Ratio Of Mass Energy Absorption Coefficients Research Articles

PHANTOM EXPERIMENTAL STUDY ON PATIENT DOSES OBTAINED FROM 320-MULTIDETECTOR-ROW COMPUTED TOMOGRAPHY IN WHOLE-BRAIN PERFUSION SCAN.

This study aimed to precisely evaluate organ dose and effective dose (E) obtained from a 320-multidetector-row computed tomography (CT) scanner in brain perfusion scans and to estimate the conversion factor (k) between E and dose length product (DLP). A total of 270 thermoluminescent dosemeters were implanted in a male anthropomorphic phantom to measure air kerma. The ratios of mass-energy absorption coefficients were used to convert air kerma into organ doses. The organ doses ranged from 0.01 to 150mGy. Doses in brain, salivary glands and red bone marrow were relatively high, and dose in eye lens reached about 110mGy. The resulting effective dose was 5.30mSv. The resulting conversion factor k=(0.0022±0.0002) mSv·(mGy·cm)-1 was not significantly different from that of 0.0021mSv·(mGy·cm)-1 reported for head CT scan in ICRP Publication 102.

On the use of the absorbed depth-dose measurements in the beam calibration of a surface electronic high-dose-rate brachytherapy unit, a Monte Carlo-based study.

To evaluate the use of the absorbed depth-dose as a surrogate of the half-value layer in the calibration of a high-dose-rate electronic brachytherapy (eBT) equipment. The effect of the manufacturing tolerances and the absorbed depth-dose measurement uncertainties in the calibration process are also addressed. The eBT system Esteya® (Elekta Brachytherapy, Veenendaal, The Netherlands) has been chosen as a proof-of-concept to illustrate the feasibility of the proposed method, using its 10mm diameter applicator. Two calibration protocols recommended by the AAPM (TG-61) and the IAEA (TRS-398) for low-energy photon beams were evaluated. The required Monte Carlo (MC) simulations were carried out using PENELOPE2014. Several MC simulations were performed modifying the flattening filter thickness and the x-ray tube potential, generating one absorbed depth-dose curve and a complete set of parameters required in the beam calibration (i.e., HVL, backscatter factor (Bw ), and mass energy-absorption coefficient ratios (µen /ρ)water,air ), for each configuration. Fits between each parameter and some absorbed dose-ratios calculated from the absorbed depth-dose curves were established. The effect of the manufacturing tolerances and the absorbed dose-ratio uncertainties over the calibration process were evaluated by propagating their values over the fitting function, comparing the overall calibration uncertainties against reference values. We proposed four scenarios of uncertainty (from 0% to 10%) in the dose-ratio determination to evaluate its effect in the calibration process. The manufacturing tolerance of the flattening filter (±0.035mm) produces a change of 1.4% in the calculated HVL and a negligible effect over the Bw , (µen /ρ)water,air , and the overall calibration uncertainty. A potential variation of 14% of the electron energies due to manufacturing tolerances in the x-ray tube (69.5±~10keV) generates a variation of 10% in the HVL. However, this change has a negligible effect over the Bw and (µen /ρ)water,air , adding 0.1% to the overall calibration uncertainty. The fitting functions reproduce the data with an uncertainty (k=2) below 1%, 0.5%, and 0.4% for the HVL, Bw , and (µen /ρ)water,air , respectively. The four studied absorbed dose-ratio uncertainty scenarios add, in the worst-case scenario, 0.2% to the overall uncertainty of the calibration process. This work shows the feasibility of using the absorbed depth-dose curve in the calibration of an eBT system with minimal loss of precision.

