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R Nath

Publications and source records attributed to R Nath.

At least 307 records · Page 17Linked to original sources
PubMedDate not supplied

Yale University School of Medicine, New Haven, Connecticut.

The response and wall correction factors for various ionization chambers in a cobalt-60 gamma-ray field have been calculated using a Monte Carlo photon-electron transport code. Among the chamber parameters studied are chamber wall material and its thickness, central electrode material and its dimensions, and the shape and size of the sensitive volume. The calculations show that the response and wall correction factors are sensitive to the shape and volume of the ionization chamber, but relatively independent of the choice of material for the chamber wall and electrode when these are compared on the basis of electron density. Data are presented for cylindrical, plane-parallel, and spherical ionization chambers constructed from carbon, magnesium, aluminum, water, Lucite, polystyrene, and ICRU muscle, as well as for a number of commercially available ionization chambers.

Gamma Rays↗
PubMedDate not supplied

Fraction of ionization from electrons arising in the wall of an ionization chamber.

The accuracy of high-energy x-ray dosimetry can be improved by taking account of differences between the compositions of the chamber wall and the buildup cap or dosimetry phantom. The fraction of the ionization due to secondary electrons arising in the chamber wall has been determined as a function of wall thickness for 60Co gamma rays and x rays in the range of 2-25 MV for Farmer-type chambers. Secondary electrons arising in the accelerator head were removed from the x-ray beams by a magnetic field placed just in front of the ionization chamber. For 60Co gamma rays, the fraction increases from 40% to 100% as the wall thickness increases from 0.05 to 0.55 g cm-2. For a 0.05 g cm-2 wall, fraction decreases from 60% to 10% as the x-ray energy is increased from 2 to 25 MV. Limited data obtained with different chambers suggest that the fraction is independent of chamber wall composition when the thickness is expressed in g cm-2.

Cobalt Radioisotopes↗
PubMedDate not supplied

An automatic seed identification technique for interstitial implants using three isocentric radiographs.

A technique for the automatic reconstruction of the spatial coordinates of seeds in an interstitial implant has been developed. Seed coordinates from three isocentric radiographs, an anterior-posterior film, and an orthogonal pair of films taken at gantry angles of +/- 45 degrees from the anterior-posterior direction are digitized and recorded in a random sequence. From this, the three-dimensional coordinates of the seeds in the implant are computed by matching the values of common coordinates along the axis of rotation. This rotate and match technique leads to an accurate and consistent identification and reconstruction of seeds with 90% of them within 2 mm of their actual location. These new computer routines have been added to an existing treatment planning system (AECL TP-11 treatment planning system, version 5A). We present our algorithm and technique along with clinical examples.

Brachytherapy↗
PubMedDate not supplied

Neutrons from high-energy x-ray medical accelerators: an estimate of risk to the radiotherapy patient.

The problem of neutrons produced by many of the high-energy x-ray therapy machines (10 MV and above) is reviewed, and the possible risk their presence poses to radiotherapy patients is estimated. A review of the regulatory background containing a summary of the recommendations of the U.S. Council of State Governments (USCSG), and of the International Electro-Technical Commission (IEC), as well as an indication that recommendations will be forthcoming from the National Council on Radiation Protection (NCRP) and the International Commission of Radiological Protection (ICRP) is presented. The neutrons in question are produced by high-energy photons (x rays) incident on the various materials of the target, flattening filter, collimators, and other essential components of the equipment. The neutron yield (per treatment dose) increases rapidly as the megavoltage is increased from 10 to 20 MV, but remains approximately constant above this. Measurements and calculations of the quantity, quality, and spatial distribution of these neutrons and their concomitant dose are summarized. Values of the neutron dose are presented as entrance dose, midline dose (10-cm depth), and integral dose, both within and outside of the treatment volume. These values are much less than the unavoidable photon doses which are largely responsible for treatment side effects. For typical equipment, the average neutron integral dose from accelerator-produced neutrons is about 4-7 g cGy (per treatment cGy), depending on the treatment plan. This translates into an average dose of neutrons [averaged over the body of a typical 70-kg (154 lb) patient] of 0.06-0.10 cGy for a treatment of 1000 cGy. Using these neutron doses and the best available neutron risk coefficients, it is estimated that 50 X 10(-6) fatal malignancies per year due to the neutrons may follow a typical treatment course of 5000 rads of 25-MV x rays. This is only about 1/60th of the average incidence of malignancies for the general population. Thus, the cancer risk to the radiotherapy patient from accelerator-produced neutrons poses an additional risk to the patient that is negligible in comparison.

