Biomedical subjects
Publications and source records attributed to S N Rustgi.
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A platinum doped n type commercial silicon electron diode/electrometer system was evaluated for use in our comprehensive external beam radiotherapy quality assurance program. Directional dependence of diode response was investigated by positioning the diode at the radiation isocenter and recording the response versus beam incident direction. Dose rate response was examined by testing for nonidealities in inverse square law behavior. Energy dependence of response was explored using 6-21 MeV beams. Dose response linearity was investigated from 50-800 cGy. Radiation perturbation was measured using localization film. Finally, the system was evaluated for initial treatment quality assurance in a number of clinical cases (N = 34). The diode response exhibited +/- 5% variation within +/- 20 degrees of the detector's marked "preferred" direction for the 6 MeV beam. Higher energy electron beams exhibited much less directional dependence. Dose rate response was found constant from 178-326 cGy/min (or 96-130 cm SSD). Energy dependence of response was constant within +/- 5.5% from 6-21 MeV. Dose response was linear (r2 = 0.999). Radiation field perturbation was significant at the depth of dose maximum (dmax). The maximum perturbation measured was -12.5% at dmax using the 6 MeV beam. Clinical trials of the system demonstrated that the diodes could be utilized for initial treatment quality assurance. The system proved effective for routine initial treatment electron beam in vivo dosimetry when the directional and energy dependent limitations of the system were respected.
The influence of high-density tissue heterogeneities in small-diameter beams used in stereotactic radiosurgery has been investigated. Dose perturbation immediately behind aluminium sheets, used to simulate a high-density tissue inhomogeneity such as bone, was studied in a solid water phantom. Dose reduction factors (DRFs), which are the ratios of the dose in the presence of the inhomogeneity to dose in a uniform density solid water phantom, were measured with a diamond detector for three thicknesses of aluminium. DRFs exhibit dependence on both the inhomogeneity thickness and the beam diameter. The DRF decreases with inhomogeneity thickness. The DRF initially decreases with increase in the beam diameter from 12.5 to 25 mm. For fields greater than 25 mm, the DRFs are nearly constant. The commonly used algorithms such as the TAR ratio method underestimate the magnitude of the measured effect. A good agreement between these measurements and Monte Carlo calculations is obtained. The influence of the high-density inhomogeneity on the tissue maximum ratio (TMR) was also measured with the inhomogeneity at a fixed depth dmax from the entrance surface. The TMR is reduced for all detector-inhomogeneity distances investigated. The dose build-up phenomenon observed in the presence of low-density air inhomogeneity is absent in the presence of a high-density inhomogeneity. The beam width (defined by 50% dose points) immediately beyond the inhomogeneity is unaffected by the high-density inhomogeneity. However, the 90%-10% and 80%-20% dose penumbra widths and the dose outside the beam edge (beyond the 50% dose point) are reduced. This reduction in dose outside the beam edge is caused by the reduced range of the secondary radiation (photons and electrons) in the high-density medium.
The feasibility of using a diamond detector for the dosimetry of brachytherapy sources has been investigated. A high-activity 192Ir source was selected for this purpose. The dosimetric characteristics measured included the photon fluence anisotropy in air, transverse dose profiles in planes parallel to the plane containing the HDR source and isodose distributions. The 'in-air' anisotropy of the photon fluence relative to seed orientation was measured at 5 and 10 cm from the source centre and compared with TLD measurements. Transverse dose distributions in planes parallel to the plane containing the source long axis were measured in a water phantom and compared with calculations performed with a treatment planning system. Isodose distributions were also measured in several planes around the 192Ir source. Measurements on two sources indicate that the 'in-air' photon fluence anisotropy measured by the diamond detector and the TLDs is very similar. Dose profiles measured at several distances from the source are also found to be in good agreement with the calculated dose profiles and isodose distributions. Results of this feasibility study indicate that the diamond detector, with its excellent spatial resolution and nearly tissue equivalent and isotropic radiation response, is an appropriate detector for dose measurements around brachytherapy sources.