Data for the dosimetry of low- and medium-energy kV x rays

Following the publication of the ICRU Report 90 (2016) on key data for measurement standards in radiation dosimetry, where ionometric air-kerma standards for kilovoltage (kV) x-ray beams are estimated to change by up to about 0.5%, an update of the backscatter factors and water/air ratios of mass energy-absorption coefficients in kV dosimetry protocols was deemed necessary for consistency through the entire dosimetry chain. In addition, numerical methods and Monte Carlo (MC) systems that did not exist at the time when air-kerma protocols were developed, are currently available.Calculations of the chamber-independent quantities required for the dosimetry of low- and medium-energy kV x rays were carried out using a consistent set of key data throughout the complete process. The quantities were based on MC calculations of a database for a dense grid of monoenergetic photons for different beam diameters and source-to-surface distances, followed by an averaging procedure to compute water/air energy-absorption coefficient ratios and backscatter factors for 342 experimental and calculated kV spectra. It was found that for a given HVL and field size the variation of backscatter factors for different kVs can be up to about 5%, a trend confirmed with independent calculations that shows the limitation of using only the HVL for the beam quality specification of kV x rays. Extensive tables as a function of beam quality in terms of kV and HVL were developed for configurations that might be encountered in clinical practice; the data are also available in the form of a GUI web app at http://52.233.195.208.Results were compared with data used at PTB for deriving low-energy ion chamber calibration coefficients, finding agreement within about 0.5%, and with independent full MC kerma calculations that agreed within better than about 1%. Compared with the data in the AAPM TG-61 protocol (Ma et al 2001 Med. Phys. 28 868–93) there was in general good agreement for the ratios of mass energy-absorption coefficients, although differences of up to 1.5% resulted when both kV and HVL were taken into account; more significant discrepancies, within about 2%–6%, were obtained for backscatter factors, the present values being generally higher.

Monte Carlo simulations of different CT X-ray energy spectra within CTDI phantom and the influence of its changes on radiochromic film measurements

PurposeIn this work we use Monte Carlo simulations to investigate change in Computed tomography (CT) X-ray energy spectra between exposures in air and within CT dose index (CTDI) phantom. While the results of these simulations will be relevant when measuring CTDI with any dosimeter, we apply the appropriate beam quality change correction for CTDI measurements using XR-QA2 model GafChromic™ film. MethodsDose profiles were measured with film strips, sandwiched between acrylic rods cut in half, placed within CTDI phantoms and scanned before and after irradiation with document scanner in reflective mode. Reference dosimetry system was calibrated in terms of air kerma in air, which was converted into absorbed dose using ratio of mass-energy absorption coefficients water-to-air for a given beam quality, following the AAPM TG-61 protocol. ResultsBeam qualities for all film positions within CTDI phantom show beam softening for HVLs above 6 mm Al and beam hardening for HVLs bellow 6 mm Al. Calculated CTDI values using HVL in air for all CTDI positions, and those calculated using the appropriate calibration curves based on beam quality correction show for Head CTDI phantom differences ranging from 0.3% to 2.1% and for Body CTDI phantom from 2.5% to 5.7%. ConclusionsWe describe method for CTDI measurements using radiochromic film dosimetry protocol corrected by the beam quality change within the phantom. Our results show differences in CTDI measurements of up to 5.7% when compared to using film calibration curves for beam quality in air.

P117] Dose estimation of eye lens for pediatric inner ear X-ray diagnosis

Purpose Recently, digital X-ray tomosynthesis using flat panel detectors (FPD) has been used to diagnose the inner ears of pediatric patients. This imaging provides intermediate functionality between X-ray photographs and X-ray computer tomography (CT) scans. However, during inner ear diagnosis, digital X-ray tomosynthesis includes the eye lens in its field of view; an organ sensitive to X-ray doses. The purpose of this research is to evaluate the X-ray dosage upon the eye lens during inner ear diagnosis using digital tomosynthesis and to compare this dosage with X-ray CT exposure. Methods Small optically stimulated luminescence (OSL) dosimeters (nanodot: Nagase Landauer, Ltd.) were positioned on the eye lens of a phantom and on the bed of an X-ray TV device to measure the kinetic energy released per unit mass of air (air kerma). We performed tomosynthesis, assuming the conditions for an inner ear examination of an infant. The irradiant conditions were a tube voltage of 60–110 kV, an added filter of 0.1 mmCu, a swing angle θ of ± 20 °, and 76 photographs of a flame. The absorbed dose for the eye lens was calculated using the mass energy absorption coefficient ratio of the eye lens and air. In addition, the eye lens absorbed dose was compared with an X-ray CT scan. Results As the tube voltage increased, the absorbed dose increased. Moreover, when placed in a posterior-anterior (PA) position, as the phantom thickness increased, the absorbed dose decreased. On the contrary, when in an anterior-posterior (AP) position, as phantom thickness increased, the absorbed dose increased. Conclusions The eye lens absorbed dose in the PA position decreased by 90% compared to AP-positioning when using digital tomosynthesis for inner ear diagnosis of pediatric patients. Additionally, the eye lens absorbed dose in PA-positioned digital tomosynthesis decreased 93% compared to X-ray CT scans. Therefore, it is useful to reduce the exposure dose of the eye lens during inner ear diagnosis of pediatric patients via the use of digital tomosynthesis imaging.

