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Biomedical subjects

D W Rogers

Publications and source records attributed to D W Rogers.

At least 91 records · Page 5Linked to original sources
PubMedDate not supplied

Calculated buildup curves for photons with energies up to 60Co.

This report investigates the buildup region of the depth-dose curve for broad parallel beams of 60Co and lower energy photons incident on slabs of tissue as defined by the International Commission on Radiation Units and Measurements (ICRU) and other materials. The EGS Monte Carlo system was used to simulate the transport of photons and electrons in the materials. Calculated buildup regions of the depth-dose curves are presented for photons incident on ICRU tissue. For 60Co beams, the primary and scatter dose components are presented, both for the buildup region and at depth in the phantom. Effects of beam radius were investigated and found to have only a small effect in the buildup region of the central axis depth-dose curve. The calculated effects of beam radius are larger at depth and in good agreement with experiment. Effects of scatter and attenuation in a small miniphantom of water were investigated as a function of photon energy and were found to cancel within 0.8%. The mean distance of electron transport was calculated for 60Co beams incident on carbon, water, polystyrene, aluminum, and lead.

Cobalt Radioisotopes↗
PubMedDate not supplied

Differences in electron depth-dose curves calculated with EGS and ETRAN and improved energy-range relationships.

For 1-50 MeV electrons incident on a water phantom there are systematic differences in the depth-dose curves calculated by the Monte Carlo codes EGS and ETRAN (and its descendants SANDYL, CYLTRAN, ACCEPT, and the ITS system). Compared to ETRAN, the EGS code calculates a higher surface dose and a slightly slower dose falloff past the dose maximum. The discrepancy in the surface dose is shown to exist because the modified Landau energy-loss straggling distribution used in ETRAN underestimates the mean energy loss by about 10% since it underestimates the number of large energy-loss events. Comparison to experimental data shows a preference for the EGS depth-dose curves at 10 and 20 MeV. Since various dosimetry protocols assign electron beam energies based on measured depth-dose curves in water, formulas based on these more accurate EGS4 calculations are presented: relating the mean energy of an incident electron beam to R50, the depth at which the dose in a water phantom falls to 50% of its maximum value; and relating the most probable energy of the incident beam to the projected range of the depth-dose curve. A study is presented of the effects of the incident electron spectrum on the calculated depth-dose curve.

Biophysical Phenomena↗
PubMedDate not supplied

The dependence of mass energy absorption coefficient ratios on beam size and depth in a phantom.

The Monte Carlo computer code "electron gamma shower" (EGS) has been used to determine photon spectra in a water phantom. Spectra used by Johns and Cunningham and for the AAPM dosimetry protocol have been used as input data and ratios of average mass energy absorption coefficients have been calculated for a number of depths and field sizes. The results show that there is a slight dependence on both of these parameters. For example, (mu en/P) water graphite for cobalt-60 varies from a value of 1.111 for the primary spectrum in air, to 1.135 at a depth of 20 cm in a phantom for a beam approximately 1 m2 in area. This variation of over 2% is relevant for dosimetry. The variation is less than this for high-energy radiation beams and in most cases can be ignored. The effect is greater for high atomic materials such as bone, where the range of variation of (mu en/P)bone water, again for cobalt radiation, may be as great as 15%. This too is less for high-energy bremsstrahlung spectra.

Cobalt Radioisotopes↗
PubMedDate not supplied

The role of humidity and other correction factors in the AAPM TG-21 dosimetry protocol.

A detailed derivation is presented of the formulas required to determine Ngas and Dmed in the AAPM TG-21 dosimetry protocol. This protocol specifies how to determine the absorbed dose in an electron or photon beam when using exposure or absorbed dose calibrated ion chambers. It is shown that the expression given in TG-21's recent letter of clarification is incorrect. Accounting for humidity correctly increases, by 0.4%, all absorbed dose determinations using an exposure calibrated ion chamber. Taking into account other correction factors in the equation for exposure could also have varying, but significant effects (possibly over 1%). These are the stem scatter correction, the axial nonuniformity correction and the electrode correction for electrodes made of different materials from the wall. Attention is drawn to differences in the definitions of the exposure and absorbed dose calibration factors, Nx and ND, respectively, as supplied by the NBS and the NRCC.

Humans↗
PubMedDate not supplied

Forward-directed bremsstrahlung of 10- to 30-MeV electrons incident on thick targets of Al and Pb.

