Kerma Approximation Research Articles

Personal dose equivalent conversion coefficients for photons to 1 GeV

The personal dose equivalent, H(p)(d), is the quantity recommended by the International Commission on Radiation Units and Measurements (ICRU) to be used as an approximation of the protection quantity effective dose when performing personal dosemeter calibrations. The personal dose equivalent can be defined for any location and depth within the body. Typically, the location of interest is the trunk, where personal dosemeters are usually worn, and in this instance a suitable approximation is a 30 × 30 × 15 cm(3) slab-type phantom. For this condition, the personal dose equivalent is denoted as H(p,slab)(d) and the depths, d, are taken to be 0.007 cm for non-penetrating and 1 cm for penetrating radiation. In operational radiation protection a third depth, 0.3 cm, is used to approximate the dose to the lens of the eye. A number of conversion coefficients for photons are available for incident energies up to several megaelectronvolts, however, data to higher energies are limited. In this work, conversion coefficients up to 1 GeV have been calculated for H(p,slab)(10) and H(p,slab)(3) both by using the kerma approximation and tracking secondary charged particles. For H(p)(0.07), the conversion coefficients were calculated, but only to 10 MeV due to computational limitations. Additionally, conversions from air kerma to H(p,slab)(d) have been determined and are reported. The conversion coefficients were determined for discrete incident energies, but analytical fits of the coefficients over the energy range are provided. Since the inclusion of air can influence the production of secondary charged particles incident on the face of the phantom, conversion coefficients have been determined both in vacuo and with the source and slab immersed within a sphere in air. The conversion coefficients for the personal dose equivalent are compared with the appropriate protection quantity, calculated according to the recommendations of the latest International Commission on Radiological Protection (ICRP) guidance.

Validation for photon beams in the energy range of 20–1000 keV

In high energy teletherapy, VMC++ is known to be a very accurate and efficient Monte Carlo (MC) code. In principle, the MC method is also a powerful dose calculation tool in other areas in radiation oncology, e.g., brachytherapy or orthovoltage radiotherapy. However, VMC++ is not validated for the low-energy range of such applications. This work aims in the validation of the VMC++ MC code for photon beams in the energy range between 20 and 1000 keV. Dose calculations were performed in different 40 x 40 x 40 cm3 phantoms of different materials. Dose distributions of monoenergetic (ranging from 20 to 1000 keV) 10 x 10 and 2 x 2 cm2 parallel beams were calculated. Voxel sizes of 4 x 4 x 4 and 1 x 1 x 1 mm3 were used for the dose calculations. The resulting dose distributions were compared to those calculated using EGSnrc, which is used as a golden standard in this work. At energies between 100 and 1000 keV, EGSnrc and VMC++ calculated dose distributions agree within the statistical uncertainty of about 1% (1sigma). At energies < or = 50 keV, dose differences of up to 1.6% (in % of D(max)) occur when VMC++ and EGSnrc are compared. Turning off Rayleigh scattering, binding effects for Compton scattering, and the atomic relaxation after photoelectric absorption in EGSnrc (all not implemented in VMC++) leads to an agreement between both MC codes within statistical uncertainty. Further, using the KERMA approximation feature implemented in VMC++ leads to very efficient simulations in the energy range between 20 and 1000 keV. Further improvements for very low energies in accuracy of VMC++ could be achieved by implementing Rayleigh scattering, binding effects for Compton scattering, and the atomic relaxation after photoelectric absorption. Implementation into VMC++ of KERMA approximation has been validated.