On the conversion of dose to bone to dose to water in radiotherapy treatment planning systems.

Background and purposeConversion factors between dose to medium (Dm,m) and dose to water (Dw,w) provided by treatment planning systems that model the patient as water with variable electron density are currently based on stopping power ratios. In the current paper it will be illustrated that this conversion method is not correct.Materials and methodsMonte Carlo calculations were performed in a phantom consisting of a 2 cm bone layer surrounded by water. Dw,w was obtained by modelling the bone layer as water with the electron density of bone. Conversion factors between Dw,w and Dm,m were obtained and compared to stopping power ratios and ratios of mass-energy absorption coefficients in regions of electronic equilibrium and interfaces. Calculations were performed for 6 MV and 20 MV photon beams.ResultsIn the region of electronic equilibrium the stopping power ratio of water to bone (1.11) largely overestimates the conversion obtained using the Monte Carlo calculations (1.06). In that region the MC dose conversion corresponds to the ratio of mass energy absorption coefficients. Near the water to bone interface, the MC ratio cannot be determined from stopping powers or mass energy absorption coefficients.ConclusionStopping power ratios cannot be used for conversion from Dm,m to Dw,w provided by treatment planning systems that model the patient as water with variable electron density, either in regions of electronic equilibrium or near interfaces. In regions of electronic equilibrium mass energy absorption coefficient ratios should be used. Conversions at interfaces require detailed MC calculations.

Collision-kerma conversion between dose-to-tissue and dose-to-water by photon energy-fluence corrections in low-energy brachytherapy

The AAPM TG-43 brachytherapy dosimetry formalism, introduced in 1995, has become a standard for brachytherapy dosimetry worldwide; it implicitly assumes that charged-particle equilibrium (CPE) exists for the determination of absorbed dose to water at different locations, except in the vicinity of the source capsule. Subsequent dosimetry developments, based on Monte Carlo calculations or analytical solutions of transport equations, do not rely on the CPE assumption and determine directly the dose to different tissues. At the time of relating dose to tissue and dose to water, or vice versa, it is usually assumed that the photon fluence in water and in tissues are practically identical, so that the absorbed dose in the two media can be related by their ratio of mass energy-absorption coefficients. In this work, an efficient way to correlate absorbed dose to water and absorbed dose to tissue in brachytherapy calculations at clinically relevant distances for low-energy photon emitting seeds is proposed. A correction is introduced that is based on the ratio of the water-to-tissue photon energy-fluences. State-of-the art Monte Carlo calculations are used to score photon fluence differential in energy in water and in various human tissues (muscle, adipose and bone), which in all cases include a realistic modelling of low-energy brachytherapy sources in order to benchmark the formalism proposed. The energy-fluence based corrections given in this work are able to correlate absorbed dose to tissue and absorbed dose to water with an accuracy better than 0.5% in the most critical cases (e.g. bone tissue).

Experimental determination of ionization chamber overall correction factor in medium-energy X-ray beams