Bremsstrahlung spectra from thick targets of Al and Pb have been measured absolutely (photons per incident electron) along the beam axis for electrons of 10-, 15-, 20-, 25-, and 30-MeV incident energy. The spectra have a 220-keV low-energy cutoff. The targets were cylinders with nominal thicknesses of 110% of the electron CSDA range. A thin transmission detector, calibrated against a toroidal current monitor, was placed upstream of the target to measure the beam current. The spectrometer was a 20-cm diameter by 25-cm-long cylindrical NaI detector. Measured spectra were corrected for pile-up, background, detector response, detector efficiency, attenuation in materials between the target and detector and the collimator effect. Spectra were calculated using the EGS4 Monte Carlo system for simulating the radiation transport. The simulation model included the small amount of material upstream of the target. This material contributed about 40% of the spectrum, but its presence or absence had little effect on the calculated bremsstrahlung yield. The shapes of the measured and calculated spectra were in excellent agreement. The ratio of the total number of photons in each measured spectrum to those in the corresponding calculated spectrum varied from 0.97 +/- 0.06 to 1.12 +/- 0.06, depending largely on the atomic number of the target. Absolute spectral measurements in the literature agreed with our calculations of spectral shape but showed a range of +/- 30% in the number of photons per incident electron relative to the calculated values, which is contrary to our result.

Aluminum↗
PubMedDate not supplied

Effects of gold and silver backings on the dose rate around an 125I seed.

Measurements of the effect of either gold or silver backing on the dose rate around an 125I seed were performed using a Therados RFA7 dosimetry system and a small diode detector which was 2.5 mm in diameter and 0.06 mm thick. It was found that the presence of the gold or silver backing modifies the diode response on the side of the 125I seed away from the backing. The effect depends on the backing material and the distance from the seed. There is a small increase close to the gold backing but a decrease further away. This decrease at distances greater than 10 mm from the seed is uniformly 10%, the same as found when the seed is backed by air. There is an increase of up to 25% observed with silver backing the seed and this increase remains significant more than 30 mm from the seed. When the response increases, the results are hard to interpret quantitatively because of variations in the diode response per unit dose with photon energy and extreme sensitivity to geometric changes. Nonetheless, except for the increase at close distances with the gold, the results are in agreement with EGS4 Monte Carlo photon transport simulations which are for a simplified geometry and account for x-ray fluorescence from the K-shell. Furthermore, the increase in the gold-backed case is qualitatively explained by Williamson's Monte Carlo calculations which take into account the L-shell fluorescent x-rays from gold.

Brachytherapy↗
PubMedDate not supplied

Calculation of water/air stopping-power ratios using EGS4 with explicit treatment of electron-positron differences.

Using the EGS4 Monte Carlo simulation program, a general purpose code has been written to calculate Bragg-Gray and Spencer-Attix stopping-power ratios for use in radiation dosimetry. The stopping-power ratios can be calculated in any material in any region in a general cylindrical geometry with a large number of source geometries possible. The calculations take into account for the first time the differences between the stopping powers and the inelastic scattering of positrons and electrons. The results show that previous calculations ignoring these effects were accurate. The present results agree, typically within 0.1%, with the Spencer-Attix water-to-air stopping-power ratios for broad parallel beams of electrons given in the AAPM and IAEA protocols except at the surface where the present calculations follow the buildup of secondary electrons in more detail and see a 2% reduction in the stopping-power ratios.

Air↗
PubMedDate not supplied

Angular distribution of bremsstrahlung from 15-MeV electrons incident on thick targets of Be, Al, and Pb.

Bremsstrahlung spectra from thick cylindrical targets of Be, Al, and Pb have been measured at angles of 0 degrees, 1 degree, 2 degrees, 4 degrees, 10 degrees, 30 degrees, 60 degrees, and 90 degrees relative to the beam axis for electrons of 15-MeV incident energy. The spectra are absolute (photons per incident electron) and have a 145-keV lower-energy cutoff. The target thickness were nominally 110% of the electron CSDA range. A thin transmission detector, calibrated against a toroidal current monitor, was placed upstream of the target to measure the beam current. The spectrometer was a 20-cm-diam by 25-cm-long cylindrical NaI detector. Measured spectra were corrected for pile-up, background, detector response, detector efficiency, attenuation in materials between the target and detector and collimator effects. Spectra were also calculated using the EGS4 Monte Carlo system for simulating the radiation transport. There was excellent agreement between the measured and calculated spectral shapes. The measured yield of photons per incident electron was 9% and 7% greater than the calculated yield for Be and Al, respectively, and 2% less for Pb, all with an uncertainty of +/- 5%. There was no significant angular variation in the ratio of the measured and calculated yields. The angular distributions of bremsstrahlung calculated using available analytical theories dropped off more quickly with angle than the measured distributions. The predictions of the theories would be improved by including target-scattered photons.

Aluminum↗
PubMedDate not supplied

A standard timing benchmark for EGS4 Monte Carlo calculations.

A Fortran 77 Monte Carlo source code built from the EGS4 Monte Carlo code system has been used for timing benchmark purposes on 29 different computers. This code simulates the deposition of energy from an incident electron beam in a 3-D rectilinear geometry such as one would employ to model electron and photon transport through a series of CT slices. The benchmark forms a standalone system and does not require that the EGS4 system be installed. The Fortran source code may be ported to different architectures by modifying a few lines and only a moderate amount of CPU time is required ranging from about 5 h on PC/386/387 to a few seconds on a massively parallel supercomputer (a BBN TC2000 with 512 processors).