Abnormalities, Radiation-Induced↗
PubMedDate not supplied

An improved electron energy-loss straggling algorithm for Monte Carlo transport codes.

The commonly used Blunck and Leisegang electron energy-loss distribution falls off too rapidly with increasing energy loss. Also, for large thicknesses and/or low-Z media, where their distribution should approach Landau's, it normalizes to 0.92 rather than 1.0, it overestimates the number of very small energy-loss events, and its peak is shifted from lambda = -0.225 to 0.1. Because of these shortcomings, calculations made using this distribution yield a mean straggled energy loss which is lower than the value predicted by the continuous slowing down approximation (CSDA). An improved version of the Blunck-Leisegang distribution, which exhibits better normalization and falloff, has been developed. Further, an algorithm was created which (depending on the CSDA energy loss, Z,A, electron energy, and transport step size) samples the electron's straggled energy loss from the more accurate of the available distribution functions.

Electrons↗
PubMedDate not supplied

Enhancement of electron beam dose distributions by longitudinal magnetic fields: Monte Carlo simulations and magnet system optimization.

A Monte Carlo electron-photon transport code was developed in order to determine the effects of static, longitudinal, magnetic fields on dose distributions produced by high-energy electron beams, and to optimize the design of a superconducting magnet system. As a result of these simulations, a 20-cm-i.d., 30-cm-o.d., 15-cm-tall, single-coil, magnet system was designed that could be incorporated into a mobile treatment table for use with a standard radiation therapy accelerator. Operating at a current density of 18 kA/cm2, the magnet would produce field strengths of 1-4 T in the phantom and 0.01 T at the accelerator exit window. Magnetically enhanced dose distributions, calculated for 20- and 30-MeV electron beams, show a pronounced Bragg peak, steeper gradients to the sides and rear, and a roughly fourfold increase in the peak dose to entrance dose ratios relative to those similarly calculated without a magnetic field. These magnetically enhanced dose distributions have the potential for sparing intervening tissue when high-energy electrons are used for the treatment of deep-seated tumors.

Electrons↗
PubMedDate not supplied

Choice of material for HVL measurements in megavoltage x-ray beams.

The relative sensitivity of the half-value layer (HVL) method as a quality index for megavoltage x-ray beams is examined by theoretical calculation and experimental measurements for 4-, 6-, 10-, and 25-MV x-ray beams. It is shown that lower atomic number materials are more sensitive to beam quality changes than higher atomic number materials, and that aluminum is a reasonable choice of material for HVL measurements in megavoltage x-ray beams. Further, it was found that the HVL in aluminum or polystyrene is a more sensitive index of spectral quality than the ionization ratio method, recommended by recent dosimetry protocols.

Aluminum↗
PubMedDate not supplied

Dose distributions around cylindrical 241Am sources for a clinical intracavitary applicator.

Encapsulated, cylindrical sources containing 2, 5, and 8 Ci of 241Am have been designed and fabricated for intracavitary irradiation of uterine cancers. Exposure rates in air and dose rates in water around these sources have been measured using an ionization chamber and a lithium fluoride thermoluminescent dosimetry system. Dose rates in water at a distance of 2.5 cm from the source center along a direction transverse to the source axis were found to be 10.4, 24.3, and 23.3 cGy/h for the 2-, 5-, and 8-Ci sources, respectively, using an ionization chamber. Under the same conditions, the thermoluminescent dosimetry system yielded the values of 10.3, 23.1, and 22.3 cGy/h. It was observed that the ratio of dose-to-water and exposure in air is sensitive to the scattering geometry and source geometry in the case of 241Am photons. This ratio was found to increase substantially as conditions of full scattering were approached. A three-dimensional integration model was employed for the determination of dose distributions around these sources. Results of this dose computation model have been compared against the measured data and were found to be in good agreement with each other. Average deviations of calculated data from measured data were in the range of 0.2 to 0.5 cGy/h and larger deviations were observed in the paraxial region, where the effects of oblique filtration are more severe.(ABSTRACT TRUNCATED AT 250 WORDS)

