Biomedical subjects

R Nath

Publications and source records attributed to R Nath.

At least 289 records · Page 16Linked to original sources

PubMedDate not supplied

Dosimetry calculation for a novel phosphorus-32-impregnated balloon angioplasty catheter for intravascular brachytherapy.

PURPOSE: A phosphorus-32-impregnated balloon angioplasty catheter was used in a novel technique of simultaneous angioplasty and vessel irradiation. The 32P radionuclides were distributed on the surface of the balloon so that a certain amount of radiation was delivered while angioplasty was performed. Three-dimensional dosimetry and dose-time relationship needs to be established for the catheter so that quantitative dosimetric information is available for both clinical treatment and research investigation. METHODS AND MATERIALS: The 32P-impregnated balloon of an angioplasty catheter was assumed to have a cylindrical shape, and the radionuclides were assumed to be distributed uniformly on the curved surface of the cylinder. The dose rate at a point in space was computed by integrating the point dose-rate kernel of 32P over the radioactive surface of the balloon. The point dose-rate kernel was computed with Monte Carlo simulation of radiation transport. The energy spectra of 32P based on a mathematical model was used in the calculations. The three-dimensional dose distributions and dose-time relationships were calculated for balloons of various lengths and radii. RESULTS: At a short radial distance (e.g., 0.2 mm) away from the balloon surface, the dose distribution was uniform across a large portion of the balloon along the longitudinal axis, and dropped off rapidly at both ends of the balloon. Uniformity became worse as the radial distance increased. Uniformity was almost independent of balloon radius. The underdosed length at each end of the balloon was also almost independent of balloon length. In the central transverse plane, the dose reached a maximum at the surface of the balloon and then dropped off rapidly as the distance increases. Relative dose coverage outside the balloon was approximately independent of balloon radius and length, and the absolute dose coverage was approximately inversely proportional to balloon radius and length, assuming same total activity. CONCLUSIONS: Point dose-rate kernel of 32P beta emitter and the three-dimensional dose distributions of a 32P-impregnated balloon from an novel angioplasty catheter were calculated. A rule of thumb for dose calculation and dose coverage was established for simultaneous angioplasty and vascular brachytherapy with a 32P-impregnated balloon catheter.

Angioplasty, Balloon↗

PubMedDate not supplied

Shielding effects of metallic encapsulations and radiographic contrast agents for catheter-based intravascular brachytherapy.

PURPOSE/OBJECTIVE: Both photon- and beta-emitting radionuclides for intravascular brachytherapy (IVB) are under active investigation for prevention of restenosis following conventional angioplasty with or without stents. High atomic number materials are usually present in the coronary vessels undergoing treatment in the form of metallic encapsulations, stents, calcified plaque, or radiographic contrast agent. The high atomic number materials are likely to interfere with the photons and betas and, thus, change the dosimetry in the treatment volume. The purpose of this study is to investigate the shielding effects caused by the presence of high atomic number materials in IVB. MATERIALS AND METHODS: Dose rates at various distances in water, with and without the presence of various high atomic number materials, were calculated using Monte Carlo simulation techniques for photon and electron transport in extended media. The high atomic number materials investigated included titanium, stainless steel, calcified plaque, Hypaque, and Omnipaque. A wide range of monoenergetic photon and electron sources and several photon- and beta-emitting radionuclides, which have been under consideration for IVB, were used. The energy of the monoenergetic photon sources was in the range from 10 keV to 1 MeV, and that of the monoenergetic electron sources in the range from 0.5 to 2 MeV. Photon-emitting radionuclides (192)Ir, (125)I, and (103)Pd and beta-emitting radionuclides (90)Y, (32)P, and (188)Re were also considered. RESULTS: It was found that the high atomic number materials interfere considerably with the transport of photons of relatively low energies (below 40 keV) and all electron sources. When the energy of photon exceeds 100 keV, the interference becomes minimum for the high atomic number materials that are likely to be present in clinical situations. The shielding correction factors (SCFs) for dose rate at 2 mm from center were essentially 1.00 for photon energies above 100 keV; as the energy decreased below 100 keV, the SCF became smaller reaching a value of almost 0 for the lowest energy studied, 10 keV. For the photon source of (192)Ir, the SCF was essentially 1.00; while for the photon sources of (125)I and (103)Pd, shielding corrections were considerably lower than 1.00 depending on the type and thickness of the high atomic number material. For the beta emitting sources, the shielding effect can be expressed as a loss in effective penetration depth. This loss depends both on the material and its thickness. For titanium and stainless steel, the loss of range was about two and four times the thickness of the metal. CONCLUSIONS: The effects of high atomic number materials, such as metallic stents, calcified plaque, and contrast agents are minimal for high energy photon emitters, such as (192)Ir. The effects are pronounced for beta emitters and low energy photon emitters, and must be included in dosimetry planning.

