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Biomedical subjects

M J Rivard

Publications and source records attributed to M J Rivard.

At least 19 recordsLinked to original sources
PubMed2005

Shielding evaluation of a medical linear accelerator vault in preparation for installing a high-dose rate 252Cf remote afterloader.

In support of the effort to begin high-dose rate 252Cf brachytherapy treatments at Tufts-New England Medical Center, the shielding capabilities of a clinical accelerator vault against the neutron and photon emissions from a 1.124 mg 252Cf source were examined. Outside the clinical accelerator vault, the fast neutron dose equivalent rate was below the lower limit of detection of a CR-39 etched track detector and below 0.14 +/- 0.02 muSv h(-1) with a proportional counter, which is consistent, within the uncertainties, with natural background. The photon dose equivalent rate was also measured to be below background levels (0.1 muSv h(-1)) using an ionisation chamber and an optically stimulated luminescence dosemeter. A Monte Carlo simulation of neutron transport through the accelerator vault was performed to validate measured values and determine the thermal-energy to low-energy neutron component. Monte Carlo results showed that the dose equivalent rate from fast neutrons was reduced by a factor of 100,000 after attenuation through the vault wall, and the thermal-energy neutron dose equivalent rate would be an additional factor of 1000 below that of the fast neutrons. Based on these findings, the shielding installed in this facility is sufficient for the use of at least 5.0 mg of 252Cf.

Brachytherapy↗
PubMed2001

A discretized approach to determining TG-43 brachytherapy dosimetry parameters: case study using Monte Carlo calculations for the MED3633 103Pd source.

It is of interest to discern the energy-dependence of American Association of Physicists in Medicine (AAPM) TG-43 brachytherapy dosimetry parameters. Using Monte Carlo calculation geometry and techniques (MCNP), dependence of these parameters was calculated as a function of photon energy, in general, and for the MED3633 103Pd source using a discretized approach. Results were weighted and summed to determine the total contribution for comparison with the 103Pd source literature. Comprehensive 2-D results are discussed, and the level of agreement with other assessments are presented.

Anisotropy↗
PubMed2001

Monte Carlo calculations of AAPM Task Group Report No. 43 dosimetry parameters for the MED3631-A/M125I source.

Brachytherapy dosimetry parameters for MED3631-A/M 125I sources have been determined in accordance with the AAPM Task Group No. 43 (TG-43) dosimetry protocol. These data were calculated using the *F8 tally from the MCNP4B2 Monte Carlo radiation transport code with the DLC-189 cross-section libraries. Due to motion of the 125I resin beads and gold-copper markers within the capsule, parameters such as the geometry function, radial dose function, dose rate constant, and anisotropy function were examined with the beads and markers having either "realistic" or "ideal" positions; the realistic position was a weighted combination of "vertical" and "diagonal" capsule orientations. The dose rate constants for the realistic and ideal geometries, lambda99std(realistic) and lambda99std(ideal) were 1.066 and 1.067 cGy h(-1) U(-1), respectively, which were within uncertainties of measured values by Wallace and Fan [Med. Phys. 26, 1925-1931 (1999)] and Li et al. [Med. Phys. 27, 1275-1280 (2000)], 1.06 and 1.067 cGy h(-1) U(-1), respectively. The calculated reference dose rate at r0= 1 and theta0= 90 degrees for the realistic source geometry was 0.7% less than for the ideal source geometry. The anisotropy constants, phian(realistic), for the realistic and ideal geometries were 0.948 and 0.965, respectively. phian(realistic) matched that (0.941) measured by Wallace and Fan, and phian(ideal) was significantly different from that (0.948) calculated by Wierzbicki et al. [Med. Phys. 25, 2197-2199 (1998)] for an ideal MED3631-A/S 125I source.

Anisotropy↗
PubMed2001

A formalism for independent checking of Gamma Knife dose calculations.

For stereotactic radiosurgery using the Leksell Gamma Knife system, it is important to perform a pre-treatment verification of the maximum dose calculated with the Leksell GammaPlan (DLGP) stereotactic radiosurgery system. This verification can be incorporated as part of a routine quality assurance (QA) procedure to minimize the chance of a hazardous overdose. To implement this procedure, a formalism has been developed to calculate the dose DCAL(X,Y,Z,dav,t) using the following parameters: average target depth (dav), coordinates (X,Y,Z) of the maximum dose location or any other dose point(s) to be verified, 3-dimensional (3-dim) beam profiles or off-centerratios (OCR) of the four helmets, helmet size i, output factor Oi, plug factor Pi, each shot j coordinates (x,y,z)i,j, and shot treatment time (ti,j). The average depth of the target dav was obtained either from MRI/CT images or ruler measurements of the Gamma Knife Bubble Head Frame. DCAL and DLGP were then compared to evaluate the accuracy of this independent calculation. The proposed calculation for an independent check of DLGP has been demonstrated to be accurate and reliable, and thus serves as a QA tool for Gamma Knife stereotactic radiosurgery.

