Fast Neutron Measurement System Using Prompt Gamma Neutron Activation Solid Converter: Monte Carlo Study.

Measuring fast neutron emission around accelerators is important for purposes of environmental monitoring and radiation safety. It is necessary to detect two types of neutrons: thermal and fast. Fast neutron spectroscopy is commonly employed using a hydrogen-recoil proportional-counter; however, its threshold is 2 MeV. The aim of this study was to expand PGNA converters based on KCl to fulfil the need to detect neutron energies ranging from 0.02 MeV to 3 MeV. In our previous research, we established a counting system comprised of a large converter of KCl with a NaI(Tl) gamma radiation spectrometer. The KCl converter is efficient for fast neutron prompt gamma emission. The potassium naturally includes a radioisotope that emits 1.460 MeV gamma rays. The presence of the constant level of 1.460 MeV gamma ray counts offers an advantage, providing a stable background for the detector. The study was carried out using MCNP simulations of the counting system with a variety of PGNA converters based on KCl. We concluded that KCl mixtures combined with other elements, such as PGNA converters, demonstrated improved detection performance for fast neutron emissions. Furthermore, an explication of how to add materials to KCl to provide a proper converter for fast neutrons was introduced.

Dosimetric characterization of Elekta stereotactic cones.

PURPOSE: Dosimetry of small fields defined by stereotactic cones remains a challenging task. In this work, we report the results of commissioning measurements for the new Elekta stereotactic conical collimator system attached to the Elekta VersaHD linac and present the comparison between the measured and Monte Carlo (MC) calculated data for the 6 MV FFF beam. In addition, relative output factor (ROF) dependence on the stereotactic cone aperture variation was studied and penumbra comparison for small MLC-based and cone-based fields was performed. METHODS: Cones with nominal diameters of 15 mm, 12.5 mm, 10 mm, 7.5 mm, and 5 mm were employed in our study. Percentage depth dose (PDD), off-axis ratios (OAR), and ROF were measured using a stereotactic field diode (SFD). BEAMnrc code was used for MC simulations. RESULTS: MC calculated and measured PDDs for all cones agreed within 1%/0.5 mm, and OAR profiles agreed within 1%/0.5 mm. ROF obtained from the measurements and MC calculations agreed within 2% for all cone sizes. Small-field correction factors for the SFD detector Kfield,3 × 3 (SFD) were derived using MC calculations as a baseline and were found to be 0.982, 0.992, 0.997, 1.015, and 1.017 for the 5, 7.5, 10, 12.5, and 15-mm cones respectively. The difference in ROF was about 10%, 6%, 3.5%, 3%, 2.5%, and 2% for ±0.3 mm variations in 5, 7.5, 10, 12.5, and 15-mm cone aperture respectively. In case of single static field, cone-based collimation produced a sharper penumbra compared to the MLC-based. CONCLUSIONS: Accurate MC simulation can be an effective tool for verification of dosimetric measurements of small fields. Due to the very high sensitivity of output factors on the cone diameter, manufacture-related variations in cone size may lead to considerable variations in dosimetric characteristics of stereotactic cones.

Initial estimates of continuous positive airway pressure (CPAP) on heart volume, position and motion in patients receiving chest radiation.

