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Biomedical subjects

Michael G Mitch

Publications and source records attributed to Michael G Mitch.

6 recordsLinked to original sources
PubMed2006

A fibre optic scintillator dosemeter for absorbed dose measurements of low-energy X-ray-emitting brachytherapy sources.

A newly developed dosemeter using a 0.5 mm diameter x 0.5 mm thick cylindrical plastic scintillator coupled to the end of a fibre optic cable is capable of measuring the absorbed dose rate in water around low-activity, low-energy X-ray emitters typically used in prostate brachytherapy. Recent tests of this dosemeter showed that it is possible to measure the dose rate as a function of distance in water from 2 to 30 mm of a (103)Pd source of air-kerma strength 3.4 U (1 U = 1 microGy m(2) h(-1)), or 97 MBq (2.6 mCi) apparent activity, with good signal-to-noise ratio. The signal-to-noise ratio is only dependent on the integration time and background subtraction. The detector volume is enclosed in optically opaque, nearly water-equivalent materials so that there is no polar response other than that due to the shape of the scintillator volume chosen, in this case cylindrical. The absorbed dose rate very close to commercial brachytherapy sources can be mapped in an automated water phantom, providing a 3-D dose distribution with sub-millimeter spatial resolution. The sensitive volume of the detector is 0.5 mm from the end of the optically opaque waterproof housing, enabling measurements at very close distances to sources. The sensitive detector electronics allow the measurement of very low dose rates, as exist at centimeter distances from these sources. The detector is also applicable to mapping dose distributions from more complex source geometries such as eye applicators for treating macular degeneration.

Brachytherapy↗
PubMed2005

Recommendations of the American Association of Physicists in Medicine regarding the impact of implementing the 2004 task group 43 report on dose specification for 103Pd and 125I interstitial brachytherapy.

In March 2004, the recommendations of the American Association of Physicists in Medicine (AAPM) on the interstitial brachytherapy dosimetry using 125I and 103Pd were reported in Medical Physics [TG-43 Update: Rivard et al., 31, 633-674 (2004)]. These recommendations include some minor changes in the dose-calculation formalism and a major update of the dosimetry parameters for eight widely used interstitial brachytherapy sources. A full implementation of these recommendations could result in unintended changes in delivered dose without corresponding revisions in the prescribed dose. Because most published clinical experience with permanent brachytherapy is based upon two widely used source models, the 125I Model 6711 and 103Pd Model 200 sources, in this report we present an analysis of the dosimetric impact of the 2004 TG-43 dosimetry parameters on the history of dose delivery for these two source models. Our analysis indicates that the currently recommended prescribed dose of 125 Gy for Model 200 103Pd implants planned using previously recommended dosimetry parameters [AAPM 103Pd dose prescription: Williamson et al., Med. Phys. 27, 634-642 (2000)] results in a delivered dose of 120 Gy according to dose calculations based on the 2004 TG-43 update. Further, delivered doses prior to October 1997 varied from 113 to 119 Gy for a prescribed dose of 115 Gy compared to 124 Gy estimated by the AAPM 2000 report. For 125I implants using Model 6711 seeds, there are no significant changes (less than 2%). Practicing physicians should take these results into account when selecting the clinically appropriate prescribed dose for 103Pd interstitial implant patients following implementation of the 2004 TG-43 update dose-calculation recommendations. The AAPM recommends that the radiation oncology community review this report and consider whether the currently recommended dose level (125 Gy) needs to be revised.

Anisotropy↗
PubMed2004

Update of AAPM Task Group No. 43 Report: A revised AAPM protocol for brachytherapy dose calculations.

