Detection system built from commercial integrated circuits for real-time measurement of radiation dose and quality using the variance method.

A small, specialised amplifier using commercial integrated circuits (ICs) was developed to measure radiation dose and quality in real time using a microdosimetric ion chamber and the variance method. The charges from a microdosimetric ion chamber, operated in the current mode, were repeatedly collected for a fixed period of time for 20 cycles of 100 integrations, and processed by this specialised amplifier to produce signal pulse heights between 0 and 10 V. These signals were recorded by a multi-channel analyser coupled to a computer. FORTRAN programs were written to calculate the dose and dose variance. The dose variance produced in the ion chamber is a microdosimetric measure of radiation quality. Benchmark measurements of different brands of ICs were conducted. Results demonstrate that this specialised amplifier is capable of distinguishing differences of radiation quality in various high-dose-rate radiation fields including X rays, gamma rays and mixed neutron-gamma radiation from the research reactor at Texas A&M University Nuclear Science Center.

GE PETtrace cyclotron as a neutron source for boron neutron capture therapy.

This paper discusses the use of a General Electric PETtrace cyclotron as a neutron source for boron neutron capture therapy. In particular, the standard PETtrace (18)O target is considered. The resulting dose from the neutrons emitted from the target is evaluated using the Monte Carlo radiation transport code MCNP at different depths in a brain phantom. MCNP-simulated results are presented at 1, 2, 3, 4, 5, 6, 7, and 8 cm depth inside this brain phantom. Results showed that using a PETtrace cyclotron in the current configuration allows treating tumors at a depth of up to 4 cm with reasonable treatment times. Further increase of a beam current should significantly improve the treatment time and allow treating tumors at greater depths.

Effect of torso adipose tissue thickness on effective dose in a broad parallel photon beam.

The effect of torso adipose tissue thickness on effective dose was studied for external broad parallel photon beams using the MCNP code and a mathematical anthropomorphic phantom. The variation of torso adipose tissue thickness was modeled by adding a layer of soft tissue (1-7 cm) around the torso of the phantom. This study found that effective dose varies almost linearly with the thickness of the adipose tissue layer. For most irradiation geometries (i.e., antero-posterior, postero-anterior, and lateral), effective dose decreases with the thickness of the adipose tissue layer due to the shielding effect of the layer. Effective dose decreases by 11-35% when the thickness of the adipose tissue layer increases from 0 to 7 cm considering all photon energies (0.08, 0.3, and 1.0 MeV) and irradiation geometries in this study. For overhead irradiation geometry, however, an increase of adipose tissue layer thickness results in an increase of effective dose. This is because the organs and tissues in the body are additionally exposed by the photons that are scattered from the added adipose tissue layer. For the overhead irradiation geometry, effective dose increases by 13-27% when the adipose tissue thickness increases from 0 to 7 cm.

Monte Carlo simulation of single and two-dosimeter approaches in a steam generator channel head.

In a steam generator channel head, it was not unusual to see radiation workers wearing as many as twelve dosimeters over the surface of the body to avoid a possible underestimation of effective dose equivalent (H(E)) or effective dose (E). This study shows that only one or two dosimeters can be used to estimate H(E) and E without a significant underestimation. MCNP and a point-kernel approach were used to model various exposure situations in a steam generator channel head. The single-dosimeter approach (on the chest) was found to underestimate H(E) and E significantly for a few exposure situations, i.e., when the major portion of radiation source is located in the backside of a radiation worker. In this case, the photons from the source pass through the body and are attenuated before reaching the dosimeter on the chest. To assure that a single dosimeter provides a good estimate of worker dose, these few exposure situations cannot dominate a worker's exposure. On the other hand, the two-dosimeter approach (on the chest and back) predicts H(E) and E very well, hardly ever underestimating these quantities by more than 4% considering all worker positions and contamination situations in a steam generator channel head. This study shows that two dosimeters are adequate for an accurate estimation of H(E) and E in a steam generator channel head.

Effect of angular response properties of personal dosemeters on the estimation of effective dose using two dosemeters.

