate a need for updating the current recommendations, which assume a constant P of unity in carbon-ion beams, to recommendations that consider chamber-induced differences.

Dedicated cone-beam breast CT: Data acquisition strategies based on projection angle-dependent normalized glandular dose coefficients.

Dedicated cone-beam breast computed tomography (CBBCT) using short-scan acquisition is being actively investigated to potentially reduce the radiation dose to the breast. This would require determining the optimal x-ray source trajectory for such short-scan acquisition. To quantify the projection angle-dependent normalized glandular dose coefficient ( ) in CBBCT, referred to as angular , so that the x-ray ray source trajectory that minimizes the radiation dose to the breast for short-scan acquisition can be determined. A cohort of 75 CBBCT clinical datasets was segmented and used to generate three breast models - (I) patient-specific breast with heterogeneous fibroglandular tissue distribution and real breast shape, (II) patient-specific breast shape with homogeneous tissue distribution and matched fibroglandular weight fraction, and (III) homogeneous semi-ellipsoidal breast with patient-specific breast dimensions and matched fibroglandular weight fraction, which corresponds to the breast model used in current radiation dosimetry protocols. For each clinical dataset, the angular was obtained at 10 discrete angles, spaced 36° apart, for full-scan, circular, x-ray source trajectory from Monte Carlo simulations. Model III is used for validating the Monte Carlo simulation results. Models II and III are used to determine if breast shape contributes to the observed trends in angular . A geometry-based theory in conjunction with center-of-mass ( ) based distribution analysis is used to explain the projection angle-dependent variation in angular . The theoretical model predicted that the angular will follow a sinusoidal pattern and the amplitude of the sinusoid increases when the center-of-mass of fibroglandular tissue ( ) is farther from the center-of-mass of the breast ( ). It also predicted that the angular will be minimized at x-ray source positions complementary to the . The was superior to the in 80% (60/75) of the breasts. From Monte Carlo simulations and for homogeneous breasts (models II and III), the deviation in breast shape from a semi-ellipsoid had minimal effect on angular and showed less than 4% variation. From Monte Carlo simulations and for model I, as predicted by our theory, the angular followed a sinusoidal pattern with maxima and minima at x-ray source positions superior and inferior to the breast, respectively. For model I, the projection angle-dependent variation in angular was 16.4%. The heterogeneous tissue distribution affected the angular more than the breast shape. For model I, the angular was lowest when the x-ray source was inferior to the breast. Hence, for short-scan CBBCT acquisition with aligned with axis-of-rotation, an x-ray source trajectory inferior to the breast is preferable and such an acquisition spanning 205° can potentially reduce the mean glandular dose by up to 52%.

Experimental determination of the effective point of measurement for cylindrical ionization chambers in megavoltage photon beams.

Current dosimetry protocols specify an effective point of measurement (EPOM) shift of 0.6r for a cylindrical ionization chamber in photon beams. However, prior studies have reported that this shift was excessively large. The objective of this study was to experimentally evaluate the EPOM shifts in photon beams for cylindrical ionization chambers, which are widely used in clinical practice, and thus determine the appropriate EPOM shift. A microdiamond detector, which is a semiconductor detector with a small sensitive volume, was used as a reference detector, and the EPOM shifts of 11 types of cylindrical ionization chambers were evaluated at 6MV and 10MV. The depth shift from the percent depth dose (PDD) of the reference detector to that of the evaluated chamber was calculated using the least-squares method and was defined as the EPOM shift. The EPOM shift of the 10 MV condition was slightly larger than that of the 6MV condition. However, because this trend was not observed for all chambers, the results of the two energies were averaged, and the EPOM shifts were determined to be 0.33r-0.43r (± 0.05) for 10 types of ionization chambers, and 0.03r (± 0.03) for the A1SL chamber. The shifts for all ionization chambers were smaller than 0.6r, indicating that the recommended EPOM shifts were overestimated and the absorbed dose was underestimated at the calibration depth. Hence, the appropriate EPOM shift of the 10 types of ionization chambers was 0.4r (the geometric center of the A1SL chamber), with a dose uncertainty of 0.05%.