Biophysical Phenomena↗
PubMed2000

Measurements and calculations of thermal neutron fluence rate and neutron energy spectra resulting from moderation of 252Cf fast neutrons: applications for neutron capture therapy.

252Cf is a neutron emitting radioisotope which has promise for both standard brachytherapy and neutron capture enhanced brachytherapy. In this study, experimental measurements and calculations were used to determine the thermal neutron fluence rate, phi(th) [n cm(-2) s(-1) mg(-1)], in the vicinity of 252Cf applicator tube (AT) type sources. Results of these measurements were confirmed with Monte Carlo calculations performed in a distributed manner on multiple workstations using MCNP. Three studies were executed: (1) relative phi(th) as a function of distance from a 252Cf AT source in an A-150 tissue equivalent plastic phantom using thermoluminescent dosimeters (TLDs) of varying 6Li/Li enrichment, (2) phi(th) measured with gold foils in a 114 liter water phantom 5 cm from two 252Cf AT sources, and (3) calculations of the impact of phantom material composition (e.g., A-150, water, brain, muscle) on phi(th) from moderated 252Cf fast neutrons. TLD results and Monte Carlo calculations in A-150 of relative phi(th) typically agreed within 1% and at most differed by 3% for distances from 1 to 6 cm. Foil measurements followed the ASTM E 262-86e protocol, and the ratio of activated plain and Cd encased gold foils (7.31) agreed well with the calculated ratio (7.26). Measured phi(th) at 5 cm (1.70+/-0.10 x 10(7) n cm(-2) s(-1) mg(-1)) was 10% greater than that determined using MCNP (1.55+/-0.12 x 10(7) n cm(-2) s(-1) mg(-1)), but was within the combined uncertainties. Compared with A-150 at a distance of 1 cm, phi(th) was 20%, 22%, and 32% less for water, brain, and muscle, respectively; these ratios decreased to 16%, 16%, and 24% less, respectively, at a distance of 5 cm from the source in a 15 cm diameter phantom. Comparisons of these results generally agreed with those in the literature for a value of 2 x 10(7) n cm(-2) s(-1) mg(-1) in water at 3 cm.

Algorithms↗
PubMed2000

Dependence of linac output on the switch rate of an intensity-modulated tomotherapy collimator.

The electro-mechanical, multivane intensity modulated collimator ("MIMiC") slit collimator with 40 vanes has been applied in the delivery of inversely planned sequential tomotherapy to over 4,000 patients. The collimator is binary in that each vane switches between fully open or closed status. Resulting beamlet patterns provide the intensity distributions imparting dose to the patient. The bouncing and damping of vanes at the two ends of their travel cause transient dose perturbations near and at the borders of the treatment field. These perturbations are not explicitly modeled by the planning system. Clinical beamlet profiles and output factors may then differ from those in the planning system and as a function of the vane switch period. A mechanical model of vane switching was developed to describe this dependency. Dose output and distribution of seven simple vane patterns with different switch times were measured with ionization chambers and radiographic films in polystyrene and anthropomorphic phantoms. Linac output dependence on switch time relative to vane open time was determined for four intensity modulated radiotherapy (IMRT) patients from measurements of an ionization chamber embedded in a cylindrical polystyrene phantom. Results demonstrate output dependence on switch time and, accordingly, on the servo mechanism for monitor units, arc length, dose rate, and gantry speed. In conclusion, the output dependence borders on clinical significance-improvements to collimator, dose calculation, commissioning, and quality assurance (QA) are suggested.

Humans↗
PubMed2000

Neutron dosimetry for a general 252Cf brachytherapy source.

