Biomedical subjects

R Nath

Publications and source records attributed to R Nath.

At least 325 records · Page 18Linked to original sources

PubMedDate not supplied

Comparison of IAEA 1987 and AAPM 1983 protocols for dosimetry calibration of radiotherapy beams.

The IAEA 1987 protocol is an international protocol which has made a number of improvements over the AAPM 1983 protocol for calibration of high-energy photon and electron beams. We present a detailed numerical comparison between the two protocols by calculating (i) Ngas and ND for PTW (PMMA wall), Capintec (air-equivalent plastic wall) and NEL (graphite wall) Farmer type ionization chambers for 60Co gamma rays; (ii) dose-to-water with chamber in water irradiated by 4- or 25-MV x rays; (iii) dose-to-water with chamber in water, PMMA, and polystyrene phantoms irradiated by 5- and 10-MeV electrons; and (iv) dose-to-water with chamber in water irradiated by 20-MeV electrons. For photons, the IAEA protocol gives results which are in good agreement with the AAPM protocol; on average the IAEA results are 0.6% smaller than the AAPM results while discrepancies between the two are in the range of -0.4% to -1.2%. For 10-MeV electrons also, the IAEA protocol gives results which are in excellent agreement with the AAPM protocol; on average the IAEA results are 0.3% smaller than the AAPM results while discrepancies between the two are in the range of -1.0% to +0.5%. In contrast to the above, for 5-MeV electrons, the IAEA protocols give results smaller than the AAPM protocol by 2.0% on average with discrepancies between protocols ranging from -4.1% to -0.7% depending upon the ionization chamber and phantom material used. For 5-MeV electrons, the discrepancies are particularly large for polystyrene phantom; the average discrepancies being -1.4%, -1.1%, and -3.6% for water, PMMA, and polystyrene, respectively. If data for 5-MeV electrons with polystyrene phantom are excluded, then the overall agreement between the two protocols for photons and electrons is within the range of -1.9% to +0.5%. Principal reasons for the observed discrepancies are (i) IAEA uses the correct expression for ND resulting in up to +0.8% correction; (ii) IAEA uses the most recent stopping power ratio for graphite-to-air resulting in up to +0.5% correction; (iii) IAEA uses a correction of up to +0.8% for the central electrode which AAPM ignores; (iv) the present estimates of the percent depth doses which arise from the differences in measurement depths in the two protocols; and (v) IAEA uses measured values of the fluence correction factor while AAPM uses a theoretical estimate resulting in corrections of up to -2.2%.

Calibration↗

PubMedDate not supplied

Dose distributions produced by shielded applicators using 241Am for intracavitary irradiation of tumors in the vagina.

Dosimetric characteristics of shielded vaginal applicators containing encapsulated 241Am sources are investigated in this work. Encapsulated 241Am sources emit primarily 60-keV photons which are more effectively shielded by thin layers of high atomic number materials than the 662-keV photons from 137Cs sources. With 241Am, it is possible to achieve almost unidirectional irradiation of localized vaginal tumors. The drastic decrease in irradiation volume on the contralateral side (uninvolved with tumor) is observed to decrease dose by up to 20%, even in the forward direction (unshielded side toward the tumor) of the applicator. A possible explanation for the observed effects of shields in both the forward and backward directions is the reduction of scattered photon fluence due to absorption of photons in the lead shield via photoelectric effect. Current theoretical models do not include this perturbation effect caused by shields on brachytherapy applicators.

Americium↗

PubMedDate not supplied

Interseed effects on dose for 125I brachytherapy implants.