Determination of beam quality correction factors for the Roos plane-parallel ionisation chamber exposed to very high energy electron (VHEE) beams using Geant4

Detailed characterisation of the Roos secondary standard plane-parallel ionisation chamber has been conducted in a novel 200 MeV Very High Energy Electron (VHEE) beam with reference to the standard 12 MeV electron calibration beam used in our experimental work. Stopping-power-ratios and perturbation factors have been determined for both beams and used to calculated the beam quality correction factor using the Geant4 general purpose MC code. These factors have been calculated for a variety of charged particle transport parameters available in Geant4 which were found to pass the Fano cavity test. Stopping-power-ratios for the 12 MeV electron calibration beam quality were found to agree within uncertainties to that quoted by current dosimetry protocols. Perturbation factors were found to vary by up-to 4% for the calibration beam depending on the parameter configuration, compared with only 0.8% for the VHEE beam. Beam quality correction factors were found to describe an approximately 10% lower dose than would be originally calculated if a beam quality correction were not accounted for. Moreover, results presented here largely resolve unphysical chamber measurements, such as collection efficiencies greater than 100%, and assist in the accurate determination of absorbed dose and ion recombination in secondary standard ionisation chambers.

Average Absorbed Breast Dose (2ABD) to Mean Glandular Dose (MGD) Conversion Function for Digital Breast Tomosynthesis: A New Approach

Background: In this work a new method for the Mean Glandular Dose evaluation in digital breast tomosynthesis (DBT) is presented. Methods: Starting from the experimental-based dosimetric index, 2ABD, which represents the average absorbed breast dose, the mean glandular dose MGD2ABD was calculated using a conversion function of glandularity f(G), obtained through the use of Monte Carlo simulations.Results: f(G) was computed for a 4.5 cm thick breast: from its value MGD2ABD for different compressed breast thicknesses and glandularities was obtained. The comparison between MGD2ABD estimates and the dosimetric index provided in the current dosimetry protocols, following the Dance's approach, MGDDance, showed a good agreement (<10%) for all the analyzed breast thicknesses and glandularities. Conclusion: The strength of the proposed method can be considered an accurate mean glandular dose assessment starting from few and accessible parameters, reported in the header DICOM of each DBT exam.

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.

Experimental determination of the recombination correction factor kS for SNC 125c, SNC 350p and SNC 600c ionization chambers in pulsed photon beams

Accurate ionization chamber measurements of the absorbed dose to water require the correction of incomplete collection of charges created within the chamber volume. According to current dosimetry protocols such as the TRS-398 or the DIN 6800-2, incomplete charge collection is accounted for by the correction factor ks, which can be determined numerically or experimentally. The method proposed by Burns & McEwen (Phys. Med. Biol., 1998) was used in this study to determine the coefficients γ and δ used for the calculation of the correction factor ks of three ionization chambers, the SNC 125c, the SNC 600c and the SNC 350p (all Sun Nuclear Corp., Melbourne, Florida) for an absorbed dose to water range of 0.2mGy to 1.6mGy per pulse in pulsed photon beams. The shift of the effective point of measurement from the reference point Δz and the correction factor kr were determined for the SNC 350p according to the draft DIN 6800-2:2019-07.

The effective point of measurement for depth-dose measurements in small MV photon beams with different detectors.