Introduction According to the IAEA TRS 398, in kilovoltage X-ray beams the calibration factor in terms of absorbed dose to water ( N D , w , Q ) for an ionization chamber can be derived from its calibration factor in terms of air kerma ( N K , Q ) if the chamber overall correction factor, p Q , is known. However, literature p Q values are lacking, especially when referring to the reference depth of 2 g/cm 2 . Purpose To determine the p Q factor at 2 g/cm 2 depth using a new absorbed-dose-to-water ( D w ) primary standard based on an in-water-phantom graphite calorimeter recently established for medium-energy X-ray beams. Materials and methods N D , w , Q factors are directly determined against the D w primary standard. Using the same beam qualities, the N K , Q factors are also determined. Values of p Q are obtained by comparing the N D , w , Q factors with the analogous calibration factors derived by N K , Q using the IAEA TRS 398 formalism. Ratios of mean mass-energy absorption coefficients, water to air, are taken from literature. Results For a Farmer type ionization chamber, the p Q factor was respectively 1.002 and 1.012 for the 180 kV and the 250 kV medium-energy quality of the CCRI series, with a relative combined standard uncertainty of 2%. This figure is going to be reduced to around 1% with the ongoing improvements on the new D w primary standard. Conclusion The new D w primary standard based on graphite calorimetry, although affected by a larger uncertainty compared to primary standards based on water calorimetry, allows independent measurements of p Q factors for ionization chambers currently used in radiotherapy dosimetry.

Backscatter factors and mass energy-absorption coefficient ratios for diagnostic radiology dosimetry

Backscatter factors, B, and mass energy–absorption coefficient ratios, (μen/ρ)w, air, for the determination of the surface dose in diagnostic radiology were calculated using Monte Carlo simulations. The main purpose was to extend the range of available data to qualities used in modern x-ray techniques, particularly for interventional radiology. A comprehensive database for mono-energetic photons between 4 and 150 keV and different field sizes was created for a 15 cm thick water phantom. Backscattered spectra were calculated with the PENELOPE Monte Carlo system, scoring track-length fluence differential in energy with negligible statistical uncertainty; using the Monte Carlo computed spectra, B factors and (μen/ρ)w, air were then calculated numerically for each energy. Weighted averaging procedures were subsequently used to convolve incident clinical spectra with mono-energetic data. The method was benchmarked against full Monte Carlo calculations of incident clinical spectra obtaining differences within 0.3–0.6%. The technique used enables the calculation of B and (μen/ρ)w, air for any incident spectrum without further time-consuming Monte Carlo simulations. The adequacy of the extended dosimetry data to a broader range of clinical qualities than those currently available, while keeping consistency with existing data, was confirmed through detailed comparisons. Mono-energetic and spectra-averaged values were compared with published data, including those in ICRU Report 74 and IAEA TRS-457, finding average differences of 0.6%. Results are provided in comprehensive tables appropriated for clinical use. Additional qualities can easily be calculated using a designed GUI interface in conjunction with software to generate incident photon spectra.

Determination of RW3-to-water mass-energy absorption coefficient ratio for absolute dosimetry

The measurement of absorbed dose to water in a solid-phantom may require a conversion factor because it may not be radiologically equivalent to water. One phantom developed for the use of dosimetry is a solid water, RW3 white-polystyrene material by IBA. This has a lower mass-energy absorption coefficient than water due to high bremsstrahlung yield, which affects the accuracy of absolute dosimetry measurements. In this paper, we demonstrate the calculation of mass-energy absorption coefficient ratios, relative to water, from measurements in plastic water and RW3 with an Elekta Synergy linear accelerator (6 and 10 MV photon beams) as well as Monte Carlo modeling in BEAMnrc and DOSXYZnrc. From this, the solid-phantom-to-water correction factor was determined for plastic water and RW3.

Monte Carlo calculated absorbed‐dose energy dependence of EBT and EBT2 film

The absorbed-dose energy dependence of GAFCHROMIC EBT and EBT2 film irradiated in photon beams is studied to understand the shape of the curves and the physics behind them. The absorbed-dose energy dependence is calculated using the EGSnrc-based EGS_chamber and DOSRZnrc codes by calculating the ratio of dose to water to dose to active film layers at photon energies ranging from 3 keV to 18 MeV. These data are compared to the mass energy absorption coefficient ratios and the restricted stopping power ratios of water to active film materials as well as to previous experimental results. In the photon energy range of 100 keV to 18 MeV the absorbed-dose energy dependence is found to be energy independent within +/- 0.6%. However, below 100 keV, the absorbed-dose energy dependence of EBT varies by approximately 10% due to changes in mass energy absorption coefficient ratios of water to film materials, as well as an increase in the number of electrons being created and scattered in the central surface layer of the film. Results are found to disagree with previous experimental studies suggesting the possibility of an intrinsic energy dependence at lower photon energies. For EBT2 film the absorbed-dose energy dependence at low photon energies varies by 50% or 10% depending on the manufacturing lot due to changes in the ratio of mass energy absorption coefficients of the active emulsion layers to water. Caution is recommended when using GAFCHROMIC EBT/EBT2 films at photon energies below 100 keV. It is recommended that the effective atomic number of future films be produced as close to that of water and that thicker active layers are advantageous.