Dose calculations in multiseed brachytherapy implants are done by adding the contribution of each individual seed and by assuming that radiation from each seed is unaffected by the presence of the other seeds. To test the validity of this assumption, dose measurements with various configurations of multiseed implants of 125I model 6702 and 125I model 6711 sources were performed. For a linear configuration of three 125I model 6702 seeds at 1-cm separation, with their transverse axes coincident, doses at distances of 3.05 and 5.09 cm from the center along the transverse axis were found to be about 8% lower than the sum of doses from the three individual seeds. However, for three seeds at 1-cm intervals with their longitudinal axes coincident, doses at 3.05 and 5.09 cm distances from the center along the longitudinal axis were found to be about equal to the dose sums from individual seeds. These initial experiments indicated that the magnitude of the interseed effect depends upon the orientation of the seed relative to each other in an implant. To evaluate the importance of this interseed effect for multiseed configurations of 125I model 6702 and 125I model 6711 seeds, dose rates at various distances from a two-plane implant (each plane containing a 3 x 3 array of sources in a 1-cm spacing square grid) were measured in a Solid Water phantom with LiF TLDs. These measurements were carried out in two different planes at different orientations relative to the implant. The average values of the interseed effect at distances ranging from 1 to 7 cm outside the implant were observed to be about the same for 125I model 6702 and model 6711 sources. The mean value of the interseed effect was 6% and the maximum was 12%. On the whole, the interseed effect reduces the dose at the periphery of the iodine implant by 6%.

Brachytherapy↗

PubMedDate not supplied

Tissue inhomogeneity correction for brachytherapy sources in a heterogeneous phantom with cylindrical symmetry.

In brachytherapy it is customary to perform dose calculations for an implant assuming that the tumor and surrounding tissues constitute a uniform, homogeneous medium equivalent to water. In this work, the validity of the above assumption is studied quantitatively for points along the transverse axis of 103Pd, 125I, and 241Am brachytherapy sources, using measured and Monte Carlo calculated dose rates in homogeneous and heterogeneous media with cylindrical symmetry. The irradiation geometry chosen was a single source implanted in a Solid Water phantom which had a 1- or 2-cm-thick cylindrical Solid Water shell replaced by a polystyrene shell. The Monte Carlo simulations were performed using the integrated tiger series CYLTRAN Code. Experimental data were obtained for the same geometry to test the validity of the Monte Carlo calculations for a heterogeneous phantom. Measured dose rates just beyond a 2-cm-thick polystyrene heterogeneity were observed to be greater than those in a homogeneous Solid Water phantom by about 130%, 55%, and 10% for 103Pd, 125I, and 241Am, respectively. Thus the effect of a relatively small polystyrene heterogeneity in Solid Water can be substantial for lower energy photons. This perturbation of dose was found to increase steeply with decreasing energy and increasing size (thickness) of inhomogeneity. A simple dose calculation formalism has been developed to predict dose rate in a heterogeneous phantom with cylindrical symmetry, which uses as input the radial dose functions of the uniform media comprising the heterogeneous phantom. Dose rate predictions using this formalism are in reasonable agreement with the experimental data and the Monte Carlo calculated values.(ABSTRACT TRUNCATED AT 250 WORDS)

Americium↗

PubMedDate not supplied

Pressure wave generated by the passage of a heavy charged particle in water.

Energy deposition around the trajectories of ionizing particles with linear energy transfer (LET) of 4, 40, and 400 keV/microns in water and subsequent diffusion of deposited heat is calculated using computational fluid dynamics. Immediately after the deposition of energy by the charged particle, the temperature and pressure in the vicinity of the particle track both increase dramatically, leading to the formation of a thermal spike and a pressure wave. Initially, the region of heat deposition is primarily localized to a region called the "thermal core," which has dimensions of 0.3, 1, and 3 nm for particles with LETs of 4, 40, and 400 keV/microns, respectively. Instantaneous peak temperatures within the thermal core were 800 degrees C-2000 degrees C and peak pressures were about 25,000 atm. This sudden deposition of heat in a localized region leads to a very strong shock wave around the particle trajectory, which is shown to last for a duration of 10(-9)-10(-8) s. Even at distances beyond 10 nm away from the particle trajectory, pressures above 100 atm could exist for a duration of up to 10(-11) s. This local and transient environment, created by the passage of a charged particle in a medium, may lead to new mechanisms of radiation action leading to cell damage, as well as to the development of new radiation detectors.