To evaluate effects of Continuous Positive Airway Pressure (CPAP) on cardiac position, volume, and motion in a cohort of patients receiving thoracic radiation therapy (RT). Patients underwent 3-dimensional (3D) and 4D-computerized tomography (CT) imaging with free-breathing (FB) and CPAP for RT planning. All scans were co-registered on the treatment planning system for contouring, identification of the center of heart volume and comparative measurements of cardiac displacement, volume and motion. Heart volume (HV) was created from 3D-CT contours. Range of heart motion was estimated by creating an internal heart volume (IHV) from 4D-CT contours. Magnitude of cardiac motion (cardiac excursion) was recorded as the difference in volume between IHV and HV. Wilcoxon signed rank test and Spearmen's rank correlation coefficient were used to assess differences between variables and correlations between lung volume and heart parameters. Results from 9 patient data sets were available for this report. Compared to FB, CPAP use was associated with caudal displacement of the HV (1 cm, p < 0.008) and IHV (1.1 cm, p < 0.008). CPAP use decreased HV 6% (p < 0.008) and IHV 13% (p < 0.008). Cardiac excursion was 49% (p < 0.01) less with CPAP than with FB. CPAP use increased mean lung volume by 30% (p < 0.008) which correlated with caudal displacement of the HV (r = 0.83, p < 0.008) and IHV (r = 0.98, p < 0.001). The use of CPAP reduced cardiac motion and volume although the reduction in volume was minimal. The increase in lung volume correlated with caudal displacement of the heart. These results suggest the mechanism for achieving dosimetric benefit was obtained by cardiac displacement and decreased lung and heart motion rather than reduction of HV. Further evaluation of CPAP as a novel technique to reduce heart exposure when offering RT is warranted.

Design and dosimetry characteristics of a commercial applicator system for intra-operative electron beam therapy utilizing ELEKTA Precise accelerator.

The design concept and dosimetric characteristics of a new applicator system for intraoperative radiation therapy (IORT) are presented in this work. A new hard-docking commercial system includes polymethylmethacrylate (PMMA) applicators with different diameters and applicator end angles and a set of secondary lead collimators. A telescopic device allows changing of source-to-surface distance (SSD). All measurements were performed for 6, 9, 12 and 18 MeV electron energies. Output factors and percentage depth doses (PDD) were measured in a water phantom using a plane-parallel ion chamber. Isodose contours and radiation leakage were measured using a solid water phantom and radiographic films. The dependence of PDD on SSD was checked for the applicators with the smallest and the biggest diameters. SSD dependence of the output factors was measured. Hardcopies of PDD and isodose contours were prepared to help the team during the procedure on deciding applicator size and energy to be chosen. Applicator output factors are a function of energy, applicator size and applicator type. Dependence of SSD correction factors on applicator size and applicator type was found to be weak. The same SSD correction will be applied for all applicators in use for each energy. The radiation leakage through the applicators is clinically acceptable. The applicator system enables effective collimation of electron beams for IORT. The data presented are sufficient for applicator, energy and monitor unit selection for IORT treatment of a patient.

Dosimetric evaluation of the gantry sag effect in clinical SRS plans.

OBJECTIVES: The gantry sag introduces a largely reproducible variation of the radiation field center around the radiation isocenter. The purpose of this work is to assess the change of the dose distribution caused by the gantry sag in clinical stereotactic plans. METHODS: Brain stereotactic radio surgery treatment plans were evaluated and grouped according to radiation therapy planning technique. Group 1 was planned with volumetric arc therapy technique using coplanar arcs while Group 2-non-coplanar arcs. To simulate the gantry sag effect in the treatment planning system, the original plan segments were divided into four groups according to corresponding gantry angles: upper, lower, left and right quadrants. Then, isocenter of the upper quadrant was shifted towards "Gun", isocenter of the lower quadrant was shifted towards "Target" and isocenter of the left and right quadrants was left at its original positions. The magnitude of the shift was 0.5, 1 and 1.5 mm in each direction, corresponding to 1, 2 and 3 mm of gantry isocenter diameter. To estimate the changes in dose distribution between the original and modified plans, the following dose-volume metrics were tracked: planning target volume (PTV) coverage (V99;PTV), hotspot dose in PTV (DPTV;0.015cc)), coldspot doses in PTV (DPTV;(V-0.015cc)), conformity and gradient indexes, maximum point doses in organs at risk (OAR, DOAR;0.015cc) and outside PTV (DoutsidePTV;0,015cc). For the second group of patients volume of brain receiving 12 Gy (V12Gy) was analyzed. RESULTS: The mean relative change of all metrics was within -2%/+2.5% range for both techniques for isocenter diameter up to 2 mm. Isocenter diameter of 3 mm causes significant changes in V99;PTV, conformity and gradient indexes for coplanar, and additionally in DPTV;(V-0.015cc) for non-coplanar plans. The largest increase of maximum point dose in OAR was 1.1, 2.1 and 3.2% for ±0.5, ±1 and ±1.5 mm shift, respectively. CONCLUSION: The results demonstrate dosimetric effect of gantry sag depending on its value. By itself, the gantry sag effect does not produce clinically perceptible dose changes neither for PTV nor for OARs for shift ranges up to ±1 mm, both for coplanar and non-coplanar delivery techniques. For the larger gantry sag magnitude dosimetric changes can become significant, especially for non-coplanar plans. It indicates that 2 mm diameter tolerance of gantry isocenter postulated in TG-142 is reasonable, as variations in excess of this value start to affect the overall dosimetric and spatial uncertainty. ADVANCES IN KNOWLEDGE: Dosimetric evaluation of the gantry sag effect in clinical stereotactic radio surgery plans is presented for the first time.

