David C. Medich

Changes in Occupational Radiation Exposures after Incorporation of a Real-time Dosimetry System in the Interventional Radiology Suite

AbstractA statistical pilot study was retrospectively performed to analyze potential changes in occupational radiation exposures to Interventional Radiology (IR) staff at Lawrence General Hospital after implementation of the i2 Active Radiation Dosimetry System (Unfors RaySafe Inc, 6045 Cochran Road Cleveland, OH 44139-3302). In this study, the monthly OSL dosimetry records obtained during the eight-month period prior to i2 implementation were normalized to the number of procedures performed during each month and statistically compared to the normalized dosimetry records obtained for the 8‐mo period after i2 implementation. The resulting statistics included calculation of the mean and standard deviation of the dose equivalences per procedure and included appropriate hypothesis tests to assess for statistically valid differences between the pre and post i2 study periods. Hypothesis testing was performed on three groups of staff present during an IR procedure: The first group included all members of the IR staff, the second group consisted of the IR radiologists, and the third group consisted of the IR technician staff. After implementing the i2 active dosimetry system, participating members of the Lawrence General IR staff had a reduction in the average dose equivalence per procedure of 43.1% ± 16.7% (p = 0.04). Similarly, Lawrence General IR radiologists had a 65.8% ± 33.6% (p=0.01) reduction while the technologists had a 45.0% ± 14.4% (p=0.03) reduction.

A brachytherapy photon radiation quality index Q(BT) for probe-type dosimetry.

INTRODUCTION In photon brachytherapy (BT), experimental dosimetry is needed to verify treatment plans if planning algorithms neglect varying attenuation, absorption or scattering conditions. The detectors response is energy dependent, including the detector material to water dose ratio and the intrinsic mechanisms. The local mean photon energy E¯(r) must be known or another equivalent energy quality parameter used. We propose the brachytherapy photon radiation quality indexQ(BT)(E¯), to characterize the photon radiation quality in view of measurements of distributions of the absorbed dose to water, Dw, around BT sources. MATERIALS AND METHODS While the external photon beam radiotherapy (EBRT) radiation quality index Q(EBRT)(E¯)=TPR10(20)(E¯) is not applicable to BT, the authors have applied a novel energy dependent parameter, called brachytherapy photon radiation quality index, defined as Q(BT)(E¯)=Dprim(r=2cm,θ0=90°)/Dprim(r0=1cm,θ0=90°), utilizing precise primary absorbed dose data, Dprim, from source reference databases, without additional MC-calculations. RESULTS AND DISCUSSION For BT photon sources used clinically, Q(BT)(E¯) enables to determine the effective mean linear attenuation coefficient μ¯(E) and thus the effective energy of the primary photons Eprim(eff)(r0,θ0) at the TG-43 reference position Pref(r0=1cm,θ0=90°), being close to the mean total photon energy E¯tot(r0,θ0). If one has calibrated detectors, published E¯tot(r) and the BT radiation quality correction factor [Formula: see text] for different BT radiation qualities Q and Q0, the detectors response can be determined and Dw(r,θ) measured in the vicinity of BT photon sources. CONCLUSIONS This novel brachytherapy photon radiation quality indexQ(BT) characterizes sufficiently accurate and precise the primary photons penetration probability and scattering potential.

Comparison of methods to estimate water‐equivalent diameter for calculation of patient dose

Abstract Modern CT systems seek to evaluate patient‐specific dose by converting the CT dose index generated during a procedure to a size‐specific dose estimate using conversion factors that are related to patient attenuation properties. The most accurate way to measure patient attenuation is to evaluate a full‐field‐of‐view reconstruction of the whole scan length and calculating the true water‐equivalent diameter (D w) using CT numbers; however, due to time constraints, less accurate methods to estimate D w using patient geometry measurements are used more widely. In this study we compared the accuracy of D w values calculated from three different methods across 35 sample scans and compared them to the true D w. These three estimation methods were: measurement of patient lateral dimension from a pre‐scan localizer radiograph; measurement of the sum of anteroposterior and lateral dimensions from a reconstructed central slice; and using CT numbers from a central slice only. Using the localizer geometry method, 22 out of 35 (62%) samples estimated D w within 20% of the true value. The middle slice attenuation and geometry methods gave estimations within the 20% margin for all 35 samples.

Dosimetric characterization of the M-15 high-dose-rate Iridium-192 brachytherapy source using the AAPM and ESTRO formalism