Even though the two-dosemeter approach successfully solved the underestimation problem of the single-dosemeter approach for posterior incident radiations, this approach significantly overestimates effective dose for the lateral and overhead beam directions when isotropic-responding dosemeters are used for measurement. This kind of overestimation can be reduced by using anisotropic-responding dosemeters whose responses decrease as the incident angle increases, i.e. from 0 degree (normal incidence) to 90 degrees (lateral incidence). To quantify the reduction of overestimation by using anisotropic-responding dosemeters, this study applied the two-dosemeter approach to several types of anisotropic-responding dosemeters--both ideal and commercial--and then compared the results with those of isotropic-responding dosemeters. This study also derived a set of angular response factors (ARF) which can be used to develop a personal dosemeter with ideal angular response properties for use in the two-dosemeter approach.

Improved estimates of effective dose equivalent using two optimal anisotropic responding dosimeters.

Although the use of two dosimeters, one on the chest and the other on the back, successfully solved the underestimation problem for posterior incident photon beams, the two-dosimeter approach still has some problems-significant overestimations for lateral, overhead, and underfoot beam directions when isotropic-responding dosimeters are used for measurement. A solution to this problem is to intentionally construct the dosimeters to under-respond as the beam direction departs from normal incidence and approaches lateral, overhead, or underfoot beam directions. The objective of this study is to develop a dosimeter that does not significantly overestimate effective dose equivalent (H(E)) for lateral, overhead, and underfoot beam directions, while maintaining good performance for anterior and posterior beam directions. Several dosimeter geometries were investigated using Monte Carlo simulation to find the best geometry using aluminum oxide (Al2O3) as dosimeter attenuator material. Then, the developed Optimal Anisotropic Responding dosimeters were tested for 0.08, 0.30 and 1.00 MeV photon beams of various beam directions. The dosimeters did not overestimate H(E) by more than 80% considering all photon energies and beam directions, which is much less than the overestimation of isotropic-responding dosimeters (202%). The dosimeters also showed similar performance compared to isotropic-responding dosimeters for anterior and posterior beam directions. Finally, the dosimeters were applied to effective dose (E) and the results are compared with those of H(E).

Monte Carlo calculations of the dose backscatter factor for monoenergetic electrons.

Recently, there has been growing interest in beta emitters for therapeutic uses, especially in connection with so-called endovascular (or intravascular) brachytherapy. Since accurate dose estimation is necessary for the success of such applications, some problems in beta-ray dosimetry need further study. Among these problems, we have investigated the effect of electron backscattering on dose, which has significance not only for accurate dose estimation but also for new source design. In this study, an empirical measure of electron backscattering, known as the dose backscatter factor, was calculated using EGS4 Monte Carlo calculations for monoenergetic electrons and various scattering materials. Electron energies were 0.1, 0.5, 1.0, 2.0 and 3.0 MeV in combination with Al (Z = 13), Ti (Z = 22), Sr (Z = 38), Ag (Z = 47) and Pt (Z = 78) scatterers. The dose backscatter factor ranged from 10% to 60%, depending on electron energy and material, and was found to increase with the atomic number Z by a log(Z + 1) relationship. A method is presented for calculating the beta-ray dose backscatter factors using the results of this study. To demonstrate the efficacy of this method, a dose backscatter factor depth profile for 32P near a water/aluminium interface was calculated and these calculated results were found to generally reproduce the depth profile obtained from direct EGS4 calculations using the 32P spectrum. The data presented in this study can be used to calculate dose backscatter factors for any combination of beta emitter/scatterer whose atomic number ranges from 13 to 78.

Calculation of effective doses for broad parallel photon beams.