Energy Transfer↗

PubMedDate not supplied

Anisotropy functions for 103Pd, 125I, and 192Ir interstitial brachytherapy sources.

Anisotropy of dose distributions around 103Pd, 125I, and 192Ir sources for interstitial brachytherapy was examined. Dose rates around 125I models 6702 and 6711 and 192Ir sources were measured using lithium fluoride thermoluminescence dosimeters (LiF TLDs) in a water-equivalent, solid phantom. From these measured data for 125I and 192Ir and the previously published measured data for 103Pd, isodose rate contours were determined using a bivariate interpolation and smooth surface fitting algorithm. The anisotropy functions, F(r,theta), as defined by the Interstitial Collaborative Working Group (ICWG) for each source, were determined. Also, 4 pi-averaged anisotropy factors, phi an(r), for use in point source approximation, have been calculated at radial distances varying from 1-10 cm for 103Pd, 125I, and 192Ir sources. The anisotropy factors had average values of 0.90, 0.93, 0.95, and 0.98 for 103Pd, 125I model 6711, 125I model 6702, and 192Ir, respectively. The anisotropy factors determined from dose measurements in phantom are observed to be closer to unity than from those determined previously from in-air measurements. This can be attributed to the smoothing of two-dimensional dose distributions due to the presence of more scattered photons in the phantom measurements compared to in-air measurements. Because in-phantom measurements simulate more closely the brachytherapy patient, data from these experiments are recommended for a more accurate determination of dose distributions around clinical brachytherapy implants. In this work, we present a complete set of source data for two-dimensional dosimetry following the ICWG formalism.

Anisotropy↗

PubMedDate not supplied

Calibration of high-energy photon and electron beams for radiotherapy using AAPM 1983 and IAEA 1987 dosimetry protocols.

To follow up on the theoretical comparison of the IAEA 1987 and AAPM 1983 protocols for dosimetry calibration of high-energy photons and electrons [Med Phys. 18, 26-35 (1991)], results of a set of dosimetric measurements made with a Farmer type PTW and Capintec ionization chambers in solid water, PMMA, and polystyrene phantoms and exposed to a 4 MV photon beam from a Varian Clinac 4S at Yale, a 10 MV photon beam and 6 and 15 MeV electron beams from a Varian Clinac 1800 at Phelps Radiation Center, University of Connecticut, and a 25 MV photon beam from a Sagittaire at Yale, are presented. Because different methods are used for the determination of electron beam energies, the values of mean electron energy determined by the two protocols are different by up to 8%. However, for dose inter-comparison, the overall agreement between the two protocols is within 1% in most cases, with a maximum discrepancy of 3.3% in one case. For photons, the IAEA results are smaller than the AAPM results by 0.7% on the average, while maximum discrepancies are in the range of -0.4%-(-1%). In the case of 15 MeV electrons, the discrepancies between the two protocols are found to be in the range of -0.1%-1% and have an average value of 0.5%. In contrast to the above, a large discrepancy is observed between the two protocols for 6 MeV electrons. Depending upon the choice of phantom and ion chamber, this discrepancy is found to be in the range of -0.1%-(-3.3%).(ABSTRACT TRUNCATED AT 250 WORDS)

Calibration↗

PubMedDate not supplied

Superheated drop detector for determination of neutron dose equivalent to patients undergoing high-energy x-ray and electron radiotherapy.

The superheated drop detector (SDD) consists of thousands of superheated drops dispersed in a small vial of gel, which vaporize upon exposure to high LET radiation, thereby providing a directly observable indication of neutron dose. This detector possesses high sensitivity to neutrons and insensitivity to high-energy photons and electrons, making it suitable for the determination of neutron dose equivalent rates around high-energy photon and electron radiotherapy beams. In the present work, the SDD was used to measure the neutron dose equivalent in and around the radiotherapy beams produced by a 32-MeV linear accelerator. For both x-ray and electron beams, the neutron dose profiles were observed to follow the photon/electron radiotherapy beam profiles. For 25-MV x rays, the neutron dose equivalent per photon dose on the central axis increased by a factor of about 3 as field size increased from 5 x 5 to 30 x 30 cm. However, the neutron dose equivalent rate at 50 cm off-axis in the patient plane was essentially independent of field size. The neutron dose equivalent per electron dose was essentially zero for electron beams with energies below 15 MeV, but increased rapidly above 15 MeV. For 25-MeV electrons, neutron dose equivalent on the central axis was about 1/5 that for 25-MV x rays. Analogous to the data for 25-MV x rays, the neutron dose equivalent rate on the central axis of a 25-MeV electron beam exhibited a similar field size dependence and outside the beam it was essentially independent of field size.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrons↗