The Source Production & Equipment Co. (SPEC) model M−15 is a new Iridium−192 brachytherapy source model intended for use as a temporary high‐dose‐rate (HDR) brachytherapy source for the Nucletron microSelectron Classic afterloading system. The purpose of this study is to characterize this HDR source for clinical application by obtaining a complete set of Monte Carlo calculated dosimetric parameters for the M‐15, as recommended by AAPM and ESTRO, for isotopes with average energies greater than 50 keV. This was accomplished by using the MCNP6 Monte Carlo code to simulate the resulting source dosimetry at various points within a pseudoinfinite water phantom. These dosimetric values next were converted into the AAPM and ESTRO dosimetry parameters and the respective statistical uncertainty in each parameter also calculated and presented. The M−15 source was modeled in an MCNP6 Monte Carlo environment using the physical source specifications provided by the manufacturer. Iridium−192 photons were uniformly generated inside the iridium core of the model M−15 with photon and secondary electron transport replicated using photoatomic cross‐sectional tables supplied with MCNP6. Simulations were performed for both water and air/vacuum computer models with a total of 4×109 sources photon history for each simulation and the in‐air photon spectrum filtered to remove low‐energy photons below δ=10%keV. Dosimetric data, including D(r,θ),gL(r),F(r,θ),Φan(r), and φ¯an, and their statistical uncertainty were calculated from the output of an MCNP model consisting of an M−15 source placed at the center of a spherical water phantom of 100 cm diameter. The air kerma strength in free space, SK, and dose rate constant, Λ, also was computed from a MCNP model with M−15 Iridium−192 source, was centered at the origin of an evacuated phantom in which a critical volume containing air at STP was added 100 cm from the source center. The reference dose rate, D˙(r0,θ0)≡D˙(1cm,π/2), is found to be 4.038±0.064 cGy mCi−1 h−1. The air kerma strength, SK, is reported to be 3.632±0.086 cGy cm2 mCi−1 g−1, and the dose rate constant, Λ, is calculated to be 1.112±0.029 cGy h−1 U−1. The normalized dose rate, radial dose function, and anisotropy function with their uncertainties were computed and are represented in both tabular and graphical format in the report. A dosimetric study was performed of the new M−15 Iridium−192 HDR brachytherapy source using the MCNP6 radiation transport code. Dosimetric parameters, including the dose‐rate constant, radial dose function, and anisotropy function, were calculated in accordance with the updated AAPM and ESTRO dosimetric parameters for brachytherapy sources of average energy greater than 50 keV. These data therefore may be applied toward the development of a treatment planning program and for clinical use of the source. PACS numbers: 87.56.bg, 87.53.JwThe Source Production & Equipment Co. (SPEC) model M-15 is a new Iridium-192 brachytherapy source model intended for use as a temporary high-dose-rate (HDR) brachytherapy source for the Nucletron microSelectron Classic afterloading system. The purpose of this study is to characterize this HDR source for clinical application by obtaining a complete set of Monte Carlo calculated dosimetric parameters for the M-15, as recommended by AAPM and ESTRO, for isotopes with average energies greater than 50 keV. This was accomplished by using the MCNP6 Monte Carlo code to simulate the resulting source dosimetry at various points within a pseudoinfinite water phantom. These dosimetric values next were converted into the AAPM and ESTRO dosimetry parameters and the respective statistical uncertainty in each parameter also calculated and presented. The M-15 source was modeled in an MCNP6 Monte Carlo environment using the physical source specifications provided by the manufacturer. Iridium-192 photons were uniformly generated inside the iridium core of the model M-15 with photon and secondary electron transport replicated using photoatomic cross-sectional tables supplied with MCNP6. Simulations were performed for both water and air/vacuum computer models with a total of 4×109 sources photon history for each simulation and the in-air photon spectrum filtered to remove low-energy photons below δ=10%keV. Dosimetric data, including D(r,θ),gL(r),F(r,θ),Φan(r), and φ¯an, and their statistical uncertainty were calculated from the output of an MCNP model consisting of an M-15 source placed at the center of a spherical water phantom of 100 cm diameter. The air kerma strength in free space, SK, and dose rate constant, Λ, also was computed from a MCNP model with M-15 Iridium-192 source, was centered at the origin of an evacuated phantom in which a critical volume containing air at STP was added 100 cm from the source center. The reference dose rate, D˙(r0,θ0)≡D˙(1cm,π/2), is found to be 4.038±0.064 cGy mCi-1 h-1. The air kerma strength, SK, is reported to be 3.632±0.086 cGy cm2 mCi-1 g-1, and the dose rate constant, Λ, is calculated to be 1.112±0.029 cGy h-1 U-1. The normalized dose rate, radial dose function, and anisotropy function with their uncertainties were computed and are represented in both tabular and graphical format in the report. A dosimetric study was performed of the new M-15 Iridium-192 HDR brachytherapy source using the MCNP6 radiation transport code. Dosimetric parameters, including the dose-rate constant, radial dose function, and anisotropy function, were calculated in accordance with the updated AAPM and ESTRO dosimetric parameters for brachytherapy sources of average energy greater than 50 keV. These data therefore may be applied toward the development of a treatment planning program and for clinical use of the source. PACS numbers: 87.56.bg, 87.53.Jw.

Variation in the calibrated response of LiF, Al2O3, and silicon dosimeters when used for in-phantom measurements of source photons with energies between 30 KeV AND 300 KeV.