The effect of air inhomogeneity on dose distribution in small diameter beams used in stereotactic radiosurgery (SRS) has been investigated. The measurements of the surface dose and central axis dose were made for a 6 MV photon beam with a diamond detector which has suitable radiologic properties for such measurements. Measurements made in a uniform density solid water phantom and in the presence of four air gaps indicate significant dose perturbation immediately beyond the air-solid water interface. The reduction in dose at the surface for a 12.5 mm diameter field is 11%, 17%, 23% and 33% for air gap thicknesses of 3, 4.6, 6 and 9.2 mm, respectively. The corresponding dose reduction for a 25 mm diameter field is 3%, 4%, 7% and 13%, respectively. The ratio of the dose with and without air inhomogeneity is highly dependent on field diameter and approaches 1.0 as the field diameter increases from 12.5 mm to 40 mm. The dose perturbation also increases with increase in air inhomogeneity thickness for all field diameters investigated. A dose buildup phenomenon is observed beyond the air gap with a shallow dmax of approximately 4-6 mm. Beyond the buildup region, a higher dose value compared to a homogeneous phantom is observed at all depths due to reduced photon attenuation in the air gap. The dose profiles beyond the air gap, measured with radiographic films, demonstrate no significant increase in the beam diameter but a pronounced broadening of the beam penumbra (80%-20% and 90%-10%) leading to enhanced dose outside the primary beam geometric edge and reduced dose inside the edge. The dose enhancement outside the beam edge increases with increase in air gap thickness.
The dosimetric properties of a new diamond detector for the measurement of relative dose in photon beams have been investigated and compared to those of a silicon p-type photon diode and a 0.14 cm3 ionization chamber. The mass energy absorption ratio of carbon to water is nearly constant over a wide energy range making the diamond detector nearly tissue equivalent. The directional dependence of the radiation response of the diamond detector for cobalt 60, 6 MV and 18 MV photon beams was more uniform than that of the diode. As the incident photon beam moves from 0 degree (parallel to the detector stem) to a direction transverse to the detector stem (90 degrees), the diamond detector sensitivity remains nearly uniform whereas the diode sensitivity diminishes by approximately 15%-22%. The spatial resolution of the diamond detector, as measured by penumbra width, is slightly less than that of the diode detector but clearly superior to that of the 0.14 cm3 ionization chamber. The tissue maximum ratio measurements for small size photon fields (diameter < or = 4 cm) with the diamond, diode, and a Markus parallel plate chamber are in excellent agreement. The diamond detector with high radiation sensitivity and spatial resolution is an excellent choice as a detector in photon fields with high dose gradients such as brachytherapy and radiosurgery.
Dosimetric characteristics of small diameter 6 MV photon beams for a commercial radiosurgery system have been measured in a solid water phantom with a new diamond detector and compared with measurements using a p-type Si photon diode, small volume cylindrical and parallel plate ionization chambers, and radiographic films. Tissue maximum ratios (TMR) and total scatter factors (S c.p) were measured with the three ionometric (diamond, diode, ion chamber) detectors for 12 circular beams ranging in diameter from 12.5 to 40 mm. The TMR values obtained with the three ionometric detectors agreed well (+/- 1%) for all cone sizes and depths investigated when the displacement of the sensitive volume of the detector from its front surface is taken into account. The S c.p factors obtained with the ionometric detectors also agreed well (+/- 1.2%) for field sizes greater than 20 mm in diameter. For smaller field sizes, the cylindrical and parallel plate chambers measure a smaller S c.p factor, as a result of the steep dose gradients across their sensitive volumes. Cross-beam profiles acquired with the diamond detector agree well with the measurements with the diode detector and radiographic film. A distortion in the measured profiles in terms of broadened penumbra is observed with a small volume cylindrical ionization chamber.
Patients with basal cell skin cancer can be treated with afterloading 192Ir surface molds constructed from dental wax. High activity 192Ir seeds in ribbons are distributed in the mold to uniformly irradiate the target volume. The treatment is preplanned with a treatment planning system to achieve a uniform dose distribution in the planned target volume. The implant parameters optimized include the seed strength and number of seeds, inter- and intracatheter spacing between 192Ir seeds, and the distance between the implant and treatment planes. The radioactive 192Ir strands are afterloaded in the catheters embedded in the wax mold and the position secured with buttons. The treatment area drawn on the patient surface is visually overlapped with the uniformly irradiated area sketched on the mold surface. The mold is taped to the head to secure this position. The dose rate on the surface of the mold in contact with the patient skin is measured with calibrated LiF TLD chips and is within +/- 5% of the computer preplanned dose rate value. This technique is a viable alternative to external beam treatments when daily treatments are not feasible and a dose distribution conforming to the treatment area is desirable.
