Dose Rate Reduction Research Articles

Flux and dose rate evaluation of iter system using MCNP5

The International Thermonuclear Experimental Reactor (ITER) will perform Deuterium-Tritium (DT) plasma experiments and the neutrons production rate at 14.1 MeV will achieve the level of 1013 n.cm-2.s-1. In this work, the neutron flux and the dose rate during ITER operation has been calculated using the one-dimensional model of the Monte Carlo code MCNP5 and the FENDL/MC-2.1 nuclear data library. The neutron flux and dose rate associated during normal ITER operation were determined along the radial machine direction. Calculations for two different types of concrete compositions were performed to investigate the impact of the bioshield filling materials on the dose rate estimation. The results show that the dose rate level near to the outer wall of the tokamak hall is close to the allowable limit dose. Taking into account the use of large boron concentration in the biological shield concrete (2.9% weight fraction), it was obtained a dose rate reduction by one order of magnitude. The dose rate is dominated by the secondary gamma ray. The magnitude of the dose rate on the outside hall of bioshield during normal ITER operation can not be considered low in accordance with the result found in the simulation performed in this work, i.e., 1 µSv.h-1.

Strahlenwirkung am Normalgewebe

SummaryThe curative effectivity of external or internal radiotherapy necessitates exposure of normal tissues with significant radiation doses, and hence must be associated with an accepted rate of side effects. These complications can not a priori be considered as an indication of a too aggressive therapy. Based on the time of first diagnosis, early (acute) and late (chronic) radiation sequelae in normal tissues can be distinguished. Early reactions per definition occur within 90 days after onset of the radiation exposure. They are based on impairment of cell production in turnover tissues, which in face of ongoing cell loss results in hypoplasia and eventually a complete loss of functional cells. The latent time is largely independent of dose and is defined by tissue biology (turnover time). Usually, complete healing of early reactions is observed. Late radiation effects can occur after symptom-free latent times of months to many years, with an inverse dependence of latency on dose. Late normal tissue changes are progressive and usually irreversible. They are based on a complex interaction of damage to various cell populations (organ parenchyma, connective tissue, capillaries), with a contribution from macrophages. Late effects are sensitive for a reduction in dose rate (recovery effects).A number of biologically based strategies for protection of normal tissues or for amelioration of radiation effects was and still is tested in experimental systems, yet, only a small fraction of these approaches has so far been introduced into clinical studies. One advantage of most of the methods is that they may be effective even if the treatment starts way after the end of radiation exposure. For a clinical exploitation, hence, the availability of early indicators for the progression of subclinical damage in the individual patient would be desirable. Moreover, there is need to further investigate the molecular pathogenesis of normal tissue effects in more detail, in order to optimise biology based preventive strategies, as well as to identify the precise mechanisms of already tested approaches (e. g. stem cells).

Cancer and Non-cancer Risks in Normal and Cancer-ProneTrp53Heterozygous Mice Exposed to High-Dose Radiation

This report tests the hypotheses that cancer proneness elevates risk from a high radiation exposure and that the risk response to high doses is qualitatively similar to that from low doses. Groups of about 170 female mice heterozygous for Trp53 (Trp53(+/-)) and their normal female littermates (Trp53(+/+)) were exposed at 7-8 weeks of age to (60)Co gamma-radiation doses of 0, 1, 2, 3 or 4 Gy at a high dose rate (0.5 Gy/min) or 4 Gy at a low dose rate (0.5 mGy/min). In the absence of radiation exposure, Trp53 heterozygosity reduced life span approximately equally for death from either cancer or non-cancer disease. Heterozygosity alone produced a 1.5-fold greater shortening of life span than a 4-Gy acute exposure. Per unit dose, life shortening from cancer or non-cancer disease was the same for normal mice and Trp53 heterozygous animals, indicating that, contrary to previous reports, Trp53 heterozygosity did not confer radiation sensitivity to high doses of gamma rays. In Trp53(+/-) mice with cancer, life shortening from acute doses up to 4 Gy was related to both increased tumor formation and decreased tumor latency. A similar tumor response was observed in normal mice, but only up to 2 Gy, indicating that above 2 Gy, normal Trp53 function protected against tumor initiation, and further life shortening reflected only decreased latency for cancer and non-cancer disease. Dose-rate reduction factors were 1.7-3.0 for both genotypes and all end points. We conclude that Trp53 gene function influences both cancer and non-cancer mortality in unexposed female mice and that Trp53-associated cancer proneness in vivo is not correlated with elevated radiation risk. Increased risk from high acute radiation doses contrasts with the decreased risk seen previously after low doses of radiation in both Trp53 normal and heterozygous female mice.