Values of effective dose (E) were calculated for the entire range of incident directions of broad parallel photon beams for selected photon energies using the Monte Carlo N-Particle (MCNP) transport code with a hermaphroditic phantom. The calculated results are presented in terms of conversion coefficients transforming air kerma to effective dose. This study also compared the numerical values of E and H(E) over the entire range of incident beam directions. E was always less than H(E) considering all beam directions and photon energies, but the differences were not significant except when a photon beam approaches some specific directions (overhead and underfoot). This result suggests that the current H(E) values can be directly interpreted as E or, at least, as a conservative value of E without knowing the details of irradiation geometries. Finally, based on the distributions of H(E) and E over the beam directions, this study proposes ideal angular response factors for personal dosimeters that can be used to improve the angular response properties of personal dosimeters for off-normal incident photons.

Effective dose equivalent and effective dose for photon exposures from point and disk sources on the floor.

A complete set of H(E) and E values were calculated for photon exposures from point and disk sources on the floor using the MCNP code and a "hermaphroditic" phantom. It was found that a male can receive a higher H(E) or health risks than a female by a factor of two from an identical point source on the floor when source distance is less than 50 cm. Conversely, if the source distance becomes larger than 100 cm, the female receives H(E) higher than the male by up to 40%. For identical sources, both the male and female experience significantly higher H(E) from front-located sources than from back- or side-located sources. For a 100-cm source distance, male H(E) from a front-located source is greater than that from a side-located source by factors of 4, 3, and 2 for 0.08, 03, and 1.0 MeV photons, respectively. In the female cases, the differences are somewhat smaller but still differ by factors of 3, 2, and 1.7. It was also found that both the highest male and female H(E) values occur when a source is within 40-60 cm in front of the phantom. The maximum male H(E) is 1.8 x 10(-18), 6.6 x 10(-18), and 2.1 x 10(-17) Sv per photon emission for 0.08, 03, and 1.0 MeV photons, respectively. For females, these maximum values are slightly smaller, 1.4 x 10(-18), 5.3 x 10(-18), and 1.9 x 10(-17) Sv/photon, respectively. Tissue kerma free-in-air at 100 cm above a disk source (Ktissue) was found to greatly overestimate H(E) if the source radius is less than 200 cm. For radii larger than 200 cm, the Ktissue gives a relatively better estimate of H(E), overestimating by not more than 100%. The point source H(E) values were directly integrated to estimate H(E) for simple non-self-shielding sources such as disk, circle, and line sources. This simple approach was found to overestimate H(E) by less than 10% for these irradiation geometries. Finally, the comparison of H(E) and E showed that for most cases these values are almost identical. For point sources, when source distance is larger than 50 cm, the difference between H(E) and E was always less than 23% over photon energies between 0.08 and 1.0 MeV. For disk sources of radius larger than 50 cm, the difference was even smaller (<12%).

Calculation of the dose distribution in water from 71Ge K-shell x-rays.

The dose distribution in water from 71Ge K-shell x-rays (Eave = 9.44 keV) was calculated for various source configurations using both analytic and EGS4 Monte Carlo calculations. The point source kernel and the buildup factor are presented. The buildup factor for a point source in water has been found to increase up to about 1.1 as radial distance approaches 1 cm. Comparison between 71Ge and 90Sr/Y shows a similarity between their relative dose distribution in water. The dose distribution from a disc source was calculated using the EGS4 code and compared with the results from analytic calculation. Excellent agreement was observed, confirming the validity of analytic calculations. The dose rate at 0.01 cm from a 71Ge disc source was calculated to be about 1.3 x 10(-5) Gy MBq-1 s-1. Based on the results from this study, 71Ge activity of the order of 3.7 x 10(10) Bq (approximately 1 Ci) might be necessary to obtain dose rates typical of 90Sr/Y ophthalmic applicators. The possibility of using 71Ge as a source of radioactive stents was also investigated. A 71Ge stent was modelled as a cylindrical shell source and the dose rates were determined by Monte Carlo calculations. Some calculated results are compared with published values for a 32P-coated stent. The dose rate at 0.01 cm from a 71Ge stent has been calculated to be about 6.5 x 10(-3) Gy MBq-1 h-1, which is much lower than the reported dose rate at the same distance from a 32P-coated stent. However, an initial source activity of the order of 3.7 x 10(7) Bq (approximately 1 mCi) would easily result in a typical target dose (approximately 24 Gy) needed for intravascular stent applications. In conclusion, 71Ge sources could be used as alternatives to beta sources and, unlike high-energy (approximately MeV) beta sources, may provide easily predictable dose distributions in heterogeneous media and low dose rates, which might be beneficial for some clinical applications.