SU-GG-I-64: MicroCT-Based Methods of Assessing Imaging Dose to Active Marrow and Endosteum in CT, Fluoroscopy, and Nuclear Medicine

Purpose: To present newly developed microCT-based methods for assessing radiation dose to radiosensitive tissues of the skeleton within computational patient phantoms for CT, fluoroscopy, and nuclear medicine imaging. Methods and Materials: In this study, 13 skeletal sites were harvested from a 40-year male and a 45-year female cadaver from which 32 samples of trabecular spongiosa were cored from each and imaged under microCT. These images were thresholded and coupled to voxelized models of skeletal sites taken from the UF adult hybrid phantoms. EGSnrc-based Paired Image Radiation Transport was then performed for a series of monogenetic electrons localized in various spongiosa and cortical source tissues. The resultant electron absorbed fractions were used to assess (1) radionuclide S values for nuclear medicine dosimetry, and (2) bone-specific fluence-to-dose response functions for CT and fluoroscopic x-ray dosimetry. Results: Radionuclide S values presented in this study uniquely account for the trabecular microstructure of adult bone, account for the finite size and shape of trabecular spongiosa, and allow for electron cross-fire from particle emissions within the cortical regions of mineral bone. For assessment of bone marrow and endosteum dose during CT imaging and fluoroscopic procedures, a detailed library of photon fluence-to-dose response functions is presented for assessing both regional and skeletal average dose to active marrow and endosteum. Conclusions: Calculations of secondary electron transport in the microstructure of trabecular spongiosa of the adult skeleton reveal an enhancement of dose due to secondary electron disequilibrium for x-rays entering the skeleton at energies below 200 keV. Percent excess dose to active marrow over that predicted under the kerma approximation range from 1–5% in bones such as the pelvis to a high of 25% in the cranium. Percent excess dose to the skeletal endosteum can approach 100% in some bone sites over that predicted by the kerma approximation.

Critical Discretization Issues in 3-D SN Simulations Relevant to Dosimetry and Medical Physics

Dosimetry problems inherently involve dose determinations among widely varying materials and densities, and may require complex, detailed investigations of the angular, spatial, and energy behavior of the applied radiation transporting throughout the simulation geometry. Traditionally, Monte Carlo codes have been implemented in solving these types of problems using voxelized geometries and phantoms. The motivation of this work is to investigate the discretization requirements for deterministic radiation transport simulations for these problems via direct solutions of the linear Boltzmann transport equation, focusing on the discrete ordinates (SN) method. The SN method can yield accurate global solutions, provided the inherent discretizations among the angular, spatial, and energy domains properly represent problem physics. In this paper, the SN approach is implemented using a three-dimensional (3-D) 60Co photon transport simulation to highlight the critical issues encountered in performing deterministic photon simulations in dosimetry problems. Calculations were performed using the PENTRAN parallel SN code to obtain a 3-D distribution of flux and dose computed using a collisional kerma approximation. For an acceptable result, we determined that a minimum angular Legendre-Chebychev quadrature of S32 with P3 anisotropy is required, with block-adaptive meshes on the order of 1 cm, even in air regions, implemented with an adaptive differencing scheme (implemented in the PENTRAN code) to yield optimal solution convergence. Also, photon cross-section libraries should be carefully evaluated for the problem studied; for our test problem, the BUGLE-96 photon library yielded the closest results to Monte Carlo (MCNP5) among those tested. Overall, this work details the levels of discretization involved in performing deterministic computations in dosimetry problems and will be useful in enabling future efforts to perform rapid deterministic computations of phantom doses.

Dosimetry Parameters of the IRH10 192Ir High Dose Rate Brachytherapy Source

The dosimetry parameters of the IRH10 (192)Ir high dose rate brachytherapy source were obtained from the dose calculation formalism recommended in the AAPM Task Group No. 43 report using the Monte Carlo code PENELOPE. The absorbed doses to water and air originating from the photons of the IRH10 (192)Ir brachytherapy source were calculated by the collision kerma approximation. The dose rate constant was evaluated to be (1.110 +/- 0.011) cGy/h U(-1). The dose rate per unit air kerma strength around the (192)Ir IRH10 brachytherapy source and the anisotropy function were given in tables and figures.