Step and shoot IMRT to mobile targets and techniques to mitigate the interplay effect

The purpose of this work is to evaluate a method to mitigate temporal dose variation due to the interplay effect as well as investigate the effect of randomly varying motion patterns. The multi-leaf collimator (MLC) settings from 5, 9 and 11 field step and shoot intensity modulated radiation therapy (IMRT) of non-small cell lung cancer (NSCLC) treatment plans with tumor motion of 1.53, 1.03 and 1.95 cm, respectively, were used. Static planar dose distributions were determined for each treatment field using the Planar Dose Module in the Pinnacle3 treatment planning system. The MotionSIM XY/4D robotic diode array was used to recreate the tumor motion orthogonal to each treatment beam. Dose rate modulation was investigated as a method to mitigate temporal dose variation due to the interplay effect. Computer simulation was able to identify individual fields where interplay effects are greatest. Computer simulation and physical measurement have shown that temporal dose variation can be mitigated by the selection of the dose rate or by selective dose rate modulation within a given IMRT treatment field. Selective dose rate modulation within a given IMRT treatment field reduced temporal dose variation to levels comparable to whole field dose rate reduction, while also producing shorter radiation delivery times in six of the seven cases investigated. For the cases considered, the interplay effect did not appear to have a greater effect on hypofractionation compared to traditional fractionation even though fewer fractions were delivered. Randomized motion kernel variation was also considered. For this portion of the study, a nine field step and shoot IMRT configuration was considered with a 1.03 cm tumor motion rather than the five field case. In general, if the extent of the variant motion pattern was mostly contained within the target volume, limited impact on the temporal dose variation was observed. In cases where the variant motion kernels increasingly exceeded the target volume limits, increases in temporal dose variation were observed.

The Effects of a Custom-Made Lead Collar on Dose Rates of Patients Treated with 131I

We studied the effects of a custom-made lead collar on dose rates and hospitalization days of patients treated with high doses (>400 MBq) of radioiodine (131I). Between October 2005 and March 2006, the dose rates of 28 consecutive patients were studied. Dose rates of patients with and without the collar (5.5 mm lead) were measured daily until the rate dropped below the discharge dose rate of 20 microSv-1 h at 1 m. Age, sex, 131I dose, uptake, thyroid disorder, and days of isolated hospitalization were recorded. The difference in mean dose rate with or without the collar was analyzed by t test or Mann-Whitney test, as appropriate. Twenty-eight patients (7 male, 21 female; mean age, 57 y; range, 29-82 y) were analyzed. Nine patients suffered from Graves' disease, 9 from toxic nodular goiter, and 10 from non-toxic nodular goiter. Patients were treated with a mean dose of 1,647 MBq 131I (range, 511-3,276 MBq). The mean anterior dose rates were 28 +/- 12 microSv-1 h at 1 m and 11 +/- 6 microSv-1 h at 1 m without and with the lead collar, respectively (p < 0.001). The mean reduction in dose rate for patients wearing the collar was 61 +/- 13%. The mean hospital stay was 2.6 +/- 1.3 days without the collar vs. 1.2 +/- 0.6 days with the collar (p < 0.001). A custom-made lead collar significantly reduced dose rates and could reduce compulsory hospitalization, especially in toxic- and non-toxic nodular goiter.