The effective point of measurement (EPOM) of cylindrical ionization chambers differs from their geometric center. The exact shift depends on chamber construction details, above all the chamber size, and to some degree on the field-size and beam quality. It generally decreases as the chamber dimensions get smaller. In this work, effective points of measurement in small photon fields of a range of cylindrical chambers of different sizes are investigated, including small chambers that have not been studied previously. In this investigation, effective points of measurement for different ionization chambers (Farmer type, scanning chambers, micro-ionization chambers) and solid state detectors were determined by measuring depth-ionization curves in a 6MV beam in field sizes between 2×2cm2 and 10×10cm2 and comparing those curves with curves measured with plane-parallel chambers. It was possible to average the results to one shift per detector, as the results were sufficiently independent of the studied field sizes. For cylindrical ion chambers, shifts of the EPOM were determined to be between 0.49 and 0.30 times the inner chamber radius from the reference point. We experimentally confirmed the previously reported decrease of the EPOM shift with decreasing detector size. Highly accurate data for a large range of detectors, including new very small ones, were determined. Thus, small chambers noticeably differ from the 0.5-times to 0.6-times the inner chamber radius recommendations in current dosimetry protocols. The detector-individual EPOMs need to be considered for measurements of depth-dose curves.

Calibration of Therapy Level Ionization Chamber at <sup>60</sup>Co Teletherapy Beam Used for Radiation Therapy

The accuracy and traceability of absorbed dose to water measurement of radiotherapy beam is a critical issue to achieve the curative outcome of cancer patients. The current dosimetry protocols for radiotherapy beams TRS-398, TG-51 and DIN-6800-2 are based on the calibration factor of ionization chamber in terms of absorbed dose to water for 60Co beam. The accuracy of the calibration factor of ionization chamber as well as output of radiotherapy beam is the primary requirements of precisional dose deliver to the tumor which is the QA part of radiotherapy dosimetry. In the present study, we have calibrated 9 different ionization chambers (8 thimbles and 1 parallel plate) of various active volumes for 60Co beam against reference standard NE2571 and compared with manufacturer’s values. The Percentage Depth Dose (PDD) and Output Factors (OF) of two cobalt units were measured with standard calibration system by following IAEA dosimetry protocol TRS-398 and compare with 6 MV photon beam from medical linear accelerator. The traceability of the dosimetry was verified by the participation of postal dose IAEA/WHO intercomparison program. The aim of the participation was to investigate uncertainties involved in the calibration of Ionization Chamber (IC) and absorbed dose measurement. The percentage of deviation relative to IAEA mean dose was found to be -0.2% (traceable limit ±5%), which shows an excellent agreement of calibration of beam as well as ionization chamber with international standard. The deviation of factors of ionization chambers between the measured and manufacturer’s values were found within 0.07-2.81% with an uncertainty of ±1.5% (k=1).

Calibration of Therapy Level Ionization Chamber at <sup>60</sup>Co Teletherapy Beam Used for Radiation Therapy

The accuracy and traceability of absorbed dose to water measurement of radiotherapy beam is a critical issue to achieve the curative outcome of cancer patients. The current dosimetry protocols for radiotherapy beams TRS-398, TG-51 and DIN-6800-2 are based on the calibration factor of ionization chamber in terms of absorbed dose to water for 60Co beam. The accuracy of the calibration factor of ionization chamber as well as output of radiotherapy beam is the primary requirements of precisional dose deliver to the tumor which is the QA part of radiotherapy dosimetry. In the present study, we have calibrated 9 different ionization chambers (8 thimbles and 1 parallel plate) of various active volumes for 60Co beam against reference standard NE2571 and compared with manufacturer’s values. The Percentage Depth Dose (PDD) and Output Factors (OF) of two cobalt units were measured with standard calibration system by following IAEA dosimetry protocol TRS-398 and compare with 6 MV photon beam from medical linear accelerator. The traceability of the dosimetry was verified by the participation of postal dose IAEA/WHO intercomparison program. The aim of the participation was to investigate uncertainties involved in the calibration of Ionization Chamber (IC) and absorbed dose measurement. The percentage of deviation relative to IAEA mean dose was found to be -0.2% (traceable limit ±5%), which shows an excellent agreement of calibration of beam as well as ionization chamber with international standard. The deviation of factors of ionization chambers between the measured and manufacturer’s values were found within 0.07-2.81% with an uncertainty of ±1.5% (k=1).