PubMedDate not supplied

Dosimetric characteristics of a double wall 125I source for interstitial brachytherapy.

Recently, a newly designed encapsulated source of 125I has become commercially available for use in permanent and temporary interstitial brachytherapy. The 125I sources in current use come in two different configurations: the Model 6711 source (Medi Physics/Amersham) for permanent implants has radioactive iodine adsorbed on the surface of a silver wire, and the Model 6702 (Medi Physics/Amersham) source for temporary implants has radioactive iodine absorbed in three spherical resin balls. Both of these iodine sources are encapsulated in a thin-walled shell (0.05-mm thick) made of titanium. The newly designed 125I source (Best Industries Model 2300 series) contains radioactive iodine adsorbed on a tungsten wire that is encapsulated by two walls of titanium. This double-walled 125I source offers the following potential advantages: (i) Because it contains radioactive iodine on the ends as well as the circular surface of the tungsten wire, it can produce a more isotropic dose distribution than the sources in current use; (ii) because it is available in a wider range of source strengths, it is suitable for both temporary and permanent implantation; (iii) because it has a tungsten radiographic marker, source localization is considerably easier than the 125I Model 6702 source that has no radiographic marker; and (iv) because it uses a double-walled encapsulation the risk of radioactive contamination due to source rupture is considerably reduced. In this work, dose distributions produced by the new design 125I source (Model 2300) for interstitial brachytherapy have been measured using LiF TLD's in a Solid Water phantom. Dosimetric characteristics of the new 125I sources are compared with those of the currently available 125I sources.(ABSTRACT TRUNCATED AT 250 WORDS)

Anisotropy↗

PubMedDate not supplied

Response time and safety profile of pulsed oral methotrexate therapy in idiopathic retinal periphlebitis.

PURPOSE: To evaluate the response time and safety profile of low-dose oral methotrexate pulsed therapy in idiopathic retinal periphlebitis (Eales' disease). METHODS: A tertiary care center-based prospective interventional study, based on visual acuity grading, was undertaken. Twenty-one consecutive patients with idiopathic retinal periphlebitis were administered 12.5 mg methotrexate as a single oral dose, once per week for 12 weeks (cumulative dose = 150 mg). Each patient was assessed for change in visual acuity grades. Time of first therapeutic response was also noted. Drug safety was monitored by laboratory tests that included twice-weekly white blood cells and differential counts, twice-weekly platelet counts, and monthly liver function tests. RESULTS: Twenty-one eyes were assessed. Mean follow-up period was 6 months. All showed improvement in visual acuity grades. An excellent visual outcome (6/6 or better) was achieved in 18 (69%) eyes. Time of first therapeutic response varied from 2 to 6 weeks with a majority of eyes (80%) showing response by 4 weeks (median = 3 weeks). All the side effects of methotrexate were mild or moderate in severity and rapidly reversible on dose reduction or discontinuation. No patient had any constitutional symptoms severe enough to necessitate cessation of therapy. CONCLUSIONS: Low dose oral methotrexate pulse therapy (at a dose of 12.5 mg/week) is clinically effective within 4 weeks, and is associated with an acceptable safety profile.

Administration, Oral↗

PubMedDate not supplied

Bilateral lens subluxation associated with atopic eczema.