Temporal reduction of the external gamma dose rate due to 137Cs mobility in sandy beaches

In the present paper the contribution to the external gamma dose rate due to 137Cs in soil as a function of time is presented. Sampling sites were selected along the Calabria and Basilicata Regions coastal beaches (southern part of Italy) to assess the external gamma dose rate in air, 1 m above the ground level. A convection-dispersion model, with constant parameters was used to approximate the radiocesium soil vertical migration. The model was calibrated using the initial 137Cs activity deposition in this region (Chernobyl fallout) and 137Cs activity concentration down the soil profile, measured 10 years later. The dispersion coefficient and the advection velocity values, were respectively: 2.17 cm2 y−1 and 0.32 cm y−1. The Radionuclide Software Package (RSP), which uses a Monte Carlo simulation code, was used to determine the primary 137Cs gamma dose contribution in air 1 m above the ground surface. The resulting 137Cs external dose rate ranged from 0.42 nGy h−1 in 1986, to 0.05 nGy h−1 in 2007.

The Cosmic Silence experiment: on the putative adaptive role of environmental ionizing radiation

Previously we reported that yeast and Chinese hamster V79 cells cultured under reduced levels of background environmental ionizing radiation show enhanced susceptibility to damage caused by acute doses of genotoxic agents. Reduction of environmental radiation dose rate was achieved by setting up an underground laboratory at Laboratori Nazionali del Gran Sasso, central Italy. We now report on the extension of our studies to a human cell line. Human lymphoblastoid TK6 cells were maintained under identical in vitro culture conditions for six continuous months, at different environmental ionizing radiation levels. Compared to "reference" environmental radiation conditions, we found that cells cultured in the underground laboratories were more sensitive to acute exposures to radiation, as measured both at the level of DNA damage and oxidative metabolism. Our results are compatible with the hypothesis that ultra-low dose rate ionizing radiation, i.e. environmental radiation, may act as a conditioning agent in the radiation-induced adaptive response.

A Monte Carlo model of an industrial gauge for radiation protection purposes

The Mahlo Gravimat densitometers/gauges located at the Feltex carpet factory in Christchurch, New Zealand, contain the radionuclide strontium-90 (Sr-90). Accurate dose/dose rate estimation is always an important concern from a radiation protection point of view. The EGSnrc/BEAMnrc Monte Carlo code was used to create a model of one gauge to determination of the radiation dose distributions and dose rates in air from the Sr-90 source within the gauge. The model was then modified to include a Perspex shield on the surface of the gauge as a possible method of lowering the radiation levels. With this addition, the model predicted an overall reduction in dose rates of 30%. Although no experimental benchmarking was able to be performed due to simplifications in the model and the lack of reliable experimental data, we believe that Monte Carlo methods could be a valuable addition in the design process of any devices, industrial or otherwise, that contain or use radioactive materials. Furthermore, such methods may aide or guide any investigations undertaken as part of radiation protection surveys.

Cancer and Low Dose Responses in vivo: Implications for Radiation Protection

The Linear No Threshold (LNT) hypothesis states that ionizing radiation risk is directly proportional to dose, without a threshold. This hypothesis, along with a number of additional derived or auxiliary concepts such as radiation and tissue type weighting factors, and dose rate reduction factors, are used to calculate radiation risk estimates for humans, and are therefore fundamental for radiation protection practices. This system is based mainly on epidemiological data of cancer risk in human populations exposed to relatively high doses (above 100 mSv), with the results linearly extrapolated back to the low doses typical of current exposures. The system therefore uses dose as a surrogate for risk. There is now a large body of information indicating that, at low doses, the LNT hypothesis, along with most of the derived and auxiliary concepts, is incorrect. The use of dose as a predictor of risk needs to be re-examined and the use of dose limits, as a means of limiting risk needs to be re-evaluated. This re-evaluation could lead to large changes in radiation protection practices.