Examining the influence of humidity on reference ionization chamber performance.

International dosimetry protocols require measurements made with a vented ionization chamber to be corrected for the influence of air density by using the standard temperature-pressure correction factor. The effect of humidity, on the other hand, is generally ignored with the provision that the relative humidity (RH) is within certain limits (typically 20% to 80%). However, there is little experimental data in the published literature as to the true effect of humidity on modern reference-class ionization chambers. This investigation used two different radiation beams - a Co-60 irradiator and an Sr-90 check source - to examine the effect of humidity on several Farmer-type ionization chambers. An environmental cabinet controlled the humidity. For the Co-60 beam, the irradiation was external, whereas for the Sr-90 measurements, the source itself was placed within the cabinet. Extensive measurements were carried out to ensure that the experimental setup provided reproducible readings. Four chamber types were investigated: IBA FC65-G, IBA FC65-P, PTW 30013 and Exradin A19. The different wall materials provided potentially different mechanical responses (i.e., in terms of expansion/contraction) to the water content in the air. The relative humidity was varied between 8% and 98% and measurements were made with both increasing and decreasing humidity to investigate any possible hysteresis effects. Measurements in Co-60 were consistent with the published data obtained with primary standard cavity chambers in ICRU Report 31 (i.e., a very small variation <0.1% for RH >10%). The measurements in the Sr-90 field showed no dependence with the relative humidity, within the measurement uncertainties (0.05%, k = 1). Very good repeatability of the ionization current was obtained over successive wet/dry cycles, no hysteresis was observed, and there was no dependence on chamber type. These results indicate that humidity has no significant effect on these particular types of ionization chambers, consistent with recommendations in current megavoltage dosimetry protocols.

TH‐AB‐201‐06: Examining the Influence of Humidity On Reference Ion Chamber Performance

Purpose:International dosimetry protocols require measurements made with a vented ionization chamber to be corrected for the influence of air density by using the standard temperature‐pressure correction factor. The effect of humidity, on the other hand, is generally ignored with the provision that the relative humidity is between certain limits (15% to 80%). However, there is little experimental data in the published literature as to the true effect of humidity on modern reference‐class ion chambers. This investigation used two different radiation beams – a Co‐60 irradiator and a Sr‐90 check source – to examine the effect of humidity on several versions of the standard Farmer‐type ion chamber.Methods:An environmental cabinet controlled the humidity. For the Co‐60 beam, the irradiation was external, whereas for the Sr‐90 measurements, the source itself was placed within the cabinet. Extensive measurements were carried out to ensure that the experimental setup provided reproducible readings. Four chamber types were investigated: IBA FC65‐G (×2), IBA FC65‐P, PTW30013 &amp; Exradin A19. The different wall materials provided potentially different mechanical responses (i.e., in terms of expansion/contraction) to the water content in the air. The relative humidity was varied between 8 % and 97 % and measurements were made with increasing and decreasing humidity to investigate possible hysteresis effects.Results:Measurements in Co‐60 were consistent with the published data obtained with primary standard cavity chambers in ICRU Report 31. Ionization currents with Sr‐90 showed no dependence with the relative humidity, within the measurement uncertainties. Very good repeatability of the ionization current was obtained over successive wet/dry cycles, no hysteresis was observed, and there was no dependence on chamber type.Conclusion:This null result is very encouraging as it indicates that humidity has no significant effect on these particular types of ionization chambers, consistent with recommendations in current megavoltage dosimetry protocols.

Monte Carlo calculations of electron beam quality conversion factors for several ion chamber types.