Americium↗
PubMedDate not supplied

A comparison of solid phantoms with water for dosimetry of 125I brachytherapy sources.

Dosimetry of brachytherapy sources is critically dependent on precise measurement of the source-detector distance. A solid phantom can be precisely machined and hence distances can be accurately determined. In this work LiF thermoluminescent chips are used for absolute dose rate measurements in solid water, polymethylmethacrylate (PMMA), and polystyrene. These media are examined for their suitability in the dosimetry of 125I by comparing depth doses in each phantom. Measurements and Monte Carlo calculations show that solid water is equivalent to water for the dosimetry of 125I seeds, however, polystyrene and PMMA are not equivalent to water. Also, photon energy spectra for several depths in each phantom material have been calculated and are used to determine average photon energy and mass energy absorption coefficients as a function of depth.

Biophysical Phenomena↗
PubMedDate not supplied

A model of heat production and transport in a water calorimeter.

The heat diffusion equation for a water calorimeter under isothermal and adiabatic boundary conditions has been solved analytically for a general source function. Based upon this model an interactive computer program has been developed and employed to predict the temporal variation of the temperature distribution in a water calorimeter irradiated with 250 kV, 4-, 6-, and 25-MV x rays and 7-, 13-, and 32-MeV electrons. The model requires central-axis depth dose curves as inputs and allows for any number of alternate heating and cooling periods. It is found that the shape of the cooling curve following an irradiation period depends upon the location of the point of measurement, the choice of boundary conditions, and the thermal history of the calorimeter.

Calorimetry↗
PubMedDate not supplied

Dosimetry on transverse axes of 125I and 192Ir interstitial brachytherapy sources.

Dose rates along the transverse axes of 125I model 6702, 125I model 6711 and 192Ir 0.2-mm steel sources for interstitial brachytherapy have been measured in a solid-water phantom for distances up to 10 cm using LiF thermoluminescent dosimeters (TLDs). Specific dose rate constants, the dose rates in water per unit source strength 1 cm along the perpendicular bisector of the source, are determined to be 0.90 +/- 0.03, 0.85 +/- 0.03, and 1.09 +/- 0.03 cGy h-1 U-1 for 125I model 6702, 125I model 6711 and 192Ir 0.2-mm steel sources, respectively (1 U = unit of air kerma strength = 1 microGy m2 h-1 = 1 cGy cm2 h-1). In older and obsolete units of source strength (i.e., mCi apparent), these are 1.14 +/- 0.03, 1.08 +/- 0.03, and 4.59 +/- 0.15 cGy h-1 mCi-1 (apparent). Currently accepted values of specific dose rate constant for 125I sources are up to 20% higher than our measured values which are in good agreement with the results of our Monte Carlo simulations. But for 192Ir there is good agreement between our measured value of the specific dose rate constant and currently accepted values. The radial dose function for 125I model 6702 is found to be consistently larger than that for 125I model 6711, with an increasing difference as the distance from the source increases. Our measured values for the radial dose function for 125I sources are in good agreement with the results of our Monte Carlo simulation as well as the measured values of Schell et al. [Int. J. Radiat. Oncol. Biol. Phys. 13, 795-799 (1987)] for model 6702 and Ling et al.(ABSTRACT TRUNCATED AT 250 WORDS)

Biophysical Phenomena↗
PubMedDate not supplied

A dose computation model for 241Am vaginal applicators including the source-to-source shielding effects.