Reference radiochromic film dosimetry in kilovoltage photon beams during CBCT image acquisition

A common approach for dose assessment during cone beam computed tomography (CBCT) acquisition is to use thermoluminescent detectors for skin dose measurements (on patients or phantoms) or ionization chamber (in phantoms) for body dose measurements. However, the benefits of a daily CBCT image acquisition such as margin reduction in planning target volume and the image quality must be weighted against the extra dose received during CBCT acquisitions. The authors describe a two-dimensional reference dosimetry technique for measuring dose from CBCT scans using the on-board imaging system on a Varian Clinac-iX linear accelerator that employs the XR-QA radiochromic film model, specifically designed for dose measurements at low energy photons. The CBCT dose measurements were performed for three different body regions (head and neck, pelvis, and thorax) using humanoid Rando phantom. The authors report on both surface dose and dose profiles measurements during clinical CBCT procedures carried out on a humanoid Rando phantom. Our measurements show that the surface doses per CBCT scan can range anywhere between 0.1 and 4.7 cGy, with the lowest surface dose observed in the head and neck region, while the highest surface dose was observed for the Pelvis spot light CBCT protocol in the pelvic region, on the posterior side of the Rando phantom. The authors also present results of the uncertainty analysis of our XR-QA radiochromic film dosimetry system. Radiochromic film dosimetry protocol described in this work was used to perform dose measurements during CBCT acquisitions with the one-sigma dose measurement uncertainty of up to 3% for doses above 1 cGy. Our protocol is based on film exposure calibration in terms of "air kerma in air," which simplifies both the calibration procedure and reference dosimetry measurements. The results from a full Monte Carlo investigation of the dose conversion of measured XR-QA film dose at the surface into dose to water (or water kerma) at the surface of the phantom indicate that, for typical beam qualities used in CBCT, this conversion can be approximated by simple mass-energy absorption coefficient ratios water-to-air.

Investigation of the Air Kerma Response of Spherical Ionization Chambers for Unfolded Pulse Height Distributions of 60Co and 137Cs using the EGS4 Monte Carlo Code

Data required for the determination of the absolute air kerma rate for (60)Co and (137)Cs gamma-rays using spherical cavity chambers were calculated using the EGS4 Monte Carlo system. Mass energy-absorption coefficient ratio and the stopping power ratio were calculated for a 10 cm(3) primary standard graphite-walled ionization chamber from the unfolded energy pulse height distributions of (60)Co and (137)Cs sources. Wall correction factors and non-uniformity correction factors for two graphite and one air equivalent plastic walled ionization chambers were also calculated with EGS4 code. The wall correction factors were compared with those determined by an experimental extrapolation method. To check the accuracy of the calculations the results were compared with those obtained from other primary standard laboratories such as NIST and NRCC. For a 10 cm(3) graphite ionization chamber, the mass energy-absorption coefficient ratios were 0.99917 for (60)Co and 1.0004 for (137)Cs. The values differed by 0.02-0.05 % for (60)Co and 0.11 % for (137)Cs from those of two laboratories. The stopping power ratios were 0.99984 for (60)Co and 1.0087 for (137)Cs. Comparison with NIST values showed differences of 0.06 % for (60)Co and 0.04 % for (137)Cs. The wall correction factors were obtained and they were different by 0.6-1.1 % for (60)Co and (137)Cs compared to the experimental linear extrapolation method. These values were compared with Monte Carlo derived values from other laboratories. The non-uniformity correction factors were also calculated and they differed from unity, the traditional value used in most standard national metrology laboratories.