This paper extends previous work to characterize neutron dosimetry in the vicinity of 252Cf brachytherapy sources. A general source is examined with an arbitrary length, diameter, and encapsulation using Monte Carlo methods. Fast neutron dosimetry and thermal neutron fluence rates were determined in a variety of clinically relevant media of varying dimensions. Applicator Tube, point source, high dose rate VariSource, and high dose rate muSelectron source geometries were analyzed. Fast neutron dosimetry was relatively independent of encapsulation thickness for an assortment of encapsulation materials less than 2 mm thick. Large variations in phantom size made minimal differences in the fast neutron dose close to the source. Specific source geometries were compared with dosimetry obtained from a simplified point model. The consequence of these results is a convenient means of accurately predicting clinical fast neutron dosimetry characteristics around a general 252Cf brachytherapy source in a variety of media without requiring neutron transport. Thermal neutron fluence rates were determined for a variety of source encapsulation materials, encapsulation thicknesses, and phantom sizes. At a distance of 3 cm from the source center, the thermal neutron fluence rate for a 30 cm diameter phantom was a 2.65 times greater than for a 10 cm diameter water phantom. These results demonstrate 252Cf thermal neutron fluence rate is relatively independent of encapsulation thickness and composition, yet highly dependent on hydrogen mass density and phantom size for phanta with diameters <30 cm.

Air↗
PubMed2000

Burst calculations for 252Cf brachytherapy sources.

Due to helium production following alpha decay, it is necessary to demonstrate the structural integrity of new 252Cf sources at elevated temperatures for special form certification by the U.S. Department of Transportation. Effects of temperature, capsule composition, and capsule dimensions are examined and reduced to a simple mathematical model. This highly conservative model assumes that all gas products leaving the radioactive source wire are retained by the capsule, and upon elevation to a temperature of 800 degrees C the capsule exhibits negligible expansion or change in internal volume and no increase in gas-phase components due to vaporization of spontaneous fission products. The calculated maximum loadings for the ORNL-made Applicator Tube and three proposed high dose rate 252Cf sources encapsulated in Pt/Ir-10% (VariSource, microSelectron classic, and a novel design) were 10.8, 0.508, 0.708, and 2.12 mg 252Cf, respectively.

Brachytherapy↗
PubMed2000

Recommendations of the American Association of Physicists in Medicine on 103Pd interstitial source calibration and dosimetry: implications for dose specification and prescription.

The National Institute of Standards and Technology (NIST) introduced a national standard for air kerma strength of the ThreaSeed Model 200 103Pd source (the only 103Pd seed available until 1999) in early 1999. Correct implementation of the NIST-99 standard requires the use of dose rate constants normalized to this same standard. Prior to the availability of this standard, the vendor's calibration procedure consisted of intercomparing Model 200 seeds with a 109Cd source with a NIST-traceable activity calibration. The AAPM undertook a comprehensive review of 103Pd source dosimetry including (i) comparison of the vendor and NIST-99 calibration standards; (ii) comparison of original Task Group 43 dosimetry parameters with more recent studies; (iii) evaluation of the vendor's calibration history; and (iv) evaluation of administered-to-prescribed dose ratios from the introduction of 103Pd sources in 1987 to the present. This review indicates that for a prescribed dose of 115 Gy, the administered doses were (a) 124 Gy for the period 1988-1997 and (b) 135 Gy for the period 1997-1999. The AAPM recommends that the following three steps should be undertaken concurrently to implement correctly the 1999 dosimetry data and NIST-99 standard for 103Pd source: (1) the vendor should provide calibrations in terms of air kerma strength traceable to NIST-99 standard, (2) the medical physicist should update the treatment planning system with properly normalized (to NIST-99) dosimetry parameters for the selected 103Pd source model, and (3) the radiation oncologist in collaboration with the medical physicist should decide which clinical experience they wish to duplicate; the one prior to 1997 or the one from 1997 to 1999. If the intent is to duplicate the experience prior to 1997, which is backed by the long-term follow-up and published outcome studies, then the prior prescriptions of 115 Gy should be replaced by 124 Gy to duplicate that experience.

Brachytherapy↗
PubMed1999

Mass attenuation coefficients of clear-Pb for photons from 125I, 103Pd, 99mTc, 192Ir, 137Cs and 60Co.

The mass attenuation coefficients, mu/rho, for Clear-Pb for photon energies ranging from 10 keV to 10 MeV were determined using Monte Carlo methods and simple equations used to manipulate elemental mass attenuation coefficients. It was determined that the effectiveness of Clear-Pb as a radiation shielding material was greater than plain acrylic for all photon energies, especially those less than 150 keV, and for deep penetration problems where the differences in mu/rho between Clear-Pb and acrylic became more significant. Finally, the usefulness of Clear-Pb as a shielding material when compared with acrylic was determined for the following commonly used radionuclides: 125I, 103Pd, 99mTc, 192Ir, 137Cs, and 60Co.

Cesium Radioisotopes↗
PubMed1999

Clinical brachytherapy with neutron emitting 252Cf sources and adherence to AAPM TG-43 dosimetry protocol.