Angioplasty, Balloon, Coronary↗

PubMedDate not supplied

The American Brachytherapy Society perspective on intravascular brachytherapy.

BACKGROUND: Recent clinical studies indicate that intravascular brachytherapy (IVB) can reduce the rate of restenosis substantially after angioplasty procedures. However, no clinical guidelines exist for optimal therapy. METHODS: The members of the IVB Subcommittee of the American Brachytherapy Society (ABS) identified the areas of consensus and controversies in IVB to issue the ABS perspective on IVB, based on analysis of published reports and the clinical experience of the members in brachytherapy. RESULTS: IVB is still experimental. The long-term efficacy, toxicity, the target tissue, and dose required for IVB are not established. The ABS recommends that IVB procedures must be performed, with careful attention to radiation-related issues, in the context of controlled multidisciplinary clinical trials with the approval of the institutional review board, the Nuclear Regulatory Commission, the Food and Drug Administration, and under an Investigational Device Exemption. The therapeutic radiologist, with a qualified radiation physicist, is responsible for dose prescription and delivery and needs to be present during the IVB procedure as part of this multidisciplinary team. The long-term outcome from these studies should be reviewed critically and published in peer-reviewed journals. The ABS endorsed the dosimetric guidelines of the American Association of Physicists in Medicine Task Group 60 (AAPM TG-60) report. The ABS recommends that dose specification be defined clearly; to allow comparisons between studies, the dose should be prescribed at 2 mm from the source for intracoronary brachytherapy and at an average luminal radius of +2 mm for peripheral vascular brachytherapy. The prescription doses at the above point is generally in the 12-18 Gy range. Comprehensive procedures for quality assurance, radiation protection, and emergencies should be in place before initiating an IVB program. Higher energy beta sources, lower energy gamma sources, dose-volume histograms, and correlation of three-dimensional reconstructions of delivered dose with patterns of failure are areas for further research. CONCLUSION: The ABS perspective on IVB is presented to assist the interventional team in developing protocols for the use of IVB in the prevention of restenosis. Long-term outcome data with a standardized reporting system are needed to establish the role of brachytherapy in preventing vascular restenosis. Endovascular brachytherapy is a new and evolving modality, and these recommendations are subject to modifications as new data become available.

Brachytherapy↗

PubMedDate not supplied

On the depth of penetration of photons and electrons for intravascular brachytherapy.