Since publication of the American Association of Physicists in Medicine (AAPM) Task Group No. 43 Report in 1995 (TG-43), both the utilization of permanent source implantation and the number of low-energy interstitial brachytherapy source models commercially available have dramatically increased. In addition, the National Institute of Standards and Technology has introduced a new primary standard of air-kerma strength, and the brachytherapy dosimetry literature has grown substantially, documenting both improved dosimetry methodologies and dosimetric characterization of particular source models. In response to these advances, the AAPM Low-energy Interstitial Brachytherapy Dosimetry subcommittee (LIBD) herein presents an update of the TG-43 protocol for calculation of dose-rate distributions around photon-emitting brachytherapy sources. The updated protocol (TG-43U1) includes (a) a revised definition of air-kerma strength; (b) elimination of apparent activity for specification of source strength; (c) elimination of the anisotropy constant in favor of the distance-dependent one-dimensional anisotropy function; (d) guidance on extrapolating tabulated TG-43 parameters to longer and shorter distances; and (e) correction for minor inconsistencies and omissions in the original protocol and its implementation. Among the corrections are consistent guidelines for use of point- and line-source geometry functions. In addition, this report recommends a unified approach to comparing reference dose distributions derived from different investigators to develop a single critically evaluated consensus dataset as well as guidelines for performing and describing future theoretical and experimental single-source dosimetry studies. Finally, the report includes consensus datasets, in the form of dose-rate constants, radial dose functions, and one-dimensional (1D) and two-dimensional (2D) anisotropy functions, for all low-energy brachytherapy source models that met the AAPM dosimetric prerequisites [Med. Phys. 25, 2269 (1998)] as of July 15, 2001. These include the following 125I sources: Amersham Health models 6702 and 6711, Best Medical model 2301, North American Scientific Inc. (NASI) model MED3631-A/M, Bebig/Theragenics model I25.S06, and the Imagyn Medical Technologies Inc. isostar model IS-12501. The 103Pd sources included are the Theragenics Corporation model 200 and NASI model MED3633. The AAPM recommends that the revised dose-calculation protocol and revised source-specific dose-rate distributions be adopted by all end users for clinical treatment planning of low energy brachytherapy interstitial sources. Depending upon the dose-calculation protocol and parameters currently used by individual physicists, adoption of this protocol may result in changes to patient dose calculations. These changes should be carefully evaluated and reviewed with the radiation oncologist preceding implementation of the current protocol.

Air↗
PubMed2004

Evaluation of the new cesium-131 seed for use in low-energy x-ray brachytherapy.

Characterization measurements and calculations were performed on a new medical seed developed by IsoRay Inc. in Richland, Washington, that utilizes the short-lived isotope 131Cs. This model has recently received FDA 510(k) clearance. The objective of this work was to characterize the dosimetric properties of the new seed according to the AAPM Task Group 43 recommendations. Cesium-131 is a low-energy x-ray emitter, with the most prominent peaks in the 29 keV to 34 keV region. The intended application is brachytherapy for treating cancers in prostate, breast, head and neck, lung, and pancreas. The evaluations performed included air-kerma strength, radial dose function, anisotropy in phantom, half-life, energy spectra, and internal activity. The results indicate the CS-1 seeds have a dose-rate constant of 0.915 cGy hr(-1) U(-1) in water, dose penetration characteristics similar to 125I and 103Pd, anisotropy function values on the order of 0.71 at short distances and small angles, and an average anisotropy factor of 0.964. The overall dosimetric characteristics are similar to 125I and 103Pd seeds with the exception of half-life, which is 9.7 days, as compared to 17 days for 103Pd and 60 days for 125I. The shorter half-life may offer significant advantages in biological effectiveness.

Air↗
PubMed2002

Mapping the distribution of (90)Sr in teeth with a photostimulable phosphor imaging detector.

The present communication describes the technical aspects of the first application of an imaging plate for visualization of (90)Sr deposited in human teeth. The teeth were obtained from Techa River area residents who were exposed as a result of releases of radioactivity into the Techa River by the first Soviet nuclear plant Mayak in the early 1950s. The investigations form the basis for an experimental procedure for accurate mapping of the distribution of (90)Sr in teeth with an imaging plate. This new method can be used as an individual indicator of radionuclide intake. Its advantages are its high sensitivity (0.02 Bq/g mm(-2) of (90)Sr), it ability to examine small detectable cross-sectional areas of dental tissue (dentin) contaminated with (90)Sr (from 0.01 mm(2)), the nondestructive method of analysis, and the simplicity of use. The combined application of this method with EPR tooth biodosimetry can provide more accurate dose reconstruction and may lead to more effective radiation risk assessment.

Calibration↗