A dose computation model has been developed for the determination of dose distributions around vaginal plaque applicators containing encapsulated 241Am sources. Encapsulated sources of 241Am emit primarily 60-keV photons which have a half-value layer thickness of 1/8 mm of lead. This makes possible highly effective in vivo shielding of normal tissues at risk, by placing thin lead shields at appropriate places on the applicator. However, self-absorption of photons in the source material itself is intense, requiring bulky sources of about 1 cm diameter. These sources also produce considerable source-to-source shielding which must be taken into account in dose calculations. Our dose computation model for a single source employs three-dimensional integration of dose contributions from volume elements of the source including the effects of absorption and scattering of photons in the source material, titanium encapsulation, and water. An empirical correction to Berger's data on buildup factors of point, isotropic sources is made to account for the effects of anisotropic photon emission by cylindrical 241Am sources. The second part of our dose computation model takes into account source-to-source shielding effects on both primary and scattered photons for the vaginal plaque geometry. The results of the model have been verified for accuracy by comparisons with extensive dosimetry measurements using lithium fluoride thermoluminescent dosimeters.

Americium↗
PubMedDate not supplied

Photon energy dependence of the sensitivity of radiochromic film and comparison with silver halide film and LiF TLDs used for brachytherapy dosimetry.

There is a new radiochromic film, a highly uniform, thin (100-microns) detector whose sensitive layer (6 microns thick) changes from colorless to blue by dye polymerization without processing, upon exposure to ionizing radiation. Because the dose gradients around brachytherapy sources are steep, the high spatial resolution offered by film dosimetry is an advantage over other detectors such as thermoluminescent dosimeters (TLDs). This compares the photon energy dependence of the sensitivities of GafChromic film, silver halide verification film (Kodak X-Omat V Film), and lithium fluoride TLDs (Harshaw), over the photon energy range 28 keV to 1.7 MeV, which is of interest in brachytherapy. Sensitivity of the radiochromic film is observed to decrease by about 30% as effective photon energy decreases from 1710 keV (4-MV x rays) to 28 keV (60-kV x rays, 2-mm A1 filter). In contrast, the sensitivity of verification film increases by 980% and that of LiF TLDs increases by 41%. The variation of the sensitivity of radiochromic film with photon energy is considerably less than that for silver halide film and similar to that for LiF TLDs, but in the opposite direction. Radiochromic film, like LIF TLDs, does not exhibit the drastic sensitivity changes below 127 keV that silver halide film exhibits. Dose distribution in the immediate vicinity of a high activity (370 GBq) brachytherapy 192Ir source has been mapped using radiochromic film and is presented to illustrate the applicability of this new technology to brachytherapy dosimetry.

Brachytherapy↗
PubMedDate not supplied

Clinical implementation of AAPM Task Group 32 recommendations on brachytherapy source strength specification.

Historically the strength of sealed brachytherapy sources has been described by many physical quantities, including true activity, apparent activity, and equivalent mass of radium. Recently, the AAPM Task Group 32 recommended that these quantities be replaced by a single quantity, air-kerma strength, with units of muGy m2h-1. A set of equations has been developed for unambiguously converting source strength estimates and renormalizing published dose-rate tables, which assume traditional quantities and units, into forms consistent with air-kerma strength. For commonly used brachytherapy sources, multiplicative conversion factors for each source-strength formalism and set of units are given. To convert equivalent mass of radium to air-kerma strength requires a single multiplicative factor, 7.23 muGy m2h-1/mgRaEq, applicable to all sources. Based upon a review of vendor source specification practices, the factors for converting source strength of 198Au, 103Pd, and 125I seeds from apparent mCi to air-kerma strength are 2.06, 1.29, and 1.27 muGy m2h-1/mCi(apparent), respectively. These factors are independent of source geometry but depend on the nominal exposure rate constant value selected by the vendor. Conversion factors applicable to mass of radium or true activity depend upon both source geometry and radionuclide identity. Because many of these conversion factors depend upon vendor choices of physical constants and exposure rate constants, readers are cautioned to carefully review vendor source strength specification practices before adopting these values clinically. Finally, the relationships between the various source strength quantities and absorbed dose rate in the medium surrounding the source are elucidated.

Brachytherapy↗