To provide a comprehensive investigation of electron beam reference dosimetry using Monte Carlo simulations of the response of 10 plane-parallel and 18 cylindrical ion chamber types. Specific emphasis is placed on the determination of the optimal shift of the chambers' effective point of measurement (EPOM) and beam quality conversion factors. The EGSnrc system is used for calculations of the absorbed dose to gas in ion chamber models and the absorbed dose to water as a function of depth in a water phantom on which cobalt-60 and several electron beam source models are incident. The optimal EPOM shifts of the ion chambers are determined by comparing calculations of R50 converted from I50 (calculated using ion chamber simulations in phantom) to R50 calculated using simulations of the absorbed dose to water vs depth in water. Beam quality conversion factors are determined as the calculated ratio of the absorbed dose to water to the absorbed dose to air in the ion chamber at the reference depth in a cobalt-60 beam to that in electron beams. For most plane-parallel chambers, the optimal EPOM shift is inside of the active cavity but different from the shift determined with water-equivalent scaling of the front window of the chamber. These optimal shifts for plane-parallel chambers also reduce the scatter of beam quality conversion factors, kQ, as a function of R50. The optimal shift of cylindrical chambers is found to be less than the 0.5 rcav recommended by current dosimetry protocols. In most cases, the values of the optimal shift are close to 0.3 rcav. Values of kecal are calculated and compared to those from the TG-51 protocol and differences are explained using accurate individual correction factors for a subset of ion chambers investigated. High-precision fits to beam quality conversion factors normalized to unity in a beam with R50 = 7.5 cm (kQ (')) are provided. These factors avoid the use of gradient correction factors as used in the TG-51 protocol although a chamber dependent optimal shift in the EPOM is required when using plane-parallel chambers while no shift is needed with cylindrical chambers. The sensitivity of these results to parameters used to model the ion chambers is discussed and the uncertainty related to the practical use of these results is evaluated. These results will prove useful as electron beam reference dosimetry protocols are being updated. The analysis of this work indicates that cylindrical ion chambers may be appropriate for use in low-energy electron beams but measurements are required to characterize their use in these beams.

TH‐E‐BRE‐06: Challenges in the Dosimetry of Flattening Filter Free Beams

Purpose:In current dosimetry protocols [AAPM TG51, IAEA TRS‐389] the beam quality correction factor kQ and the water‐to‐air restricted mass collision stopping‐power ratio SPR are related to beam quality specifiers %dd(10)x respectively TPR20,10 Determining kQ and SPR using these regular beam quality specifiers for conventional accelerators (WFF) and flattening filter free accelerators (FFF) similarly could lead to systemic bias. The influence of the flattening filter on the relationship between various beam quality specifiers and SPR respectively kQ was studied using Monte Carlo simulations with realistic beam sources.Methods:All Monte Carlo simulations were performed using the BEAMnrc/EGSnrc code system. Radiation transport through nine linear accelerator heads modeled according to technical drawings given by the manufactures and a 6 0 Co therapy source was simulated with BEAMnrc and then used as a radiation source for further simulations. FFF beam sources were implemented by removing the fattening filter from the WFF model. SPR was calculated applying the user code SPRRZnrc. The mean photon energy below the accelerator head and the mean energies of photons and electrons at the measuring point within the water phantom were calculated using FLURZnrc. Dose calculations within a small water voxel and the thimble ionization chamber PTW‐31010 in a water depth of 10 cm were made using the egs_chamber code.Results:SPR and kQ as a function of fluence spectra based beam quality specifiers as well as conventional beam quality specifiers differ systematically between FFF and WFF beams. According to the results the specifier %dd(10)x revealed the smallest deviation (max. 0.4%) between FFF and WFF beams.Conclusion:The results show that %dd(10)x is an acceptable beam quality specifier for FFF beams. Nevertheless the results confirm the expected bias between FFF and WFF beams which must by further investigated.

Sci-Thur PM: YIS - 07: Monte Carlo simulations to obtain several parameters required for electron beam dosimetry.