Measurements of the central axis depth dose curves, isodose profiles and field size dependence of the output factors for small field electron beams from a Varian Clinac 18 linear accelerator have been performed. Energies of electron beams studied were 6, 9, 12, 15, and 18 MeV. Circular fields of 1, 2, 3, 5, and 8 cm diameter were obtained from cerrobend shields attached to the bottom face of a 15 x 15 cm2 electron cone applicator. Measurements were carried out in a water phantom with an ionization chamber and a silicon diode detector and in a polystyrene phantom with Kodak XV-2 film. For all energies studied, as the field size of the electron beam becomes smaller: (1) the depth of maximum dose shifts toward the surface, (2) the depth of 90% and 80% dose, which are common dose prescription depths, becomes smaller, (3) the surface dose increases, and (4) the dose fall-off region becomes more gradual. The output of the electron beam reduces significantly with reduction in electron beam field size as a consequence of lack of lateral equilibrium. Hence, for accurate treatment planning for small electron fields, it is essential that the beam characteristics be individually measured.
There has been considerable interest in the use of high activity 125I sources as a substitute for 192Ir seeds for removable implants of the breast and prostate. 125I seeds with an initial activity of approximately 5 mCi per seed, loaded in special afterloading nylon catheters, are used to improve dose distribution in the tumor volume and minimize dose to the adjacent critical organs and normal tissues. Seed spacing in strands is adjusted to maintain a dose rate of 40-60 cGy per hour at a distance of 5 mm from the plane of the implant. Implants custom loaded with 125I sources achieve superior isodose distribution compared to implants loaded with standard 192Ir seed strands. High activity 125I seeds also offer the advantage of reduced exposure to radiation oncology staff, nurses, and visitors leading to better patient care. Due to reduced exposure to personnel, the accuracy of the actual implant geometry can be verified by taking a localization film with actual 125I sources placed in the tumor bed.
Liver metastases from colorectal carcinomas occur frequently. While surgical resection offers the only hope for long-term cure, unsuspected bilobar metastases or extrahepatic metastatic disease may be found at laparotomy, precluding hepatic resection for cure. In this setting intraoperative interstitial hepatic irradiation using the Gamma Med II (Mick Radio-Nuclear Instruments, Bronx, New York) remote afterloading irradiator and an Iridium-192 source permits delivery of a tumoricidal dose to liver tumor(s) with a limited radiation dose to adjacent normal liver. Six patients underwent laparotomy for potential resection of hepatic metastases in a shielded operating room equipped with remote anesthesia monitoring capability and were found to be unresectable. An upper hand retractor facilitated liver exposure during the exploratory and subsequent radiation phases of the procedure. Intraoperative interstitial radiation therapy was performed in each patient. No significant complications occurred on follow-up from 2 to 9 months. Hepatic tumor regression or stabilization occurred on sonography and/or CT scan in each case with a median follow-up of 5 months. The technique offers the potential to ablate discrete tumor nodules within the liver. Ongoing clinical trials will determine the role of intraoperative interstitial radiation in the treatment of hepatic metastases.
Effectiveness of eye shields in reducing the dose to the eye lens from 6 and 9 MeV electron beams from a linear accelerator has been evaluated. The thickness of the shields made from cerrobend was such that only bremsstrahlung photons were transmitted. A shield with a diameter of 1.3 cm and thickness of 1 cm was adequate for the 9 MeV electron beam. The optimum shield to phantom surface distance was 1 cm or less. The same shield with a thickness of 0.5 cm was found to be ineffective with a 6 MeV electron beam. The dose under the shield is greater than predicted by transmission measurements because of the contribution of phantom and electron cone generated scattered electrons.