Three‐dimensional quantitative dose reduction analysis in MammoSite balloon by Monte Carlo calculations

Current treatment planning systems (TPSs) for partial breast irradiation using the MammoSite brachytherapy applicator (Cytyc Corporation, Marlborough, MA) often neglect the effect of inhomogeneity, leading to potential inaccuracies in dose distributions. Previous publications either have studied only a planar dose perturbation along the bisector of the source or have paid little attention to the anisotropy effect of the system. In the present study, we investigated the attenuation‐corrected radial dose and anisotropy functions in a form parallel to the updated American Association of Physicists in Medicine TG‐43 formalism. This work quantitatively delineates the inaccuracies in dose distributions in three‐dimensional space. Monte Carlo N‐particle transport code simulations in coupled photon–electron transport were used to quantify the changes in dose deposition and distribution caused by the increased attenuation coefficient of iodine‐based contrast solution. The source geometry was that of the VariSource wire model VS2000 (Varian Medical Systems, Palo Alto, CA). The concentration of the iodine‐based solution was varied from 5% to 25% by volume, a range recommended by the balloon's manufacturer. Balloon diameters of 4, 5, and 6 cm were simulated. Dose rates at the typical prescription line (1 cm away from the balloon surface) were determined for various polar angles. The computations showed that the dose rate reduction throughout the entire region of interest ranged from 0.64% for the smallest balloon diameter and contrast concentration to 6.17% for the largest balloon diameter and contrast concentration. The corrected radial dose function has a predominant influence on dose reduction, but the corrected anisotropy functions explain only the effect at the MammoSite system poles. By applying the corrected radial dose and anisotropy functions to TPSs, the attenuation effect can be reduced to the minimum.PACS number: 87.53.‐j

On the use of high dose rate Ir192 and Yb169 sources with the MammoSite® radiation therapy system.

Ample literature exists on the dose overestimation by commercially available treatment planning systems in MammoSite® applications using high dose rate Ir192 sources for partial breast brachytherapy as monotherapy, due to their inability to predict the dose reduction caused by the radiographic contrast solution in the balloon catheter. In this work Monte Carlo simulation is used to verify the dose rate reduction in a balloon breast applicator which does not vary significantly with distance and it is 1.2% at the prescription distance for the reference simulated geometry of 10% diluted radiographic contrast media and 2.5cm balloon radius. Based on these findings and the minimal hardening of the initially emitted photon spectrum for Ir192, a simple analytical method is proposed and shown capable for correcting dosimetry planning in clinical applications. Simulations are also performed to assess the corresponding dose reduction in applications of balloon breast applicators using high dose rate Yb169 sources that have recently become available. Results yield a far more significant and distance dependent dose reduction for Yb169 (on the order of 20% at the prescription distance for the abovementioned reference simulation geometry). This dose reduction cannot be accounted for using simple analytical methods as for Ir192 due to the significant hardening of the initially emitted Yb169 photons within the diluted radiographic contrast media. Combined with results of previous works regarding the effect of altered scatter conditions (relative to treatment planning system assumptions) on breast treatment planning accuracy, which is more pronounced for Yb169 relative to Ir192, these findings call for the amendment of dose treatment planning systems before using Yb169 high dose rate sources in balloon breast applicators.

Dose Perturbation Caused by Stents: Experiments with a Model 90Sr/90Y Source

Biological effects of intravascular brachytherapy are very sensitive to discrepancies between the prescription and the applied dose. If brachytherapy is aimed at in-stent restenosis, shielding and shadowing effects of metallic stents may change the dose distribution relative to that produced by the bare source. The development of new generations of stents inspired us to a new experimental study in this field. The effect was studied for 14 stents which we have recently encountered in clinical practice. The model source was a continuous 20-mm column of (90)Sr/(90)Y solution sealed in a 1-mm-I.D. Plexiglas capillary. The dose distribution in the Plexiglas phantom was mapped using GafChromic MD-55-2 film. The stent masses varied from 2.5 to 25 mg; the strut thicknesses, from 0.075 to 0.15 mm; and the atomic numbers of stent materials, from 24 (Cr) to 79 (Au). Dose perturbations depend on a variety of stent features. Local reduction of the mean dose rates near the reference distance (r(0) = 2 mm) varied from 11% to 47%. No simple correlation was found between these data and stent characteristics, but it seems that the atomic number of the stent material is less important than the strut thickness and mesh density. The results provide a warning that clinical indications for in-stent radiation therapy must always be confronted with another aspect of the patient's history: the kind of implanted stent. Intravascular brachytherapy using pure beta sources may be recommended only for patients "wearing" light, thin-strut stents. The presence of thick-strut stents is a contraindication for this modality, due to excessive dose perturbation.