AbstractThe MCNP5 radiation transport code was used to quantify changes in the absorbed dose conversion factor for LiF, Al2O3, and silicon-based electronic dosimeters calibrated in-air using standard techniques and summarily used to measure absorbed dose to water when placed in a water phantom. A mono-energetic photon source was modeled at energies between 30 keV and 300 keV for a point-source placed at the center of a water phantom, a point-source placed at the surface of the phantom, and for a 10-cm radial field geometry. Dosimetric calculations were obtained for water, LiF, Al2O3, and silicon at depths of 0.2 cm and 10 cm from the source. These results were achieved using the MCNP5 *FMESH photon energy-fluence tally, which was coupled with the appropriate DE/DF card for each dosimetric material studied to convert energy-fluence into the absorbed dose. The dosimeter’s absorbed dose conversion factor was calculated as a ratio of the absorbed dose to water to that of the dosimeter measured at a specified phantom depth. The dosimeter’s calibration value also was obtained. Based on these results, the absorbed dose conversion factor for a LiF dosimeter was found to deviate from its calibration value by up to 9%, an Al2O3 dosimeter by 43%, and a silicon dosimeter by 61%. These data therefore can be used to obtain LiF, Al2O3, and silicon dosimeter correction factors for mono-energetic and poly-energetic sources at measurement depths up to 10 cm under the irradiation geometries investigated herein.

Feasibility of Small Animal Anatomical and Functional Imaging with Neutrons: A Monte Carlo Simulation Study

A novel technique is presented for obtaining a single in-vivo image containing both functional and anatomical information in a small animal model such as a mouse. This technique, which incorporates appropriate image neutron-scatter rejection and uses a neutron opaque contrast agent, is based on neutron radiographic technology and was demonstrated through a series of Monte Carlo simulations. With respect to functional imaging, this technique can be useful in biomedical and biological research because it could achieve a spatial resolution orders of magnitude better than what presently can be achieved with current functional imaging technologies such as nuclear medicine (PET, SPECT) and fMRI. For these studies, Monte Carlo simulations were performed with thermal (0.025 eV) neutrons in a 3 cm thick phantom using the MCNP5 simulations software. The goals of these studies were to determine: 1) the extent that scattered neutrons degrade image contrast; 2) the contrasts of various normal and diseased tissues under conditions of complete scatter rejection; 3) the concentrations of Boron-10 and Gadolinium-157 required for contrast differentiation in functional imaging; and 4) the efficacy of collimation for neutron scatter image rejection. Results demonstrate that with proper neutron-scatter rejection, a neutron fluence of 2 ×107 n/cm2 will provide a signal to noise ratio of at least one ( S/N ≥ 1) when attempting to image various 300 μm thick tissues placed in a 3 cm thick phantom. Similarly, a neutron fluence of only 1 ×107 n/cm2 is required to differentiate a 300 μm thick diseased tissue relative to its normal tissue counterpart. The utility of a B-10 contrast agent was demonstrated at a concentration of 50 μg/g to achieve S/N ≥ 1 in 0.3 mm thick tissues while Gd-157 requires only slightly more than 10 μg/g to achieve the same level of differentiation. Lastly, neutron collimator with an L/D ratio from 50 to 200 were calculated to provide appropriate scatter rejection for thick tissue biological imaging with neutrons.

SU‐GG‐T‐278: Clinical Dosimetry of Photon Sources Used in Brachytherapy: Need for ISO Standardization, Based on and Extending the AAPM TG‐43U1 Formalism by Calibration in Terms of Absorbed Dose to Water

Purpose: By calibratingbrachytherapy (BT) sources to the TG‐43U1 reference position at 1cm in water, named the nominal absorbed dose‐rate to water, Ḋ w ,1, accuracy and precision for patient treatment will be increased. Traceability must be provided to Ḋ w ,1 ‐primary‐standards; which soon become available. Methods and Materials: Efforts have been made in discussions with fellow scientists from many countries, by reviewing concerned literature, and similarities are drawn from documents (e.g. extending TG‐43U1). Results: From the study of primary photon interaction mechanisms, a need was recognized to classify BT‐photon radiation qualities as: high‐energy >100keV, medium energy 40keV to 100keV, and low energy <40keV. It was further recognized that Monte Carlo simulation based primary and scatter dose separation provides characterization for radionuclide BT‐sources and electronic X‐ray BT‐sources, for BT‐detectors and BT‐phantoms. A need was felt for developing reference data‐sets and calibration data of BT‐sources, ‐detectors and ‐phantoms, through which the end‐user medical‐physicist could critically evaluate the data supplied by the manufacturer by using established methods, prior to clinical application. Plastic scintillators appeared to be a choice of detector as future high precision transfer‐standard and high resolution, fast, direct reading dosemeter for detailed quality assurance of BT‐sources, ‐software, ‐planning, and ‐verification. Conclusion: There is the need for international standardization of clinical dosimetry in photon radiation brachytherapy similar to that described in ISO‐21439 (2009) for beta radiation BT‐sources. Based on AAPM TG‐43U1, this planned ISO‐standard will provide guidance for clinical BT‐dosimetry in terms of absorbed dose to water and for estimating the uncertainty of this quantity. Most standardized procedures can be given by referring to AAPM‐ and ESTRO‐reports. Recommendations will be prepared to replace the reference air‐kerma‐rate (air‐kerma strength) by the nominal absorbed dose‐rate to water as basic dosimetric quantity, to increase brachytherapy accuracy and precision and to become consistent with external beam radiotherapy.