When current dosimetry protocols were written, electron beam data were limited and had uncertainties that were unacceptable for reference dosimetry. Protocols for high-energy reference dosimetry are currently being updated leading to considerable interest in accurate electron beam data. To this end, Monte Carlo simulations using the EGSnrc user-code egs_chamber are performed to extract relevant data for reference beam dosimetry. Calculations of the absorbed dose to water and the absorbed dose to the gas in realistic ion chamber models are performed as a function of depth in water for cobalt-60 and high-energy electron beams between 4 and 22 MeV. These calculations are used to extract several of the parameters required for electron beam dosimetry - the beam quality specifier, R50 , beam quality conversion factors, kQ and kR50 , the electron quality conversion factor, k'R50 , the photon-electron conversion factor, kecal , and ion chamber perturbation factors, PQ . The method used has the advantage that many important parameters can be extracted as a function of depth instead of determination at only the reference depth as has typically been done. Results obtained here are in good agreement with measured and other calculated results. The photon-electron conversion factors obtained for a Farmer-type NE2571 and plane-parallel PTW Roos, IBA NACP-02 and Exradin A11 chambers are 0.903, 0.896, 0.894 and 0.906, respectively. These typically differ by less than 0.7% from the contentious TG-51 values but have much smaller systematic uncertainties. These results are valuable for reference dosimetry of high-energy electron beams.

Beam quality corrections for parallel-plate ion chambers in electron reference dosimetry

Current dosimetry protocols (AAPM, IAEA, IPEM, DIN) recommend parallel-plate ionization chambers for dose measurements in clinical electron beams. This study presents detailed Monte Carlo simulations of beam quality correction factors for four different types of parallel-plate chambers: NACP-02, Markus, Advanced Markus and Roos. These chambers differ in constructive details which should have notable impact on the resulting perturbation corrections, hence on the beam quality corrections. The results reveal deviations to the recommended beam quality corrections given in the IAEA TRS-398 protocol in the range of 0%–2% depending on energy and chamber type. For well-guarded chambers, these deviations could be traced back to a non-unity and energy-dependent wall perturbation correction. In the case of the guardless Markus chamber, a nearly energy-independent beam quality correction is resulting as the effects of wall and cavity perturbation compensate each other. For this chamber, the deviations to the recommended values are the largest and may exceed 2%. From calculations of type-B uncertainties including effects due to uncertainties of the underlying cross-sectional data as well as uncertainties due to the chamber material composition and chamber geometry, the overall uncertainty of calculated beam quality correction factors was estimated to be <0.7%. Due to different chamber positioning recommendations given in the national and international dosimetry protocols, an additional uncertainty in the range of 0.2%–0.6% is present. According to the IAEA TRS-398 protocol, the uncertainty in clinical electron dosimetry using parallel-plate ion chambers is 1.7%. This study may help to reduce this uncertainty significantly.

Initial recombination in the track of heavy charged particles: Numerical solution for air filled ionization chambers

Introduction. Modern particle therapy facilities enable sub-millimeter precision in dose deposition. Here, also ionization chambers (ICs) are used, which requires knowledge of the recombination effects. Up to now, recombination is corrected using phenomenological approaches for practical reasons. In this study the effect of the underlying dose distribution on columnar recombination, a quantitative model for initial recombination, is investigated. Material and methods. Jaffé's theory, formulated in 1913 quantifies initial recombination by elemental processes, providing an analytical (closed) solution. Here, we investigate the effect of the underlying charged carrier distribution around a carbon ion track. The fundamental partial differential equation, formulated by Jaffé, is solved numerically taking into account more realistic charge carrier distributions by the use of a computer program (Gascoigne 3D). The investigated charge carrier distributions are based on track structure models, which follow a 1/r2 behavior at larger radii and show a constant value at small radii. The results of the calculations are compared to the initial formulation and to data obtained in experiments using carbon ion beams. Results. The comparison between the experimental data and the calculations shows that the initial approach made by Jaffé is able to reproduce the effects of initial recombination. The amorphous track structure based charge carrier distribution does not reproduce the experimental data well. A small additional correction in the assessment of the saturation current or charge is suggested by the data. Conclusion. The established model of columnar recombination reproduces the experimental data well, whereas the extensions using track structure models do not show such an agreement. Additionally, the effect of initial recombination on the saturation curve (i.e. Jaffé plot) does not follow a linear behavior as suggested by current dosimetry protocols, therefore higher order corrections (such as the investigated ones) might be necessary.