EXAMINATION OF UNDERGROUND MINER DATA FOR RADON PROGENY SIZE REDUCTION AS CAUSE OF HIGH RADON “INVERSE” DOSE RATE EFFECT

Radon progeny measurements of particle size distributions show that at high radon levels the distributions decrease in arithmetic mean diameters (AMD)--apparently due to increased neutralization rates affected by high radon concentrations. The radon concentration threshold for this diameter decrease effect (enhanced deposition effect, EDE), decreases with increased humidity. From graded screen array measurements, in a test chamber the AMD's are known vs. radon concentration. Knowing all the necessary parameters and using the ICRP 66 human respiratory tract model, the reduction in human lung dose rate per unit radon concentration was computed to be a factor of 2.5 in the test chamber. Supported by other work, this implies that the underground miners must experience a similar lung dose reduction. A prior estimate of a factor of 4 was obtained for the miners, but with only an estimate of the crucially important mine humidity distributions. Here a re-evaluation, using more realistic values for a representative mine work area surface to volume ratio and using in-mine measured humidity data at 108 mine work locations in 15 different mines, the re-computed estimate for the EDE miner lung dose rate reduction was 4.3. Analyzing separately in-mine data in unventilated locations finds a greater transition rate of the EDE lung dose reduction. These results are affirmed by measured mine progeny to radon concentration ratios, mine deposition rates and miners' individual work shift exposures at the North Alice, Utah mine. In comparing our data with other radon "inverse" dose rate effect (IDRE) data, a significant portion of the IDRE must be from reduction in lung dose from this EDE and not from decreased lung cancer incidence per progeny lung dose. We offer the only currently plausible answer for IDRE. This does not negate any bystander effects at the very low radon concentrations where other data are least accurate.

TH‐C‐M100F‐09: Design and Dosimetry Validation for Micro‐Irradiator Using Monte Carlo Simulation

Purpose: New tools are needed to improve the effectiveness of small specimen irradiation performed in radiobiology research. This work is focused to support the design and dosimetry of a new micro‐irradiator, capable to deliver fields as small as 1 mm, using Monte Carlo simulation. Method and Materials: A miniature x‐ray source was introduced into a brass collimator with 3.0 cm in diameter and 3.0 cm in length. A pinhole of 1.0 mm in diameter in the central axis of the collimator allowed a pencil photon beam. MCNP5 was used to characterize this collimated beam. Pulse height, flux and point detector were the tallies used to find dose rate at the surface of water, energy spectrum, PDD curves, penumbra, source position effect and radiation levels at 50 cm from the source at determined collimator‐surface distance (CSD). Relative importance in cell sub‐division, population control and angular distribution biasing were used to reduce computation time and keep the statistical fluctuation within 2%. Results: The designed micro‐irradiator has the following characteristics: the photon energy fluence was proportional to the source area projected in the pinhole and the average energy was 26.6 keV. At CSD=6.0 mm, the dose rate may go up to 1 Gy/min and it decayed about 10% per millimeter depth in water. The penumbra in the beam increases about 40 μm per millimeter away from the collimator. A source displacement of 0.5 mm respect to the pinhole axis resulted in a dose rate reduction of 20%. Radiation level at 50 cm was 10−7 Sv/min. Conclusion: The characteristics of the photon beam produced by the miniature x‐ray source and the pinhole collimator system are adequate to irradiate total and partial body of zebrafish embryos, cell cultures and other small specimens used in radiobiology research.