Electrons↗
PubMedDate not supplied

Calibration of parallel-plate chambers: resolution of several problems by using Monte Carlo calculations.

For five commonly used parallel-plate ion chambers, Monte Carlo calculations are presented of the wall attenuation and scatter correction factors (Kwall = Awall-1 or katt-1) and the correction for nonhomogeneous composition of the chamber (Kcomp or related to km). The chambers are assumed to have 0.5 g/cm2 buildup caps made of the predominant material in each chamber. These correction factors are needed if air-kerma calibration factors are used to deduce the chamber's cavity-gas calibration factor, Ngas. The scatter from the material around the cavity more than compensates for the attenuation in the front wall and hence Kwall values are less than unity. The corrections for the nonhomogeneous composition can be significant. Thin collectors or insulators behind the collecting volume can have a major effect because electron backscattering depends strongly on material. The calculations agree well with experimental data and explain previous results that had not been understood. Calculations for different buildup caps predict trends in chamber in-air responses, which go in the opposite direction to the km values for homogeneous chambers of the same materials. It is argued that Pwall, the wall correction factor for photon-beam measurements in-phantom is, to first order, the same correction factor as Kcomp, calculated for a buildup cap of the same material as the phantom material and the initial assumption that the wall material is made of the same material. Hence, Pwall can also be significantly different from unity. Recommended values of kmkatt or the equivalent Ngas/Nx, and Pwall (in 60Co beams) are presented for the 5 chambers. These values treat the Exradin P11, Markus and Holt parallel-plate chambers as homogeneous in construction whereas the values for the NACP and Capintec PS-033 chambers demonstrate that the chambers are distinctly nonhomogeneous.

Calibration↗
PubMedDate not supplied

The advantages of absorbed-dose calibration factors.

A formalism for clinical external beam dosimetry based on use of ion chamber absorbed-dose calibration factors is outlined in the context and notation of the AAPM TG-21 protocol. It is shown that basing clinical dosimetry on absorbed-dose calibration factors ND leads to considerable simplification and reduced uncertainty in dose measurement. In keeping with a protocol which is used in Germany, a quantity kQ is defined which relates an absorbed-dose calibration factor in a beam of quality Q0 to that in a beam of quality Q. For 38 cylindrical ion chambers, two sets of values are presented for ND/NX and Ngas/ND and for kQ for photon beams with beam quality specified by the TPR20(10) ratio. One set is based on TG-21's protocol to allow the new formalism to be used while maintaining equivalence to the TG-21 protocol. To demonstrate the magnitude of the overall error in the TG-21 protocol, the other set uses corrected versions of the TG-21 equations and the more consistent physical data of the IAEA Code of Practice. Comparisons are made to procedures based on air-kerma or exposure calibration factors and it is shown that accuracy and simplicity are gained by avoiding the determination of Ngas from NX. It is also shown that the kQ approach simplifies the use of plastic phantoms in photon beams since kQ values change by less than 0.6% compared to those in water although an overall correction factor of 0.973 is needed to go from absorbed dose in water calibration factors to those in PMMA or polystyrene. Values of kQ calculated using the IAEA Code of Practice are presented but are shown to be anomalous because of the way the effective point of measurement changes for 60Co beams. In photon beams the major difference between the IAEA Code of Practice and the corrected AAPM TG-21 protocol is shown to be the Prepl correction factor. Calculated kQ curves and three parameter equations for them are presented for each wall material and are shown to represent accurately the kQ curve for all ion chambers in this study with a wall of that specified material and a thickness less than 0.25 g/cm2. Values of kQ can be measured using the primary standards for absorbed dose in photon beams.

Electrons↗
PubMedDate not supplied

How accurately can EGS4/PRESTA calculate ion-chamber response?

Ion-chamber responses are calculated for graphite, PMMA, and aluminum-walled ion chambers free-in-air in 60Co beams and 200-keV beams for a graphite chamber. The EGS4 Monte Carlo system is used with various electron step-size algorithms, in particular the PRESTA algorithm, the much simpler ESTEPE constraint on the energy loss per step, and in combination. Contrary to previous reports, it is found that there are variations in the calculated ion chamber response of up to 3% in 60Co beams and up to 8% in the 200-keV beam. It is recommended that all ion chamber calculations be done with the PRESTA algorithm plus an additional constraint on the energy losses per step of 1% (or less for higher-Z materials). The systematic uncertainty in the calculations for 60Co beams is found to be 1% (1 sigma) and somewhat higher in 200-keV beams because of sensitivity to transport parameters such as AE and ECUT.

Algorithms↗