PURPOSE: Ectopia lentis remains a therapeutic challenge for ophthalmologists. It classically presents with a preceding history of blunt or penetrating ocular trauma, or it may be associated with other ocular disorders such as congenital glaucoma and aniridia, or concomitant hereditary systemic diseases such as Marfan syndrome and homocystinuria. METHODS: Case report. RESULTS: The authors describe a previously unreported mechanism of ocular trauma associated with continuous eye rubbing, resulting in bilateral recurrent subluxation of both intraocular lens and crystalline lens. CONCLUSIONS: It is useful for the ophthalmologist to be aware of this uncommon cause of ectopia lentis, since early advice and appropriate medical or surgical intervention may prevent more severe, sight-threatening complications.

Aged↗

PubMedDate not supplied

Dosimetric penumbra effects in catheter-based intravascular brachytherapy using a centered photon or beta line source.

PURPOSE: In catheter-based intravascular brachytherapy, either photon or beta emitters are often used in a linear arrangement so that blood vessels of 10-30 mm lengths can be treated. With a line source, the dose gradient in the radial direction and longitudinal direction depend on the type of radionuclides used in the treatment. The purpose of this study was to investigate the dose fall-off at the edges of a linear source in a blood vessel for different types of photon and beta emitters. MATERIALS/METHODS: Dose distributions were calculated on cylindrical blood vessels of various radii. Radioactive sources of 192Ir, 125I, 103Pd, 188Re, 32P, and 90Y/Sr were studied. All the sources were assumed to be in the form of a line. The dose rate at a point in space produced by a radioactive source was computed by integrating the point dose rate kernel of the corresponding radionuclide over the radioactive line. The point dose rate kernel was computed with Monte Carlo simulation of radiation transport. The edge effects were characterized with three newly defined quantities: longitudinal dose uniformity (LDU), effective coverage length (ECL), and margin length (ML). LDU was defined as the ratio of dose at a distance along the long axis of the vessel to the dose at center. ECL was defined as the length over which the LDU was greater than 0.95. ML was defined as half of the length difference between source length L and ECL, which is essentially the length segment at each edge that is covered by the source physical length but is being underdosed. RESULTS: All beta emitters provided more uniform dose distributions and covered a larger portion of blood vessels longitudinally than photon emitters. Typical MLs were 2-3 mm for beta emitters and 4-6 mm for gamma emitters. As the radial depth of the point of interest increased, both the LDU and ECL decreased and ML increased. The ML increased from 2 to 3 mm for beta emitters and from 4 to 6 mm for photon emitters when the radial depth of the point of interest increased from 1.5 to 2.5 mm (typical proximal and distal media points for a 3-mm diameter lumen). The ML increased with increasing source length for all radionuclides. For beta emitters the ML increased initially from 1.5 mm to more than 2.5 mm as source length increased from 5 to 10 mm. When the source length was longer than 15 mm, the ML remains nearly constant, about 3 mm. For photon emitters, ML increased continuously from 1.5 mm to more than 6.0 mm, as source length increased from 5 to 50 mm. CONCLUSIONS: A formalism to quantify the dose uniformity along the length of a blood vessel undergoing catheter-based intravascular brachytherapy has been developed. This formalism was used to study the edge effects at the ends of several beta and photon sources. The results indicated that for a centered source the ML at each end due to penumbra effects was about 2 to 3 mm for beta emitters; about 4-6 mm for photon emitters. The ML increases as the radial depth of point of interest in the vessel increases. The ML increases also with increasing source length, especially for photon sources.

Brachytherapy↗

PubMedDate not supplied

On the need for massive additional shielding of a catheterization laboratory for the implementation of high dose rate 192Ir intravascular brachytherapy.