Pulsed reduced dose-rate radiotherapy: case report

The initial management of malignant gliomas is multimodality in nature, consisting of surgery, radiation therapy and chemotherapy. However, once progression has occurred, treatment options are limited both in terms of selection and efficacy. We report a case of a 37 year-old male diagnosed with a Grade II astrocytoma initially treated with surgery and external beam radiation therapy consisting of 54 Gy delivered in 1.8 Gy fractions that subsequently progressed to a Grade IV astrocytoma. This was managed with temozolomide chemotherapy until the patient exhibited further progression. Although the patient had received prior full dose radiotherapy, he was re-treated with external beam radiotherapy delivered at a substantially reduced dose-rate. This reduction in dose-rate is obtained by delivering treatment in a series of 0.2 Gy pulses separated by 3 min time intervals, creating an apparent dose rate of 0.0667 Gy/min. The region of recurrence was treated to a dose of 50 Gy delivered using 25 daily fractions of 2.0 Gy. The patient had both a radiographic response and clinical improvement following re-irradiation using pulsed reduced dose-rate radiotherapy with no apparent acute or late neurologic toxicities at a time when other treatment options were not available. Despite delivering 104 Gy to the tumor bed and the surrounding brain parenchyma, at no time was there radiographic evidence of radiation-induced normal tissue necrosis. The radiobiologic basis for the use of pulsed reduced dose-rate external beam radiotherapy in the management of recurrent glioma patients is discussed.

Evaluating shielding effectiveness for reducing space radiation cancer risks

We discuss calculations of probability distribution functions (PDF) representing uncertainties in projecting fatal cancer risk from galactic cosmic rays (GCR) and solar particle events (SPE). The PDFs are used in significance tests for evaluating the effectiveness of potential radiation shielding approaches. Uncertainties in risk coefficients determined from epidemiology data, dose and dose–rate reduction factors, quality factors, and physics models of radiation environments are considered in models of cancer risk PDFs. Competing mortality risks and functional correlations in radiation quality factor uncertainties are included in the calculations. We show that the cancer risk uncertainty, defined as the ratio of the upper value of 95% confidence interval (CI) to the point estimate is about 4-fold for lunar and Mars mission risk projections. For short-stay lunar missions ( < 180 d ), SPEs present the most significant risk, however one that is mitigated effectively by shielding, especially for carbon composites structures with high hydrogen content. In contrast, for long duration lunar ( > 180 d ) or Mars missions, GCR risks may exceed radiation risk limits that are based on acceptable levels of risk. For example, the upper 95% CI exceeding 10% fatal risk for males and females on a Mars mission. For reducing GCR cancer risks, shielding materials are marginally effective because of the penetrating nature of GCR and secondary radiation produced in tissue by relativistic particles. At the present time, polyethylene or carbon composite shielding cannot be shown to significantly reduce risk compared to aluminum shielding based on a significance test that accounts for radiobiology uncertainties in GCR risk projection.

Comparison of radiation shielding requirements for HDR brachytherapy using and sources

has received a renewed focus lately as an alternative to sources for high dose rate (HDR) brachytherapy. Following the results of a recent work by our group which proved to be a good candidate for HDR prostate brachytherapy, this work seeks to quantify the radiation shielding requirements for HDR brachytherapy applications in comparison to the corresponding requirements for the current HDR brachytherapy standard. Monte Carlo simulation (MC) is used to obtain and broad beam transmission data through lead and concrete. Results are fitted to an analytical equation which can be used to readily calculate the barrier thickness required to achieve a given dose rate reduction. Shielding requirements for a HDR brachytherapy treatment room facility are presented as a function of distance, occupancy, dose limit, and facility workload, using analytical calculations for both and HDR sources. The barrier thickness required for is lower than that for by a factor of 4–5 for lead and 1.5–2 for concrete. Regarding HDR brachytherapy applications, the lead shielding requirements do not exceed , even in highly conservative case scenarios. This allows for the construction of a lead door in most cases, thus avoiding the construction of a space consuming, specially designed maze. The effects of source structure, attenuation by the patient, and scatter conditions within an actual treatment room on the above‐noted findings are also discussed using corresponding MC simulation results.