Personal dose equivalent conversion coefficients for neutron fluence over the energy range of 20-250 MeV

Monte Carlo simulations were performed to extend existing neutron personal dose equivalent fluence-to-dose conversion coefficients to an energy of 250 MeV. Presently, conversion coefficients, H(p,slab)(10,alpha)/Phi, are given by ICRP-74 and ICRU-57 for a range of angles of radiation incidence (alpha = 0, 15, 30, 45, 60 and 75 degrees ) in the energy range from thermal to 20 MeV. Standard practice has been to base operational dose quantity calculations <20 MeV on the kerma approximation, which assumes that charged particle secondaries are locally deposited, or at least that charged particle equilibrium exists within the tally cell volume. However, with increasing neutron energy the kerma approximation may no longer be valid for some energetic secondaries such as protons. The Los Alamos Monte Carlo radiation transport code MCNPX was used for all absorbed dose calculations. Transport models and collision-based energy deposition tallies were used for neutron energies >20 MeV. Both light and heavy ions (HIs) (carbon, nitrogen and oxygen recoil nuclei) were transported down to a lower energy limit (1 keV for light ions and 5 MeV for HIs). Track energy below the limit was assumed to be locally deposited. For neutron tracks <20 MeV, kerma factors were used to obtain absorbed dose. Results are presented for a discrete set of angles of incidence on an ICRU tissue slab phantom.

WE-D-BRB-02: Comparing a Grid-Based Boltzmann Solver with Monte Carlo Simulation for Voxel-Based Therapeutic Radionuclide Dose Calculations

Purpose: To compare the accuracy and speed of a deterministic grid‐based Boltzmann solver (GBBS) with Monte Carlo(MC) simulations for calculating voxel‐based absorbed dose rates from SPECT/CT imaging.Methods: A SPECT/CT image of a breast cancer patient with metastatic osteosarcoma was obtained using a tracer administration of 153 Sm EDTMP. The therapeutic activity administered was determined from MIRD estimates. DOSXYZnrc and the GBBS Acuros were used to calculate dose rate maps from the activity distribution over the full CT grid. The GBBS photondose rate was calculated using the collisional KERMA approximation rather than explicitly transporting the generated electrons; an energy cutoff was used to neglect spatial transport of electrons below a threshold of 200 keV for Acuros and 189 keV for DOSXYZnrc. The photon, beta‐particle, and total absorbed dose rates calculated using GBBS were compared with the gold standard MC simulations using the gamma index. Gamma index parameters evaluated were 3%/3mm and 5%/5mm; both used a step size of 0.5mm with a 10 mm search distance. A patient mask was created from the CT to report pixels within/near the patient. The gamma failure points (γ>1.0) within the patient mask were viewed overlaid on the activity map and on the CT for the calculated beta, photon, and total dose rates. Results: For total dose rate, 90.1% and 99.6% of pixels within the patient mask had γ⩽1.0 for 3%/3mm and 5%/5mm respectively. γ failures were noticed at the edges of the activity distribution for beta‐particles, and at various interfaces for the photons. For Acuros, the beta‐particle and photon transport required about 10 minutes each. DOSXYZnrc took approximately 15,000 times longer. Conclusions: GBBS has the potential to provide accuracy similar to MC in a much shorter time; this could be useful for voxel‐based radionuclide absorbed dose estimates in a clinical setting.

The definition of low doses and their importance in radiobiology

The aim of this work is to introduce a new dosimetric approach that is expected to contribute to characterising and distinguishing low, intermediate and high dose ranges. The emphasis will be on the low dose range, owing to the importance of the biological effects at low doses. A Monte Carlo code was used to simulate the energy degradation of photons in water, where the coherent and incoherent scattering were sampled from the Thompson and Klein-Nishina cross-sections corrected by the form factor and the incoherent scattering function, respectively. For the photoelectric effect, the kerma approximation was used, meaning that the simulation of the interactions of photons provides the initial spectra of the secondary electrons that cause damages at the cellular and intracellular levels. A new scoring method was proposed that allowed multiscale analysis of the energy deposited by radiation, inspired by the basic definition of an absorbed dose. Data for analysis were obtained from the simulation of a photon beam with extremely low dose values in a water phantom. Implications of the results in radiobiological studies are discussed.