PURPOSE: To investigate the depth dose characteristics of various radionuclides under consideration for intravascular brachytherapy (IVB). MATERIALS AND METHODS: In the past few years, various preclinical studies have shown that 10-30 Gy of ionizing radiation delivered by a brachytherapy treatment may inhibit restenosis following angioplasty. A number of new delivery systems using various radionuclides have been developed and are being investigated for IVB. Typical target size for IVB is in the range of millimeters, in contrast to conventional brachytherapy for cancer in which the target may be 1-5 cm in size. The question addressed in this paper is: whether lower energy photon emitters and even beta emitters, which are not commonly used for intracavitary brachytherapy of cancer, may provide a depth of penetration adequate for IVB. To explore this issue, radial dose functions for photons and electrons in the range of 1-10 mm in water were calculated using Monte Carlo simulation. Reference depth for normalization of the radial dose funtion was chosen to be 2 mm. RESULTS: Radial dose functions have been calculated for monoenergetic photons with energies of 0.01, 0.02, 0.04, 0.06, 0.08, 0.10, 0.20, 0.40, and 1.00 MeV and monoenergetic electrons with energies of 0.5, 1.0, 1.5 and 2.0 MeV. Also, the same calculations have been performed for 192Ir, 125I, and 103Pd gamma or x-ray sources as well as 90Sr-90Y, 32P, and 188Re beta-emitting sources. Results are also provided for selected cases in a simulated calcified lesion in water. CONCLUSIONS: It is concluded that photons above an energy of 20 keV and electrons above an energy of 1.0 MeV are acceptable from the point of view of adequate depth of penetration for IVB in tissue.

Animals↗

PubMedDate not supplied

Dose perturbations by high atomic number materials in intravascular brachytherapy.

PURPOSE: In intravascular brachytherapy, use of high atomic number materials, such as contrast agents and metallic stents, can introduce significant dose perturbations, especially for low energy photons. The purpose of this study is to investigate dose perturbation at the interfaces of high atomic number materials and tissue. METHODS: To investigate this issue, the radial dose functions across the interface between different materials and soft tissue were calculated by using Monte Carlo simulations. Various interfaces, including contrast agent to water, stainless steel to water, and bone (simulating a calcified plaque) to water, were investigated for photon energies between 20 keV and 1 MeV. RESULTS: It was found that the dose to water near the interface is enhanced considerably by photons of energies between 0.020 and 0.200 MeV. For example, the maximum dose enhancement factors for the Hypaque-tissue interface ranged from 2.2 to 18.3 for photons in this energy range. The enhancement factor is almost equal to 1 for photon energy between 0.400 and 1.000 MeV. It appears that the maximum enhancement occurs around 60 keV. For 60-keV photons, the maximum dose enhancement factors are about 18.3, 18.7, 19.1, and 3.1 for Hypaque, Omnipaque, stainless steel, and calcified plaque, respectively. The dose enhancement decreases exponentially with distance from the interface. The affected tissue thickness is dependent on the photon energy. As expected, the higher the photon energy is, the larger is the affected tissue thickness. Depending on the type of interface and the energy of photons, the dose enhancement distance (defined as the thickness receiving more than twice the dose without interface) ranges from 1.3 to 72 microm for photons of energy from 0.020 to 0.100 MeV, respectively. CONCLUSIONS: The existense of high atomic number materials could introduce significant dose enhancement at the interfaces between these materials and tissue. This dose enhancement can be higher than an order of magnitude for photon energies around 60 keV, and should be considered in evaluation of the efficacy of intravascular brachytherapy.

Brachytherapy↗

PubMedDate not supplied

A study of the anti-ulcer activity of diazepam and other tranquillosedatives in albino rats.

Anti-ulcer and sedative effects of tranquillosedatives viz. benzodiazepines (diazepam, oxazepam and nitrazepam), barbiturate (phenobarbitone), phenothiazines (chlorpromazine, trifluoperazine and thioridazine) and butyrophenone (haloperidol) were compared in albino rats. Ulceration of the glandular stomach was induced by 2 h restraint at 4 degrees C. Sedation was measured using the rotarod test. These tranquillosedatives showed dose dependent anti-ulcer and sedative effects. The relative potency and therapeutic index (ratio between rotarod ED50 and anti-ulcer ED50) of each drug were determined. Diazepam showed the highest therapeutic index (1.88). Diazepam significantly reduced the volume of gastric secretion, raised its pH and prevented the gastric ulcer formation in pylorus ligated rats but failed to prevent the acute duodenal ulceration induced by intramuscular injection of histamine or carbachol in guinea-pigs and rats, respectively. These observations suggest that benzodiazepines (diazepam) are more suitable anti-ulcer agents compared to barbiturate, phenothiazines and butyrophenone. The anti-ulcer effect of diazepam is possibly due to a combination of sedative, anti-anxiety and antisecretory actions.