An assessment of bone marrow and bone endosteum dosimetry methods for photon sources

The rather complex and microscopic histological structure of the skeletal system generally limits one's ability to accurately model this tissue during dosimetric evaluations. Consequently, various assumptions must be made to evaluate the absorbed dose from external and internal photons to the radiosensitive tissues of the red (or haematopoietically active) bone marrow and the osteogenic tissues of the skeletal endosteum. These various methods for photon skeletal dosimetry have not been inter-compared, partly due to the lack of a realistic reference model that can provide a high-resolution three-dimensional geometry for secondary electron particle transport. In the present study, the paired-image radiation transport (PIRT) model developed by Shah et al (2005 J. Nucl. Med. 45 344) was utilized to evaluate the absorbed dose per incident photon fluence to these skeletal regions from idealized parallel beams of monoenergetic photons. The PIRT model results were then used as a local reference against which absorbed doses via other methods were compared. For red bone marrow dosimetry, four approximate techniques were considered: (1) the dose response function method (DRF method) presented in ORNL/TM-8381, (2) the mass–energy absorption coefficient ratio method (two-parameter MEAC method), (3) the MEAC method with the additional use of energy-dependent dose enhancement factors from King and Spiers (1985 Br. J. Radiol. 58 345) (three-parameter MEAC method), and (4) the three-parameter MEAC method applied at the voxel level through the use image-specific CT numbers (CTN method). For the bone endosteum (i.e., bone surfaces), two approximate techniques were compared: (1) the DRF method for bone surfaces and (2) the homogeneous bone approximation (HBA) method. In each case, the local reference standard was assumed to be that of the PIRT model. Four different ex vivo bone specimens with distinctively different internal structures were used in the study: the cranium, the lumbar vertebra, the os coxae and the left middle rib, each excised from a 66 year male cadaver (body mass index, 22.7 kg m−2). High-resolution CT images of these skeletal sites were used to construct computational voxel models for Monte Carlo radiation transport. Study results indicated that skeletal sites with thick cortical regions and thick trabeculae such as in the cranium provide considerable beam attenuation at low photon energies, which is not properly accounted for in methods based on a homogeneous skeletal tissue structure (DRF, MEAC, HBA). For bone marrow dose assessment, the CTN method showed the best agreement with PIRT model results over a broad range of photon energies, while the HBA method showed better agreement with the PIRT model in assessing bone endosteum dose at energies above 100 keV. Bone surface doses were better approximately by the DRF method at energies below 50 keV. Considerable secondary electron escape at photon energies over 1–3 MeV were accounted for in RBM dose assessment only in the PIRT model, as the other methods presume either an infinite expanse of spongiosa (DRF) or the existence of charge-particle equilibrium (MEAC, CTN).

TU‐E‐I‐611‐04: Bone Marrow and Bone Endosteum Dosimetry Methods Comparison for External Photons

Purpose: To compare and verify different bone endosteum and bone marrow photon dosimetry methods through the use of a high‐resolution micro CT‐based radiation transport model. Method and Materials: Two different Monte Carlo dosimetry algorithms for bone endosteum and bone marrow were compared to a high‐resolution microCT image‐based radiation transport model developed in our laboratory. Bone marrow dosimetry methods are (1) the dose response function method by Eckerman (DRF), and (2) the mass energy absorption coefficient ratio method (MEAC). Bone endosteum dosimetry methods include (1) DRF method and (2) homogeneous bone dose approximation (HGB). Each method was compared to results obtained from the microCT‐based paired‐image radiation transport (PIRT) model. Two ex‐vivo bone samples of a 66‐year male (lumbar vertebrae and cranium) were chosen for the comparison because of their distinctively different microstructures. Simple mono‐energetic parallel photon beams were simulated from 0.01 to 4.0 MeV. Results: For the bone marrow dose, the DRF method shows good agreement with PIRT result in the lumbar vertebra, but showed over estimates of bone marrow dose in the cranium, while the MEAC method shows good agreement with PIRT in both bone sites. For the bone endosteum dose, the DRF method shows closer results to the PIRT model at lower energies, but shows significant over‐ estimates of endosteum dose in higher photon energies. This can be explained by the fact the Eckerman model assumes that secondary electrons are followed through an infinite expanse of trabecular spongiosa, with no loss of energy to the bone cortex at high energy. Conclusion: For the bone marrow dose assessment, the MEAC method seems to be the best choice among the methods considered, while for the bone endosteum dose, the HGB method shows better agreement with PIRT than is seen with the DRF method, especially at higher photon energies.