NORMAN-05 conversion coefficients for monoenergetic neutrons below 20 MeV

In recent years voxel models have been used to investigate the validity of radiation protection quantities evaluated, till now, employing stylized reference models. An ICRP task group (DOCAL) is developing a male and female reference voxel models to estimate a new set of radiation protection quantities. It should be taken into account that, due the intrinsic subject-dependent anatomical differences, the various developed voxel models can imply a certain variability in the corresponding sets of estimated conversion coefficients. Therefore a qualitative evaluation of the range of such discrepancies should be of interest in terms of the sensitivity of organ absorbed dose to the specific model used for its computation. The NORMAN-05 model, updated at ENEA-Radiation Protection Institute on the basis of the original NORMAN model, developed by P. Dimbylow at NRPB-UK (now HPA) in the 1990s, is a good representation of the standard Caucasian-type man and could be a valuable tool for comparison with other voxel models in order to state the inherent uncertainties associated to the estimated sets of conversion coefficients. The present paper summarizes the results obtained with NORMAN-05 and the Monte Carlo code MCNPX for neutron conversion coefficients calculation, in the energy domain from thermal to 20 MeV, in the kerma approximation that, for such energy range, as stated in the ICRU/ICRP recommendations, is a satisfactory approach.

The MCNP-4C2 design of a two element photon/electron dosemeter that uses magnesium/copper/phosphorus doped lithium fluoride

The Health Protection Agency is changing from using detectors made from 7LiF:Mg,Ti in its photon/electron personal dosemeters, to 7LiF:Mg,Cu,P. Specifically, the Harshaw TLD-700H card is to be adopted. As a consequence of this change, the dosemeter holder is also being modified not only to accommodate the shape of the new card, but also to optimize the photon and electron response characteristics of the device. This redesign process was achieved using MCNP-4C2 and the kerma approximation, electron range/energy tables with additional electron transport calculations, and experimental validation, with different potential filters compared; the optimum filter studied was a polytetrafluoroethylene disc of diameter 18 mm and thickness 4.3 mm. Calculated relative response characteristics at different angles of incidence and energies between 16 and 6174 keV are presented for this new dosemeter configuration and compared with measured type-test results. A new estimate for the energy-dependent relative light conversion efficiency appropriate to the 7LiF:Mg,Cu,P was also derived for determining the correct dosemeter response.

Fluence to organ dose conversion coefficients calculated with the voxel model NORMAN-05 and the MCNPX Monte Carlo code for external monoenergetic photons from 20 keV to 100 MeV

Photons conversion coefficients from 20 keV to 100 MeV have been calculated with the voxel model NORMAN-05 using the MCNPX code. Both kerma approximation and electronic transport were employed and the results compared with published data. In the near future, ICRP group DOCAL will issue a new computational model, based on the GSF-GOLEM, which is intended to be the reference adult male voxel model for ICRP. NORMAN-05 well approximates the western-caucasian standard man characteristics, in terms of body height (176 cm) and mass (73 kg) and masses of the organs. It is not intended to substitute, or to be an alternative, to the future official ICRP voxel model, but thanks to its accuracy and its "standard man structure" can be useful to evaluate the intrinsic uncertainties associated with the dose quantities evaluated adopting different voxel models. For such reason data obtained with NORMAN-05 could be easily compared with those that will be derived from the future ICRP model.

SU-FF-T-272: Low-Energy Photon Dose Calculations Using Various MCNP Photon Databases: Dose Comparison Between MCNP, PENELOPE, and EGSnrc

Purpose: Recent releases of the MCNP5 and PENELOPE (2001) Monte Carlo codes use photo‐atomic data from the most updated EPDL97 library, and include Doppler broadening in Compton scattering processes. Such advances in photondatabases and transport algorithm are particularly important in low‐energy photon dose calculations. Method and Materials: We computed radial dose distributions for r = 0.2–10 cm from a point source in a 50 cm‐diameter sphere of water. Nine discrete energies for primary photon sources were chosen in the range of 10–150 keV. To isolate the effects of updated photoelectric data, Doppler broadening, bound electron model, and kerma approximation, we varied transport parameters and photondatabases in the same simulation geometry using MCNP5 and MCNP4 with LIB04, LIB03, or LIB02 photondatabases, PENELOPE, and EGSnrc. Results: The results from MCNP5/LIB04 agreed with those of PENELOPE within statistical uncertainties (±1%) over the entire ranges of energies and radial distances investigated. They also agree well with EGSnrc data within about ±2% (except for doses at 10 keV). MCNP/LIB02 or LIB03, on the other hand, produced doses up to 8% lower in the range of 20–80 keV than MCNP/LIB04 and PENELOPE. Such differences stem primarily from the differences in the photoelectric data used. The dosimetric effects of Doppler broadening and bound electron model for Compton interactions appear to be insignificant in the energy range investigated. Kerma calculated by photon only transport well approximates absorbed dose in water within statistical uncertainties (±1%) except for primary photons of E = 10 keV. Conclusion: Low‐energy photon dose and kerma in water calculated by MCNP5 and MCNP4 with the updated photondatabase (LIB04) are comparable to doses by PENELOPE and EGSnrc.