Animals↗

PubMedDate not supplied

Serum and urinary zinc in fulminant hepatic failure.

Patients with chronic hepatic encephalopathy have been shown to have low serum zinc levels. Moreover, in a controlled study, significant improvement was seen in these patients on oral zinc supplementation. Information on zinc status in fulminant hepatic failure is insufficient. Serum and urinary zinc abnormalities were studied in 22 patients with fulminant hepatic failure (FHF) and they were compared with age- and sex-matched controls. The mean serum zinc values were significantly less in patients with FHF (72.7 +/- 3.7 micrograms/100 mL versus 107.9 +/- 6.2 micrograms/mL) while the urinary zinc values were significantly higher compared with controls (603.5 +/- 9.3 micrograms/24 h versus 334.4 +/- 10 micrograms/24 h). The serum zinc levels significantly and progressively decreased, while urinary zinc significantly increased after admission in patients with FHF. The serum zinc values in the group that survived were significantly higher than those in the group of patients who died. Correspondingly, urinary zinc was lower in survivors than in the group that expired. This study indicates that serum and urinary zinc levels could be used as a prognostic indicator in FHF. A therapeutic trial with zinc supplementation is justified in this group of patients.

Adult↗

PubMedDate not supplied

Determination of high-energy x-ray spectra by photoactivation.

The determination of high-energy x-ray spectra has required scintillation spectrometers with massive shielding, neutron time-of-flight spectrometers, or the tedious counting of electron tracks in nuclear emulsions. A new approach has been developed which takes advantage of the energy dependence of photoactivation cross sections. Radioactivity is produced in a small packet of C, Cu, Co, Y, Zr, and Au foils by approximately 5000 rad (tissue). Since the amount of radioactivity produced in each foil is given by the integral of the product of photonuclear cross section and differential photon fluence, a numerical method for unfolding the spectrum is required, and the orthonormal expansion has been employed for this purpose. The photoactivation method has been used to determine the x-ray spectra produced by 30-MeV electrons incident upon thin and thick tungsten targets, and filtered by equivalent amounts of lead and aluminum. These spectra have been compared to calculated thin-target spectra as well as to those determined by a neutron time-of-flight spectrometer. The central-axis and off-axis x-ray spectra produced by a 33-MeV betatron have also been determined.

Radiometry↗

PubMedDate not supplied

Photoactivation ratios for specification of high-energy x-ray quality: part I, 15-40-MV x rays.

A sensitive and relatively simple method for obtaining an index of the spectral quality of high-energy x rays is presented. The method is based upon the use of photoactivation threshold detectors. Photonuclear reactions produce measurable amounts of radioactivity for radiation exposures of several thousand roentgens. Since the threshold energies for photonuclear reactions vary for different nuclei, the ratio of radioactivity induced in two appropriately selected foils is very sensitive to the x-ray spectrum. Photoactivation ratios (PAR) have been measured for 20-35-MV x rays using Co, Cu, Y, Zr, and Au activation foils. It is shown that the PAR method offers a sensitive and practical means for quality control of x-ray spectra, comparison of high-energy accelerators, and the measurement of variations of spectral quality control at different point in an irradiated volume.

Energy Transfer↗

PubMedDate not supplied

Photoactivation ratios for specification of high-energy x-ray quality: part II, 4-15-MV x rays.