Using Monte Carlo methods, neutron dosimetry for 252Cf Applicator Tube (AT) type medical sources available from Oak Ridge National Laboratory (ORNL) has for the first time been determined in terms of TG-43 formalism. This approach, as compared to previous "along-away" formalisms, demonstrates the relative angular independence of dose rate data, when the geometry factor has been removed. As the ORNL-made 252Cf AT type sources are considerably physically larger than most clinical sources used today, the radial dose function increases for radii less than 3.0 mm due to breakdown of the line source model. A comparison of the 252Cf neutron radial dose function with those for other medical sources revealed similarities with that from 137Cs. Differences with respect to previous 252Cf AT source neutron dosimetry data generally increased at increasing distances. This was attributed to differences in the various 252Cf AT source models and phantom compositions. The current status of 252Cf medical source fabrication and calibration procedures at ORNL is presented.

Anisotropy↗
PubMed1999

Dosimetry for 252Cf neutron emitting brachytherapy sources: protocol, measurements, and calculations.

The mixed-field dosimetry for 252Cf Applicator Tube (AT) type medical sources available from Oak Ridge National Laboratory (ORNL) has been characterized using ionization chambers, a GM counter, and Monte Carlo methods. Unlike the AAPM Task Group No. 43 (TG-43), specification of dose to muscle instead of water is recommended for clinical dosimetry of 252Cf medical sources. A dosimetry protocol similar to ICRU 45 was formulated with parameters determined specifically for 252Cf brachytherapy. Comparisons of experimental and calculative dosimetry results with Colvett et al. [Phys. Med. Biol. 17, 356-364 (1972)] and Krishnaswamy [Phys. Med. Biol. 17, 56-63 (1972)] were performed, and correction factors were determined to compare the different dosimetry formalisms. Using a Maxwellian model for the 252Cf neutron energy spectrum, kerma relative to muscle was determined for a variety of materials, and compared with relative kermas for external neutron beams of three different energies. Neutron isodose distributions and data necessary for clinical implementation of 252Cf AT sources are also presented.

Biophysical Phenomena↗
PubMed1999

Refinements to the geometry factor used in the AAPM Task Group Report No. 43 necessary for brachytherapy dosimetry calculations. American Association of Physicists in Medicine.

Determination of the geometry factor is necessary for brachytherapy dosimetry calculations as recommended by the AAPM Task Group No. 43 (TG-43). The equivalence and errors associated with use of a point source approximation for an extended line segment source are examined. For all angles, the error using the point source approximation is less than 2% for distances in which the ratio of radius to active source length, (r/L), exceed about 3.6. A novel approach to determining the geometry factor using Monte Carlo methods is discussed in which the particle flux emanates from the active source and streams with no interactions occurring within the source or phantom. This method was performed for determining the geometry factor along the transverse axis for six brachytherapy sources. Differences in the geometry factor exceeding 2% between the point source approximation and that obtained using Monte Carlo methods occurred at distances ranging from 0.5 to 5 mm from the source center along the transverse plane. The merits of the Monte Carlo approach for solving the geometry factor are discussed in light of using a point or line source approximation for calculating additional brachytherapy dosimetry parameters.

Algorithms↗
PubMed1998

Calculated dosimetric parameters of the IoGold 125I source model 3631-A.

Basic dosimetric parameters as recommended by the AAPM Task Group No. 43 (TG-43) have been determined for recently available IoGold 125I brachytherapy seeds. Monte Carlo methods (MCNP) were used in the calculation of these parameters in water, and results compared with soon to be published experimental parameters also for 125I IoGold seeds as well with parameters for model 6702 and 6711 125I seeds. These parameters were the radial dose function, anisotropy factor and constant, and the dose rate constant. Using MCNP, values for the radial dose function at 0.5, 2.0, and 5.0 cm were 1.053, 0.877, and 0.443, respectively. The anisotropy factor was 0.975, 0.946, 0.945, and 0.952 at 0.5, 1.0, 2.0, and 5.0 cm, respectively, with an anisotropy constant of 0.95. The IoGold dose rate constant was determined by excluding the low energy titanium characteristic x rays produced in the IoGold titanium capsule. Using this post TG-43 revised NIST air kerma methodology, the IoGold dose rate constant was 0.96 cGy h-1 U-1. These calculatively determined parameters for IoGold seeds were compared with those determined experimentally for IoGold seeds, and also compared with parameters determined for model 6702 and 6711 seeds as presented in TG-43.

Anisotropy↗