Calculation of conversion coefficients for clinical photon spectra using the MCNP code.

In this work, the MCNP4B code has been employed to calculate conversion coefficients from air kerma to the ambient dose equivalent, H*(10)/Ka, for monoenergetic photon energies from 10 keV to 50 MeV, assuming the kerma approximation. Also estimated are the H*(10)/Ka for photon beams produced by linear accelerators, such as Clinac-4 and Clinac-2500, after transmission through primary barriers of radiotherapy treatment rooms. The results for the conversion coefficients for monoenergetic photon energies, with statistical uncertainty <2%, are compared with those in ICRP publication 74 and good agreements were obtained. The conversion coefficients calculated for real clinic spectra transmitted through walls of concrete of 1, 1.5 and 2 m thick, are in the range of 1.06-1.12 Sv Gy(-1).

Dose Equivalent Per Unit Fluence near the Surface of the ICRU Phantom by Including the Secondary Electron Transport for Photons

The dose equivalent for photon irradiation should be calculated by depositions of the secondary electrons because a photon cannot directly transfer its energy to a medium. The transport of the secondary electron is usually ignored by the kerma approximation under an assumption of secondary electron equilibrium. Ferrari and Pelliccioni calculated conversion coefficients at a depth of 10 mm in the ICRU sphere with consideration of the secondary electron transport. They showed that the conversion coefficients recommended by the ICRP were overestimated due to the kerma approximation. The electron equilibrium becomes more incomplete at depths closer to the surface. This paper considers that overestimation by assuming the kerma approximation exists not only at a depth of 10 mm but also near the surface of a phantom. Distributions of conversion coefficients in the ICRU sphere and the ICRU slab were recalculated by including the secondary electron transport and the kerma approximation. Conversion coefficients with no electron transport were overestimated at photon energies higher than 1 and 0.2 MeV at depths of 3 and 0.07 mm, respectively.

Organ Doses and Dose Equivalents for Neutrons above 20 MeV

MCNP Monte Carlo calculations with an extended cross section data set have been performed for an anthropomorphic MIRD (ADAM) phantom in the neutron energy range between 20 and 100 MeV. The irradiation conditions vary from 0° (frontal incidence) up to 75° to the left side of the MIRD phantom. From the fluence calculations, organ doses and dose equivalents are derived using the kerma approximation. The quality factor as function of neutron energy for the calculation of effective dose equivalent, H E , is derived from initial charged particle spectra produced in an ICRU tissue element by primary neutrons. Above 20 MeV an approximation was applied. Resulting organ doses and dose equivalents are compared with a data set published by Nabelssi and Hertel (1993).

Ambient Dose Equivalent Conversion Factors for High Energy Neutrons Based on the ICRP 60 Recommendations

Calculations of the ambient dose equivalent conversion factors for neutron energies from 20 MeV to 5 GeV have been performed using the Monte Carlo high energy transport code HADRON. The kerma approximation extended to the high energy region was applied in the calculations. The results obtained are compared with the previous data for an infinite slab 30 cm thick as well as with the recent calculations for the ICRU sphere. The influence of the phantom shape and dimension is expected to be fairly unimportant for high energy radiations. The discrepancies in dose equivalent data are explained mainly by incorrect quality factors of secondary charged particles used in earlier calculations. Another conclusion is that the ambient dose equivalent H*(10) satisfies the requirement of conservativity for high energy radiations in practical cases of radiation fields outside shields.