Reconstruction of the electron source intensity distribution of a clinical linear accelerator using in-air measurements and a genetic algorithm.

BACKGROUND AND PURPOSE: The electron source intensity distribution of a clinical linear accelerator has a great influence on the calculation of output factors for small radiation fields where source occlusion by the collimating devices takes place. The purpose of this study was to present a new method for the electron source reconstruction problem. MATERIALS AND METHODS: The measurements were performed in-air using diode and 6 MV 1 × 1 cm2 photon field in flattening filter-free mode. In Monte Carlo simulation, an electron target area was divided into a number of square subsources. Then, the in-air doses in 2D silicon chip array were calculated individually from each subsource. A genetic algorithm search was applied in order to determine the optimal weight factors for all subsources that provide the best agreement between simulated and measured doses. RESULTS: It was found that the reconstructed electron source intensity from a clinical linear accelerator has the two-dimensional elliptical double Gaussian distribution. The source intensity distribution consisted of two intensity components along the in-plane (x) and cross-plane (y) directions characterized by full width half-maximum (FWHM): FWHMx1 = 0.27 cm, FWHMx2 = 0.08 cm, FWHMy1 = 0.24 cm, FWHMy2 = 0.06 cm, where broader components are 81% and 53% of the total intensity along × and y axis respectively. CONCLUSIONS: The obtained results demonstrated an elliptical double Gaussian intensity distribution of the incident electron source. We anticipate that the proposed method has universal applications independent of the type of linear accelerator, modality or energy.

DNA Topoisomerase IB as a Potential Ionizing Radiation Exposure and Dose Biomarker.

In radiation exposure scenarios where physical dosimetry is absent or inefficient, dose estimation must rely on biological markers. A reliable biomarker is of utmost importance in correlating biological system changes with radiation exposure. Human DNA topoisomerase ІB (topo І) is a ubiquitous nuclear enzyme, which is involved in essential cellular processes, including transcription, DNA replication and DNA repair, and is the target of anti-cancer drugs. It has been shown that the cellular activity of this enzyme is significantly sensitive to various DNA lesions, including radiation-induced DNA damages. Therefore, we investigated the potential of topo I as a biomarker of radiation exposure and dose. We examined the effect of exposure of different human cells to beta, X-ray and gamma radiation on the cellular catalytic activity of topo I. The results demonstrate a significant reduction in the DNA relaxation activity of topo I after irradiation and the level of the reduction was correlated with radiation dose. In normal human peripheral blood lymphocytes, exposure for 3 h to an integral dose of 0.065 mGy from tritium reduced the enzyme activity to less than 25%. In MG-63 osteoblast-like cells and in human pulmonary fibroblast (HPF) cells exposed to gamma radiation from a 60Co source (up to 2 Gy) or to X rays (up to 2.8 Gy), a significant decrease in topo I catalytic activity was also observed. We observed that the enzyme-protein level was not altered but was partially posttranslational modified by ADP-ribosylation of the enzyme protein that is known to reduce topo I activity. The results of this study suggest that the decrease in the cellular topo I catalytic activity after low-dose exposure to different radiation types may be considered as a novel biomarker of ionizing radiation exposure and dose. For this purpose, a suitable ELISA-based method for large-scale analysis of radiation-induced topo I modification is under development.