On the wall perturbation correction for a parallel‐plate NACP‐02 chamber in clinical electron beams

In recent years, several Monte Carlo studies have been published concerning the perturbation corrections of a parallel-plate chamber in clinical electron beams. In these studies, a strong depth dependence of the relevant correction factors (p(wall) and P(cav)) for depth beyond the reference depth is recognized and it has been shown that the variation with depth is sensitive to the choice of the chamber's effective point of measurement. Recommendations concerning the positioning of parallel-plate ionization chambers in clinical electron beams are not the same for all current dosimetry protocols. The IAEA TRS-398 as well as the IPEM protocol and the German protocol DIN 6800-2 interpret the depth of measurement within the phantom as the water equivalent depth, i.e., the nonwater equivalence of the entrance window has to be accounted for by shifting the chamber by an amount deltaz. This positioning should ensure that the primary electrons traveling from the surface of the water phantom through the entrance window to the chamber's reference point sustain the same energy loss as the primary electrons in the undisturbed phantom. The objective of the present study is the determination of the shift deltaz for a NACP-02 chamber and the calculation of the resulting wall perturbation correction as a function of depth. Moreover, the contributions of the different chamber walls to the wall perturbation correction are identified. The dose and fluence within the NACP-02 chamber and a wall-less air cavity is calculated using the Monte Carlo code EGSnrc in a water phantom at different depths for different clinical electron beams. In order to determine the necessary shift to account for the nonwater equivalence of the entrance window, the chamber is shifted in steps deltaz around the depth of measurement. The optimal shift deltaz is determined from a comparison of the spectral fluence within the chamber and the bare cavity. The wall perturbation correction is calculated as the ratio between doses for the complete chamber and a wall-less air cavity. The high energy part of the fluence spectra within the chamber strongly varies even with small chamber shifts, allowing the determination of deltaz within micrometers. For the NACP-02 chamber a shift deltaz = -0.058 cm results. This value is independent of the energy of the primary electrons as well as of the depth within the phantom and it is in good agreement with the value recommended in the German dosimetry protocol. Applying this shift, the calculated wall perturbation correction as a function of depth is varying less than 1% from zero up to the half value depth R50 for electron energies in the range of 6-21 MeV. The remaining depth dependence can mainly be attributed to the scatter properties of the entrance window. When neglecting the nonwater equivalence of the entrance window, the variation of p(wall) with depth is up to 10% and more, especially for low electron energies. The variation of the wall perturbation correction for the NACP-02 chamber in clinical electron beams strongly depends on the positioning of the chamber. Applying a shift deltaz = -0.058 cm toward the focus ensures that the primary electron spectrum within the chamber bears the largest resemblance to the fluence of a wall-less cavity. Hence, the influence of the chamber walls on the perturbation correction can be separated out and the residual variation of p(wall) with depth is minimized.

Breast Density Effect on Target and Skin Doses in 40 Breast 103pd Seed Implants Studied using a Semiautomatic Tissue Segmentation

Current clinical dosimetry protocols based on the AAPM TG43 use water to model breast tissues while mammography standards propose the 50-50 model in which the average breast is made of 50% glandular and 50% adipose tissues. Recent studies show that the breast composition varies dramatically between patients depending on parameters such the age (Yaffe et al. Med Phys 2009 36). A recent work by our group (Afsharpour et al. Phys Med Biol June 2010; article in press) showed that the target and the skin doses are highly affected by the composition of the breast in low energy 103Pd permanent breast seed implants (PBSI).