A sensitive and relatively simple method for obtaining an index of the spectral quality of x rays in the 4-15-MV range is presented. The method is based upon the simultaneous production of 115mIn by inelastic scattering of x rays by an indium foil and the production of 116mIn by neutron capture, where the neutrons are obtained from the photodisintegration of deuterium. Since the cross sections for the production of 115mIn and 116mIn have markedly different energy response functions, the ratio of radioactivities is very sensitive to the incident x-ray spectrum. Using a detector consisting of an indium foil sandwiched between two plastic bottles of D2O, photoactivation ratios (PAR) have been measured for 4-, 6-, and 10-MV x rays. It is shown that the PAR method offers a sensitive and practical means for quality control of x-ray spectra and comparison of high-energy accelerators.

Deuterium↗

PubMedDate not supplied

Neutron spectral measurements in an intense photon field associated with a high-energy x-ray radiotherapy machine.

High-energy x-ray radiotherapy machines in the supermegavoltage region generate complex neutron energy spectra which make an exact evaluation of neutron shielding difficult. Fast neutrons resulting from photonuclear reactions in the x-ray target and collimators undergo successive collisions in the surrounding materials and are moderated by varying amounts. In order to examine the neutron radiation exposures quantitatively, the neutron energy spectra have been measured inside and outside the treatment room of a Sagittaire medical linear accelerator (25-MV x rays) located at Yale-New Haven Hospital. The measurements were made using a Bonner spectrometer consisting of 2-, 3-, 5-, 8-, 10- and 12-in.-diameter polyethylene spheres with 6Li and 7Li thermoluminescent dosimeter (TLD) chips at the centers, in addition to bare and cadmium-covered chips. The individual TLD chips were calibrated for neutron and photon response. The spectrometer was calibrated using a known PuBe spectrum Spectrometer measurements were made at Yale Electron Accelerator Laboratory and results compared with a neutron time-of-flight spectrometer and an activation technique. The agreement between the results from these independent methods is found to be good, except for the measurements in the direct photon beam. Quality factors have been inferred for the neutron fields inside and outside the treatment room. Values of the inferred quality factors fall primarily between 4 and 8, depending on location.

Neutrons↗

PubMedDate not supplied

On the choice of material for half-value-layer measurements for megavoltage x rays.

The relative sensitivity of various materials for the measurement of half-value-layer thickness has been calculated for bremsstrahlung x ray spectra in the energy range 5-40 MeV. It is concluded that low-atomic-number materials such as water are more sensitive to changes in spectral quality of megavoltage x rays than high-atomic-number materials such as lead.

Aluminum↗

PubMedDate not supplied

Modification of electron-beam dose distributions by transverse magnetic fields.

By applying a transverse magnetic field to a dosimetry phantom, an incident high-energy electron beam is made to follow a spiral path in the course of slowing down. Certain levels, determined by the electron energy and the magnetic field strength, will be traversed several times by the same electrons. The net result of this process is an enhancement of the depth dose in relation to the entrance dose, and a more sharply defined depth of penetration. Experiments with 50- and 55-MeV electrons traversing a 20.5-kG field are shown to support the predictions of a detailed Monte Carlo calculation.

Electrons↗

PubMedDate not supplied

Monte Carlo calculation of the wall correction factors for ionization chambers and Aeq for 60Co gamma rays.

The application of cavity-ionization chambers to the standadization of 60Co gamma-ray beams, in terms of exposure, requires that the specific ionization of air Jg, be corrected for the attenuation and scatter of the incident rays by the wall, central electrode, and supporting stem of the chamber. A Monte Carlo photon-electron transport code has been developed for the purpose of calculating this correction for spherical and cylindrical chambers. The code has been applied to a spherical graphite chamber having dimensions typical of the chambers used by the NBS, the calculated wall-correction factor is in close agreement with the average of the NBS factors which were determined experimentally. The code was also used to calculate Aeq, which is central to the determination of tissue-air ratios. The calculated value, 0.989 +/- 0.003, is very close to the generally accepted value, 0.985.

Cobalt Radioisotopes↗

PubMedDate not supplied

Fast and thermal neutron profiles for a 25-MV x-ray beam.