Film dosimetry calibration method for pulsed-dose-rate brachytherapy with an 192Ir source.

192Ir sources have been widely used in clinical brachytherapy. An important challenge is to perform dosimetric measurements close to the source despite the steep dose gradient. The common, inexpensive silver halide film is a classic two-dimensional integrator dosimeter and would be an attractive solution for these dose measurements. The main disadvantage of film dosimetry is the film response to the low-energy photon. Since the photon energy spectrum is known to vary with depth, the sensitometric curves are expected to be dependent on depth. The purpose of this study is to suggest a correction method for silver halide film dosimetry that overcomes the response changes at different depths. Sensitometric curves have been obtained at different depths with verification film near a 1 Ci 192Ir pulsed-dose-rate source. The depth dependence of the film response was observed and a correction function was established. The suitability of the method was tested through measurement of the radial dose profile and radial dose function. The results were compared to Monte Carlo-simulated values according to the TG43 formalism. Monte Carlo simulations were performed separately for the beta and gamma source emissions, using the EGS4 code system, including the low-energy photon and electron transport optimization procedures. The beta source emission simulation showed that the beta dose contribution could be neglected and therefore the film-depth dependence could not be attributed to this part of the source radioactivity. The gamma source emission simulations included photon-spectra collection at several depths. The results showed a depth-dependent softening of the photon spectrum that can explain the film-energy dependence.

Highly accurate prediction of specific activity using deep learning.

Building materials can contain elevated levels of naturally occurring radioactive materials (NORM), in particular Ra-226, Th-232 and K-40. Safety standards, such as IAEA Safety Standards Series No. GSR Part 3, dictate particular activities that must be fulfilled to ensure adequate safety. Traditional methods include spectral analysis of material samples measured by a HPGe detector then processed to calculate the specific activity of the NORM in Bq/Kg with 1.96 σ uncertainty. This paper describes a new method that pre-processes the raw spectrum then feeds the result into a set of pre-trained neural networks, thus generating the required specific radionuclide activity as well as the 1.96 σ uncertainty.

Selective Shielding of Bone Marrow: An Approach to Protecting Humans from External Gamma Radiation.

The current feasibility of protecting emergency responders through bone marrow selective shielding is highlighted in the recent OECD/NEA report on severe accident management. Until recently, there was no effective personal protection from externally penetrating gamma radiation. In Chernobyl, first-responders wore makeshift lead sheeting, whereas in Fukushima protective equipment from gamma radiation was not available. Older protective solutions that use thin layers of shielding over large body surfaces are ineffective for energetic gamma radiation. Acute exposures may result in Acute Radiation Syndrome where the survival-limiting factor up to 10 Gy uniform, homogeneous exposure is irreversible bone marrow damage. Protracted, lower exposures may result in malignancies of which bone marrow is especially susceptible, being compounded by leukemia's short latency time. This highlights the importance of shielding bone marrow for preventing both deterministic and stochastic effects. Due to the extraordinary regenerative potential of hematopoietic stem cells, to effectively prevent the deterministic effects of bone marrow exposure, it is sufficient to protect only a small fraction of this tissue. This biological principle allows for a new class of equipment providing unprecedented attenuation of radiation to select marrow-rich regions, deferring the hematopoietic sub-syndrome of Acute Radiation Syndrome to much higher doses. As approximately half of the body's active bone marrow resides within the pelvis region, shielding this area holds great promise for preventing the deterministic effects of bone marrow exposure and concomitantly reducing stochastic effects. The efficacy of a device that selectively shields this region and other radiosensitive organs in the abdominal area is shown here.