Determination of the reference air kerma rate for brachytherapy sources and the related uncertainty

Different methods exist to determine the air kerma calibration factor of an ionization chamber for the spectrum of a 192Ir high-dose-rate (HDR) or pulsed-dose-rate (PDR) source. An analysis of two methods to obtain such a calibration factor was performed: (i) the method recommended by [Goetsch et al., Med. Phys. 18, 462-467 (1991)] and (ii) the method employed by the Dutch national standards institute NMi [Petersen et al., Report S-EI-94.01 (NMi, Delft, The Netherlands, 1994)]. This analysis showed a systematic difference on the order of 1% in the determination of the strength of 192Ir HDR and PDR sources depending on the method used for determining the air kerma calibration factor. The definitive significance of the difference between these methods can only be addressed after performing an accurate analysis of the associated uncertainties. For an NE 2561 (or equivalent) ionization chamber and an in-air jig, a typical uncertainty budget of 0.94% was found with the NMi method. The largest contribution in the type-B uncertainty is the uncertainty in the air kerma calibration factor for isotope i, N(i)k, as determined by the primary or secondary standards laboratories. This uncertainty is dominated by the uncertainties in the physical constants for the average mass-energy absorption coefficient ratio and the stopping power ratios. This means that it is not foreseeable that the standards laboratories can decrease the uncertainty in the air kerma calibration factors for ionization chambers in the short term. When the results of the determination of the 192Ir reference air kerma rates in, e.g., different institutes are compared, the uncertainties in the physical constants are the same. To compare the applied techniques, the ratio of the results can be judged by leaving out the uncertainties due to these physical constants. In that case an uncertainty budget of 0.40% (coverage factor=2) should be taken into account. Due to the differences in approach between the method used by NMi and the method recommended by Goetsch et al., an extra type-B uncertainty of 0.9% (k= 1) has to be taken into account when the method of Goetsch et al. is applied. Compared to the uncertainty of 1% (k= 2) found for the air calibration of 192Ir, the difference of 0.9% found is significant.

Response of LiF-TLD micro-rods around 125I radioactive seed

The EGSnrc Monte Carlo system has been used to calculate the energy response of LiF-TLDs of different sizes around 125I permanent brachytherapy sources. The source model includes the effects of an encapsulation, self-absorption within the source and in the welded ends of the encapsulation. The LiF-TLD material has cylindrical geometry (micro-rod) with diameters ranging from 1 mm to 5 mm and a length of 6 mm. The energy response factor (relative to 60Co γ-rays) for a LiF-TLD calibrated in 60Co γ-rays and then irradiated by an 125I permanent brachytherapy source varies between 1.32 ± 0.2% (1 SD) and 1.406 ± 0.2% (1 SD) for 5 mm and 1 mm diameter micro-rods, respectively. The energy response factor depends on the radius and the polar angle (r, θ) of the measurement point. For a LiF-TLD of diameter 1 mm calibrated at 1 cm on the transverse axis (r = 1.0 cm, θ = 90°) of the 125I source in water, the energy response factor decreases by a maximum of 3.5% within the 6 cm × 6 cm × 6 cm calculation region. For the 5 mm diameter LiF-TLD, the energy response factor decreases by a maximum of 5% in the same region. An examination of the photon energy spectra showed that the photon spectrum does not change significantly in water within the 3D calculation region (6 cm × 6 cm × 6 cm). The mass energy absorption coefficient ratio of water to LiF-TLDs does not vary by more than 0.5% in this calculation grid. The results, however, show that there is a change in the photon spectrum with distance from the source and with polar angle for LiF-TLDs. This difference in the energy spectrum gives rise to a difference in the mass energy absorption coefficient ratio of water to LiF (calculated by taking into account the difference in photon fluence in water and LiF) and that calculated assuming that the photon spectrum in water and in the LiF-TLD is identical.