Sex-specific tissue weighting factors for effective dose equivalent calculations.

The effective dose equivalent was defined in the International Commission on Radiological Protection Publication 26 in 1977 and later adopted by the U.S. Nuclear Regulatory Commission. To calculate organ doses and effective dose equivalent for external exposures using Monte Carlo simulations, sex-specific anthropomorphic phantoms and sex-specific weighting factors are always employed. This paper presents detailed mathematical derivation of a set of sex-specific tissue weighting factors and the conditions which the weighting factors must satisfy. Results of effective dose equivalent calculations using female and male phantoms exposed to monoenergetic photon beams of 0.08, 0.3, and 1.0 MeV are provided and compared with results published by other authors using different sex-specific weighting factors and phantoms. The results indicate that females always receive higher effective dose equivalent than males for the photon energies and geometries considered and that some published data may be wrong due to mistakes in deriving the sex-specific weighting factors.

A study of the angular dependence problem in effective dose equivalent assessment.

The newly revised American National Standard N13.11 (1993) includes measurements of angular response as part of personnel dosimeter performance testing. However, data on effective dose equivalent (HE), the principle limiting quantity defined in International Commission on Radiological Protection (ICRP) Publication 26 and later adopted by U.S. Nuclear Regulatory Commission (NRC), for radiation incident on the body from off-normal angles are little seen in the literature. The absence of scientific data has led to unnecessarily conservative approaches in radiation protection practices. This paper presents a new set of fluence-to-HE conversion factors as a function of radiation angles and sex for monoenergetic photon beams of 0.08, 0.3, and 1.0 MeV. A Monte Carlo transport code (MCNP) and sex-specific anthropomorphic phantoms were used in this study. Results indicate that Anterior-posterior (AP) exposure produces the highest HE per unit photon fluence in all cases. Posterior-anterior (PA) exposure produces the highest HE among beams incident from the rear half-plane of the body. HE decreases dramatically as one departs from the AP and PA orientations. The results also indicate that overestimations caused by using isotropic dosimeters in assessing effective dose equivalent from near-overhead and near-underfoot exposures are 550%, 390%, and 254% for 0.08, 0.3, and 1.0 MeV, respectively. Comparisons of the angular dependence of HE with those based on the secondary quantities defined in International Commission on Radiation Units and Measurements (ICRU) Reports 39, 43, and 47 show significant differences. This paper discusses why more accurate assessments of HE are necessary and possible. An empirical equation is proposed which can be used as the optimum dosimeter angular response function for radiation angles ranging from 0 degrees to 90 degrees for dosimeter calibration, performance testing, and design.

Determination of photon conversion factors relating exposure and dose for several extremity phantom designs.

Dosimetric measurements were performed to determine the exposure-to-dose conversion factors (Cx) for simple extremity phantoms suitable for extremity dosimeter performance testing. The phantoms studied represented the forearm or lower leg and the finger. Measurements were performed for solid plastic phantoms and for phantoms containing simulated bone material to determine the effect of backscattered radiations from the simulated bone to the phantom surface. Photon beam energies used for the measurements ranged from 16 keV to 1.25 MeV (average). The Cx factors for the finger phantoms did not vary significantly with phantom composition. The Cx factors in the arm/leg phantoms with the bone simulant material differed significantly from those for the solid plastic phantom over the energy range of 40-100 keV. This effect was attributed to the preferential absorption of the lower energy backscattered photons by the higher atomic number material that was contained in the bone-simulant insert. The position of the bone-simulating material below the surface of the phantom was more important than its size or level of bone equivalency. For calibrations and dosimeter testing, Al was found adequate as a bone-simulating material.