SU-FF-T-68: A Three-Dimensional Quantitative Dose Reduction Analysis in MammoSite Balloon Due to Radiopaque Iodine-Based Contrast Solution in Ir-192 for HDR Brachytherapy: Monte Carlo Calculations and MOSFET Measurements

Current treatment planning systems (TPS) for partial breast irradiation (PBI) using MammoSite brachytherapy applicator often neglects the effect of inhomogeneity, leading to potential inaccuracies in dose distributions. Previous publications have investigated only a planar dose perturbation along the bisector of the source. This investigation expanded to include the attenuation‐corrected radial dose and anisotropy functions and incorporates them into a treatment planning system in a form parallel to the updated AAPM TG‐43 formalism. This will delineate quantitatively the inaccuracies in dose distributions in three‐dimensional space. The changes in dosedeposition and distribution caused by increased attenuation coefficient resulted from iodine‐based contrast solution are quantified using MCNP Monte Carlo simulations in coupled photon/electron transport. The source geometry was that of the VariSource wire model VS2000. Concentration of the iodine‐based solution was varied from 5% to 25% by volume, a range recommended by the balloon's manufacturer. Balloon diameters of 4cm through 6cm were simulated. Measurements using MOSFET were done in water, using a water equivalent jig for precision positioning of balloon and instruments. Dose rates at the typical prescription line of 1cm away from the balloon surface were determined in different polar angles. According to the computations, the dose rate reduction throughout the entire region of interest ranged from 0.33% for the smallest balloon diameter and contrast concentration to a maximum of 6.29% for the largest balloon diameter and contrast. Good agreement was observed between simulations and measurements to within the acceptable error for MOSFETdosimetry (0.9%∼2.8%).

Dose rate reduction in the Pakistan Research Reactor-1 by the utilization of high-density low enriched uranium fuel

The core conversion of Pakistan Research Reactor-1 (PARR-1) in 1992 from HEU to LEU core and also the power upgradation from 5 MW to 10 MW resulted in an increase in the dose rates at the pool surface and at the beam tubes' face. This simulation study was conducted with the aim to bring the dose rates to the same levels as before conversion without compromising on the reactor performance by employing the reactor core neutronics simulation codes WIMS-D/4, CITATION, FCAP and research reactor thermal hydraulics simulation code PARET. An equilibrium core smaller than that of existing core of PARR-1 was established to provide higher neutron flux per unit power by increasing the fuel loading from existing 12.61 g 235U per plate to 32.61 g 235U per plate and also increasing the existing coolant/moderator channel width from 2.1 to 3.5 mm. This increase in the fuel loading per plate can be achieved by replacing the existing fuel of density 3.28 gU/cm 3 with fuel of density 8.5 gU/cm 3. The designed reactor core can be operated at a lower power of 5.6 MW with the existing coolant flow rate of 1000 m 3/h to achieve neutron fluxes at the irradiation sites similar to those offered in the existing PARR-1 core at 10 MW. Due to the low operating power of this core, the dose rate will again be decreased at the various sites of the core near to the previous levels. The cost for producing the neutron flux in this small core is also similar to the cost for producing the neutron flux in the existing PARR-1 core.

Radiation risk prediction and genetics: the influence of the TP53 gene in vivo.

Risk prediction and dose limits for human radiation exposure are based on the assumption that risk is proportional to total dose. However, there is concern about the appropriateness of those limits for people who may be genetically cancer prone. The TP53 gene product functions in regulatory pathways for DNA repair, cell cycle checkpoints and apoptosis, processes critical in determining ionizing radiation risk for both carcinogenesis and teratogenesis. Mice that are deficient in TP53 function are cancer prone. This review examines the influence of variations in TP53 gene activity on cancer and teratogenic risk in mice exposed to radiation in vivo, and compares those observations to the assumptions and predictions of radiation risk inherent in the existing system of radiation protection. Current assumptions concerning a linear response with dose, dose additivity, lack of thresholds and dose rate reduction factors all appear incorrect at low doses. TP53 functional variations can further modify radiation risk from either high or low doses, or risk from radiation exposures combined with other stresses, and those modifications can result in both quantitative and qualitative changes in risk.