STUDY OF THE SUITABILITY OF ISRAELI HOUSEHOLD SALT FOR RETROSPECTIVE DOSIMETRY.

The first results of an in-depth evaluation of the practical potential of common household Israeli salt as a retrospective dosemeter in the event of a nuclear accident or terror attack are presented. Ten brands of salt were investigated with emphasis on four of the bestselling brands that constitute 76 % of the total consumer market. Eight of the ten brands show similar glow curves with two main glow peaks at maximum temperatures of ∼176°C and ∼225°C measured at a heating rate of 1°C s(-1) Chemical analysis of three major brands indicates substantial impurity levels of 200-500 ppm of Ca, K, Mg and S and significant differences of additional ppm trace impurities, which lead to an ∼50 % difference in the TL response of the three major brands. Fading in the dark is in significant but under room light is of the order of 35 % per day. The dose response is linear/supralinear with the threshold of supralinearity at ∼0.01 Gy reaching maximum value of ∼4 at 0.5-1 Gy for two of the major brands. The precision of repeated measurements is ∼10 % (1 SD), but the accuracy of dose assessment under field conditions requires further study.

Basic considerations for Monte Carlo calculations in soil.

Monte Carlo codes are extensively used for probabilistic simulations of various physical systems. These codes are widely used in calculations of neutron and gamma ray transport in soil for radiation shielding, soil activation by neutrons, well logging industry, and in simulations of complex nuclear gauges for in soil measurements. However, these calculations are complicated by the diversity of soils in which the proportions of solid, liquid and gas vary considerably together with extensive variations in soil elemental composition, morphology, and density. Nevertheless use of these codes requires knowledge of the elemental composition and density of the soil and its physical characteristics as input information for performing these calculations. It is shown that not always all of the soil parameters are critical but depend on the objectives of the calculations. An approach for identifying soil elemental composition and some simplifying assumptions for implementing the transport codes are presented.

Novel high dose rate lip brachytherapy technique to improve dose homogeneity and reduce toxicity by customized mold.

PURPOSE/OBJECTIVES: The purpose of this study is to describe a novel brachytherapy technique for lip Squamous Cell Carcinoma, utilizing a customized mold with embedded brachytherapy sleeves, which separates the lip from the mandible, and improves dose homogeneity. MATERIALS AND METHODS: Seven patients with T2 lip cancer treated with a "sandwich" technique of High Dose Rate (HDR) brachytherapy to the lip, consisting of interstitial catheters and a customized mold with embedded catheters, were reviewed for dosimetry and outcome using 3D planning. Dosimetric comparison was made between the "sandwich" technique to "classic" - interstitial catheters only plan. We compared dose volume histograms for Clinical Tumor Volume (CTV), normal tissue "hot spots" and mandible dose. We are reporting according to the ICRU 58 and calculated the Conformal Index (COIN) to show the advantage of our technique. RESULTS: The seven patients (ages 36-81 years, male) had median follow-up of 47 months. Four patients received Brachytherapy and External Beam Radiation Therapy, 3 patients received brachytherapy alone. All achieved local control, with excellent esthetic and functional results. All patients are disease free. The Customized Mold Sandwich technique (CMS) reduced the high dose region receiving 150% (V150) by an average of 20% (range 1-47%), The low dose region (les then 90% of the prescribed dose) improved by 73% in average by using the CMS technique. The COIN value for the CMS was in average 0.92 as opposed to 0.88 for the interstitial catheter only. All differences (excluding the low dose region) were statistically significant. CONCLUSION: The CMS technique significantly reduces the high dose volume and increases treatment homogeneity. This may reduce the potential toxicity to the lip and adjacent mandible, and results in excellent tumor control, cosmetic and functionality.