Calculations of Equivalent Dose and Effective Dose Conversion Coefficients for Photons from 1 MeV to 10 GeV

A calculational method is presented to evaluate equivalent doses of tissues or organs and effective dose for high energy radiations, where the radiation weighting factor given in ICRP 60 was replaced by an average quality factor estimated from the Q-L relationship in ICRP 60. The equivalent doses of tissues or organs and fluence to effective dose conversion coefficients are calculated for photons up to 10GeV with a Monte Carlo code EGS4 in which a mathematical model of an anthropomorphic phantom is included. The equivalent doses for 24 tissue and organs have been calculated for AP and PA incidence of aligned and expanded photons from I MeV to 10 GeV. The effective doses were obtained by summing the equivalent doses with tissue weighting factors from ICRP 60. It is also demonstrated that the kerma approximation overestimates significantly the effective dose for photons above 10 MeV.

Ambient Dose Equivalent, Deep Dose Equivalent Index, and ICRU Sphere Depth-Dose Calculations for Neutrons from 30 to 180 MeV

Detailed depth-dose calculations were performed in the ICRU sphere for monoenergetic parallel neutron beams with energies ranging from 30 MeV to 180 MeV. Fluence-to-dose conversion coefficients were obtained for both absorbed dose and dose equivalent quantities. Calculations of dose equivalents were performed using both ICRP 21 and ICRP 60 Q-L relationships. Ambient dose equivalents, at a depth of 10mm inside the ICRU sphere, and deep dose equivalent indices are reported. The LAHET computer code was utilised to perform the energy deposition calculations due to secondary charged particles for neutron interactions above 20 MeV. Lower energy neutrons and neutron-induced photons were transported using the HMCNP code and the ENDF/B-V continuous energy cross section data library. The kerma approximation was employed to compute the absorbed doses due to the interactions of neutrons below 20 MeV. The results are compared with the limited available depth-dose data for high energy neutron beams incident on slab phantoms.

Comparison between lucite and water as a phantom material in medical radiology

In quality assurance and quality control programs in medical radiology, Lucite (i.e. plexiglas, perspex polymethyl methacrylate (PMMA)) phantoms are often used, for convenience, instead of water to imitate the patient's body. Since Lucite has higher density and lower average atomic number than water, the energy absorption and particularly photon scattering characteristics differ. A comparison is made between water, soft tissue and Lucite phantoms of the same linear dimensions with respect to photon energy absorption and scattering at photon energies between 1 and 150 keV. Analog Monte Carlo methods are used to simulate the photon transport within phantom and receptor. Charged particle equilibrium is assumed since the electron transport is not simulated (kerma approximation). It is also assumed that coherent scattering occurs independently with the atoms of the compounds. The results show that the scatter-to-primary ratio, S/P, of the energy imparted to the receptor, with the receptior located on the rear side of the phantom, is significantly larger with the Lucite than with the water or soft tissue phantom. Also, the single-event distribution of the energy imparted to the receptor by the scattered photons transmitted through the phantom, differs between Lucite and water. The fraction of incident energy imparted to the phantom is at low photon energies (below 80 keV) lower in Lucite than in water. The opposite is true at energies above 80 keV where Compton scattering predominates, and the higher density of Lucite is decisive.

Calculations of Specified Depth Dose Equivalent in the ICRU Sphere Resulting from External Neutron Irradiation with Energies Ranging from Thermal to 20 MeV

The International Commission on Radiation Units and Measurements (ICRU) is considering a new system of operational quantities for radiation protection. This system is based on specified depth dose equivalent in a sphere, 30 cm in diameter, simulating soft tissue. Neutron fluence to depth dose equivalent conversion factors for energies ranging from thermal to 20 MeV are presented, allowing all quantities at present under discussion by ICRU to be computed. The input data and the computer code, for which the Monte Carlo method is used, are described. Recoil hydrogen nuclei and energetic electrons (reaction partners of induced photons) are discussed in detail. The kerma approximation is only used for heavy ions or photons with low energy.