A new technique is reported for the treatment of hepatic metastases using sonography-directed percutaneous placement of a 14-gauge needle applicator and a high-intensity "remote afterloading" iridium-192 (Ir-192) source for interstitial radiation therapy. The results with six patients show that the procedure is easily performed, patient tolerance is good, and there is minimal disruption of the patient's lifestyle. Hospitalizations have been less than 24 hr. Partial response or stable disease in the liver was observed in all six patients. Tumoricidal doses up to 5000 rad (cGy) in a single treatment with durations from 7 to 41 min were achieved in small volumes (less than 25 cm3) with no clinically significant toxicity on follow-up evaluations from 2-6 months. The technique appears to ablate discrete metastatic tumor deposits in the liver.
The per cent ionisation curves for the 4 MV photon beam from a Clinac 4 exhibit pronounced enhancement with field size in the build-up region. The source and nature of this increase is investigated by analysing the radiation components of the 4 MV photon beam with a 0.23 T magnetic field. The build-up ionisation curves were measured at 86 cm SSD with a parallel-plate ionisation chamber embedded in a polystyrene phantom. The surface ionisation increases from 18% to 32% as the field size is increased from 4.3 cm X 4.3 cm to 9.7 cm X 32.3 cm. With the electromagnet placed beside the treatment head, the field size dependence reduces to 16% to 24%. This and similar results at 113.2 cm SSD, clearly demonstrate that electrons from the Clinac 4 treatment head are responsible for approximately half the observed increase in the surface ionisation with field size.
Radiation fluence distribution around 192Ir and 198Au seeds of the type used for brachytherapy application was measured using a NaI crystal connected to a multi-channel analyzer. The radiation fluence, determined as a function of the angle between the seed axis and the direction of detection, is anisotropic with the fluence along the seed axis amounting to 78% of that perpendicular to the seed axis for seeds of both isotopes. The region of fluence reduction is broader for 198Au seeds, extending from 0-45 degrees; for 192Ir the decrease in fluence is evident for angles of 0-20 degrees. Integration of the angular distribution over 4 pi geometry indicated the effective activity of the implants to be 4% and 1% less than the nominal activity for the 198Au and 192Ir seeds, respectively, based on calibration measurements perpendicular to the seed axis.
The reliability of data obtained from the Norland-Cameron Bone Mineral Analyzer has been questioned and the possibility of an inherent calibration error in the machine has been raised. To make an absolute measurement of the instrumental accuracy, bone mineral content measured by the bone mineral analyzer was compared with an absolute value obtained by the ashing method. The instrument bone mineral content values were found to be higher by about 6.5%. The ash content (%), bone mineral density, and compact bone density in these same bone samples were 62.9%, 1.346 g/cm3, and 2.140 g/cm3, respectively, as determined by direct measurement.
Muscle, spleen, and kidney tissues from 4-wk-old C57 black mice were studied by proton magnetic resonance. Spin-lattice relaxation times at high fields and in the rotating frame, as well as the spin-spin relaxation times, are reported as a function of temperature in the liquid and frozen phase. Motions of large molecules and of water molecules and their changes at the freezing phase transition are studied. The shortcomings of the two-state fast-exchange relaxation model are discussed.
Reactions of the SO4- radical, generated by U.V. photolysis of Na2S2O8, were studied in aqueous solutions of amino acids, dipeptides, nucleic acid bases, nucleosides and nucleotides. The transient free radicals so formed were spin-trapped by t-nitrosobutane and identified by e.s.r. spectroscopy. The amino acids primarily undergo oxidative decarboxylation. The pKs of the ammonium groups of the spin-trapped decarboxylated radicals of glycine and alanine in D2O were determined to be 8.3 +/- 0.2. An oxidation product, which is the precursor of the decarboxylated radical, is tentatively identified for alanine, valine and isoleucine. Radicals formed by hydrogen abstraction by SO-4 are identified for leucine, serine, phenylalanine and 4-hydroxyproline. In dipeptides, SO-4 produces decarboxylation of the amino acid located at the carboxylate terminal residue. For gly-ala and ala-ala, radicals generated by hydrogen abstraction from the carboxylate terminal residue alanine were also characterized. Radicals centered on the C(5) carbon were observed for uracil, cytosine and thymine. For nucleosides and nucleotides, radicals situated on the base and/or the sugar moiety were assigned.