PURPOSE: There is a widespread belief in the cardiology and radiation oncology community that high dose rate 192Ir intravascular brachytherapy cannot be implemented without massive additional shielding of the conventional catheterization labs. The purpose of this work is to show that this is a myth, which is not based on sound radiation protection principles. METHODS: Exposure rates in air were calculated for a variety of point and line sources of 192Ir. Exposures per treatment at different distances from the source were calculated for a typical intravascular brachytherapy treatment of a 15-Gy dose at a radial distance of 2 mm from the source and for source lengths in the range of 0 to 10 cm. Additionally, exposure rates outside the catheterization lab were calculated for various lead shielding thicknesses typical of conventional X-ray facilities. These rates were used along with the NCRP recommendations on radiation facility design to assess shielding requirements. RESULTS: For a treatment dose of 15 Gy at 2 mm, the occupational exposure per treatment at 2 m in air without any tissue attenuation or shielding was 7.8 mR for a lesion length of 3.0 cm. This exposure/treatment is independent of the dose rate or the activity of the source. However, it increases as lesion length is increased, increasing from 5.4 to 24.9 mR as lesion length increased from 2 to 10 cm. Exposures in unrestricted areas outside the catheterization lab using the NCRP shielding rationale can be kept below 2 mR per treatment and using appropriate workload, use, and occupancy factors below 2 mR per week. CONCLUSIONS: The feasibility of implementing a high dose rate 192Ir intravascular brachytherapy program in a catheterization laboratory is totally independent of the dose rate or the activity of the source. If it is feasible to implement 192Ir brachytherapy in a conventional catheterization lab using low activity 192Ir seeds, then it is also feasible to do so with a high activity 192Ir afterloader.

Brachytherapy↗

PubMedDate not supplied

Angiomatoid malignant fibrous histiocytoma of the eyelid.

A 20 year old female had an angiomatoid malignant fibrous histiocytoma of her left upper eyelid extending into the orbit, frontal and temporal regions. The tumor was excised and radiotherapy was given. Nine months follow-up did not reveal any recurrence.

Adult↗

PubMedDate not supplied

A simple accurate method of cataract classification. Cataract-I.

A simple and accurate system of cataract classification using slit lamp and direct ophthalmoscope is reported. Lens opacities are classified into cortical (anterior and posterior), nuclear and posterior sub-capsular and each sub-type of opacity is graded, extent and density wise, using both slit lamp and direct ophthalmoscope. A circle representing enface view of opacity divided into 100 equal parts is used in calculating the area of each opacity. This classification takes into account both the area and depth of opacity in arriving at the total extent of sub-type of each opacity. For density determination, we do not recommend the use of a resolution target projection ophthalmoscope. Intra-observer and inter-observer variability studies using this classification system indicated that the classification system is fairly reliable.

Cataract↗

PubMedDate not supplied

Methodology for studies on medical therapy of cataracts: cataract-II.

The methodology for testing any possible effect of potential anti-cataract agents is described. This is based on slit lamp and ophthalmoscopic cataract classification and on visual acuity. The difficulties encountered in such studies are highlighted. The presented methodology is suggested to be fairly adequate in assessing usefulness of any possible medical therapy of cataracts.

Adult↗

PubMedDate not supplied

Topical glutathione therapy in senile cataracts. Cataract-III.

We undertook a prospective study in 66 cataract patients to study if topical glutathione has any effects on the progression of cataracts. The eye with more advanced cataract received topical glutathione drops q.i.d. and contralateral eye served as control. Only 32 patients completed the follow-up of three months or more. Data analysis revealed no beneficial effect of topical glutathione drops on any of the parameters studied. PSC opacity density in eyes treated with topical glutathione showed significant increase as compared to control eyes.

Administration, Topical↗

PubMedDate not supplied

Topical sulindac therapy in diabetic senile cataracts: cataract-IV.

Sulindac, a non-steroidal anti-inflammatory drug has been found to be a potent inhibitor of enzyme aldose reductase. We used sulindac topically in diabetic senile cataract patients to note if it effects the progression of cataracts. More of sulindac treated eyes maintained initial vision and fewer eyes had visual loss of up to two lines or more as compared to control eyes. The extent and density of different opacities showed less progression in sulindac treated eyes but it was not statistically significant except that the ophthalmoscopically observed density of opacity showed statistically very significant lesser mean increase in sulindac treated eyes. We suggest that sulindac is a potential drug which should be further evaluated in large double blind photodocumented studies in diabetic senile cataracts.

Administration, Topical↗