High-energy x-ray radiotherapy machines generate neutrons by photonuclear reactions in the target and the treatment head and expose the patient to a neutron flux. In order to evaluate the neutron exposure quantitatively, fast and thermal neutron profiles for 25-MV x-ray beams of the Sagittaire accelerator have been measured. An activation technique, using the reactions 31P(n, gamma)32P (thermal neutrons) and 31P(n, p)31Si (fast neutrons, E greater than 0.7 MeV), has been developed to measure fast- and thermal-neutron fluxes in an intense high-energy photon flux. The sensitivity of this activation detector to high-energy photons, which has plagued many previous neutron measurements, was carefully measured and found to be less than 4%. Neutron fluxes for various photon field sizes ranging from 5 X 5 cm to 30 X 30 cm have been measured. The fast-neutron profiles were observed to have rounded edges and the thermal fluxes were found to be relatively uniform. In the central part of the x-ray beam, the ratio of neutron dose equivalent to photon absorbed dose was found to be between 0.2% and 0.5%. Outside of the photon field, the ratio of neutron dose equivalent to the central-axis photon absorbed dose was 0.12%.

Fast Neutrons↗

PubMedDate not supplied

On the constancy in composition of polystyrene and polymethylmethacrylate plastics.

Variations in the atomic composition, and mass and electron densities of polystyrene and polymethylmethacrylate (PMM) plastics were assessed from experimentally determined mass attenuation coefficients for 125I and 137Cs gamma rays. The means and standard deviations in the mass densities of 16 samples of PMM and 10 samples of polystyrene were found to be 1.174 +/- 1.4% and 1.042 +/- 0.6% g/cm3, respectively. Based upon transmission measurements on various solutions of ethyl alcohol in water, the standard deviations in the effective atomic numbers of PMM and polystyrene were determined to be 0.77% and 1.3%, respectively. Based upon experimentally determined mass attenuation coefficients for 137Cs, the standard deviations in electron density for PMM and polystyrene were 0.5% and 1.2% respectively. Similar measurements on tap water and two grades of distilled water failed to detect any differences in atomic composition.

Methylmethacrylates↗

PubMedDate not supplied

Determination of the exposure rate constant for 125I using a scintillation detector.

A scintillation spectrometer was employed to determine the exposure rate constant for an 125I seed of the type used in therapeutic radiology. The method consisted of converting counting rate to photon fluence incident upon the detector, and then calculating the exposure rate from the photon fluence. Correlations are required for absorption of the 125I photons in the air path, in the aluminum window of the NaI crystal, and in the aluminum-oxide reflector. Corrections are also required for the escape of iodine K-characteristic radiation, and the intrinsic peak efficiency of the crystal. The experimentally determined exposure rate constant in a direction perpendicular to the long axis of the seed is 1.309 Rcm2mCi-1 h-1 +/- 5.8%, and this compares favorably with a calculated value of 1.284 R cm2mCi-1 h-1. The photon fluence as a function of angle of rotation about an axis perpendicular to the seed's long axis was also measured. These data were used to estimate an average exposure rate constant for 125I seeds of 1.089 Rcm2mC-1 h-1.

Brachytherapy↗

PubMedDate not supplied

An intercomparison of neutron measurments for a 25 MV x-ray radiotherapy accelerator.

High-energy x-ray radiotherapy machines produce neutrons by photonuclear reactions which present a potential radiation hazard to the personnel and patient. A series of measurements of the neutron flux from a 25 MV x-ray linear accelerator, inside and outside the treatment room, have been performed using a multisphere spectrometer, Nemo dosimeter, and activation detectors. These results are compared with other mixed photon-neutron field measurements for the same machine performed using an argon/propane ionization chamber, silicon diode, track-etching detectors, and Monte Carlo calculations. It is found that these measurements agree with each other within a factor of two except for silicon diode measurements in the photon beam. Measured neutron spectra at various locations in the treatment room are also compared with the results of Monte Carlo transport calculations.

Activation Analysis↗