An investigation of entrance surface dose calculations for diagnostic radiology using Monte Carlo simulations and radiotherapy dosimetry formalisms

Our aim in this work was to investigate the methodology used in the determination of the entrance surface dose (ESD) in diagnostic radiology. In kV x-rays for low-energy photons (tube potential up to 160 kV, HVL: 1–8 mm Al), the ESD is based on the use of the ratio of mass-energy absorption coefficients and backscatter factors. A full simulation of the photon and electron transport in a kilovoltage x-ray unit, using the Monte Carlo code BEAM/EGS4, was performed to obtain an accurate beam phase space for use in dose calculation. The modelled phase space was experimentally validated for the beam qualities (measured HVL: 3.3 mm Al–2.2 mm Cu) and showed good agreement between calculated and measured HVLs, air kerma and relative dose distributions. We have computed the conversion factors from air kerma to water or soft tissue absorbed dose at the surface of a phantom for beam qualities (HVL: 3.3–8.35 mm Al). The same model was also used to calculate the ESD in water and in soft tissue for the low-energy photon range considered. The results show that the numerical differences between the air kerma and the water kerma based backscatter factors are insignificant. The same conclusion was reached for the (μen/ρ) ratios, for soft tissue to air, evaluated using either the primary photon spectra or the spectra at the surface of a phantom. Furthermore, the good agreement obtained for the computation of the conversion factors with a full BEAM/EGS4 model confirms the previous studies which are based on different sources for the spectral distribution and different beam geometries (pencil beam or point source assumptions). On the other hand, the ESD in water or soft tissue is well described either with the Bair or the Bw formalism. Conversion factors from air kerma to ESD in these media are proposed in this work for several beam qualities in diagnostic radiology.

Determination of absorbed dose to water in reference conditions for radiotherapy kilovoltage x-rays between 10 and 300 kV: a comparison of the data in the IAEA, IPEMB, DIN and NCS dosimetry protocols

A comparison of four of the most commonly used dosimetry protocols for the determination of absorbed dose to water in therapeutic kilovoltage x-rays using an ionization chamber (IAEA TRS-277, IPEMB, DIN and NCS) has been carried out. Owing to the different energy ranges and HVLs recommended by each protocol, backscatter factors, water-to-air mass energy absorption coefficient ratios and perturbation correction factors have been recast to a common quality range that all protocols satisfy individually to make a comparison possible. The results of the comparison show that in the sometimes reduced quality range originally included by the different protocols, determinations of absorbed dose to water at all beam qualities agree to within ±1.0% with that obtained using the second edition of the IAEA TRS-277 code of practice (1997). The extrapolation of data to a common beam quality range practically preserves the agreement for all the protocols except for that issued by the NCS at the extremes of the range, where differences of up to 1.8% and 1.4% have been found for low and medium energies respectively. In all cases the DIN protocol yields very good agreement with TRS-277.

Photon fluence perturbation correction factors for solid state detectors irradiated in kilovoltage photon beams

Dose perturbation correction factors, (p), for LiF, CaF2 and Li2B4O7 solid state detectors have been determined using the EGS4 Monte Carlo code. Each detector was simulated in the form of a disc of diameter 3.61 mm and thickness 1 mm irradiated in a clinical kilovoltage photon beam at a depth of 1 cm in a water phantom. The perturbation correction factor (p) is defined as the deviation of the absorbed dose ratio from the average mass energy absorption coefficient ratio of water to the detector material, (en/)med,det, which is evaluated assuming that the photon fluence spectrum in the medium and in the detector material are identical. We define another mass energy absorption coefficient ratio, (en/)med,det, which is evaluated using the actual photon fluence spectrum in the medium and detector for LiF and CaF2 rather than assuming they are identical. (en/)med,det predicts the average absorbed dose ratio of the medium to the detector material within 0.3%. When the difference in atomic number between the cavity and the phantom material is large then their photon fluence spectra will differ substantially resulting in a difference between (en/)med,det and (en/)med,det. The value of (p) calculated using (en/)med,det is up to 27% greater than unity for a cavity of CaF2 in 50 kV x-rays. When the atomic number of the medium and detector are similar, their photon fluence spectra are similar, and the difference between (en/)med,det and (en/)med,det is small. For instance their difference for LiF is less than 2%. The average mass energy absorption coefficient ratio, (en()/)w,LiF, evaluated using the mean or representative energy, , is up to 8% different from (en/)w,LiF. For calcium fluoride the difference between (en/)w,CaF2 and (en()/)w,CaF2 is up to 42% in the energy range studied.