Role of plasma renin activity and the renal nerves in the natriuresis of L-NMMA infusion in rats.

The objective of this study was to determine the effect of N(G)-monomethyl-L-arginine (L-NMMA) infusion on plasma renin activity (PRA) in the presence or absence of the renal nerves in normotensive Wistar-Kyoto (WKY) rats and Okamoto spontaneously hypertensive rats (SHR). All rats were unilaterally nephrectomized two weeks before the acute experiment. On the day of the experiment, acute renal denervation (Dnx) of the remaining kidney was performed in one group of WKY rats (Dnx-WKY; n= 10) and one group of SHRs (Dnx-SHR: n=7). The renal nerves were left intact in a group of WKY rats (Inn-WKY; n=8) and SHRs (Inn-SHR; n=9). After a control clearance period, L-NMMA was administered i.v. (15 mg/kg bolus followed by 500 microg/kg/min infusion) and another clearance period of 20 min was taken. In all experimental groups L-NMMA infusion resulted in a significant natriuresis. L-NMMA infusion increased fractional excretion of sodium (FE(Na)) to a greater extent in the Inn-SHR than in the Inn-WKY (delta FE(Na) = 5.23+/-0.87% vs delta FE(Na) = 2.87+/-0.73% respectively; P=0.05), PRA did not change in the SHR with the infusion of L-NMMA. However, in the Inn-WKY group, the natriuresis of L-NMMA infusion was associated with a tendency for lower PRA levels as compared to a group of time control Inn-WKY rats. In Dnx-WKY, the natriuresis of L-NMMA infusion (delta FE(Na) = 4.60+/-0.52%) was associated with a significantly lower level of PRA (4.26+/-1.18 ng AI/ml/hr) as compared to a group of time control Dnx-WKY rats (9.83+/-1.32 ng AI/ml/hr; P<0.05). In the Dnx-SHR, the natriuretic response to L-NMMA infusion was significantly attenuated by renal denervation (delta FE(Na) = 2.36+/-0.34%) and PRA was unchanged. In conclusion, the natriuretic effect of systemic inhibition of nitric oxide (NO) synthesis was associated with decreased PRA in the Dnx-WKY suggesting that a potential interaction exists between NO and the renal nerves in the modulation of PRA in the normotensive WKY rat. Whereas, the natriuretic effect of L-NMMA infusion in the SHR in the presence and absence of the renal nerves, were independent of changes in PRA.

Leaf position error during conformal dynamic arc and intensity modulated arc treatments.

Conformal dynamic arc (CD-ARC) and intensity modulated arc treatments (IMAT) are both treatment modalities where the multileaf collimator (MLC) can change leaf position dynamically during gantry rotation. These treatment techniques can be used to generate complex isodose distributions, similar to those used in fix-gantry intensity modulation. However, a beam-hold delay cannot be used during CD-ARC or IMAT treatments to reduce spatial error. Consequently, a certain amount of leaf position error will have to be accepted in order to make the treatment deliverable. Measurements of leaf position accuracy were taken with leaf velocities ranging from 0.3 to 3.0 cm/s. The average and maximum leaf position errors were measured, and a least-squares linear regression analysis was performed on the measured data to determine the MLC velocity error coefficient. The average position errors range from 0.03 to 0.21 cm, with the largest deviations occurring at the maximum achievable leaf velocity (3.0 cm/s). The measured MLC velocity error coefficient was 0.0674 s for a collimator rotation of 0 degrees and 0.0681 s for a collimator rotation of 90 degrees. The distribution in leaf position error between the 0 degrees and 90 degrees collimator rotations was within statistical uncertainty. A simple formula was developed based on these results for estimating the velocity-dependent dosimetric error. Using this technique, a dosimetric error index for plan evaluation can be calculated from the treatment time and the dynamic MLC leaf controller file.

Improved dose homogeneity to the intact breast using three-dimensional treatment planning: technical considerations.

The current consensus is that breast-conservative treatment is superior to mastectomy because it provides survival equivalent to total mastectomy and axillary dissection while preserving the breast. This technique still has several technical issues that can adversely influence the successful outcome of breast-conservative treatment. Specifically, dose coverage and homogeneity must be maintained throughout the breast while reducing the hot-volume magnitude and normal tissue complications. A random retrospective three-dimensional treatment-planning study was conducted using computed tomography scans of 20 female patients with early-stage breast cancer. Two- and three-dimensional homogenous and heterogeneous treatment planning was conducted using all possible hard-wedge combinations and effective photon energies, with the goal of reducing the hot volumes in the breast below 110 percent of the prescribed dose. The hard-wedge combination that minimized the hot volumes uses either 15-degree wedges on the medial and lateral beams or a 30-degree wedge on the medial beam and a 15-degree wedge on the lateral beam. For patients with bridge distances less than 20 cm, this wedge combination reduces the hot volumes below 110 percent of the prescribed dose. For patients with bridge distances greater than 20 cm, low- and high-energy photon beams must be mixed to lower the maximum dose below 110 percent of the prescribed dose. The hot volumes in the breasts of 20 random patients was reduced below 110 percent of the prescribed dose without a significant reduction in tumor coverage.

The paracellular permeability of opossum kidney cells, a proximal tubule cell line.

UNLABELLED: The paracellular permeability of opossum kidney cells, a proximal tubule cell line. BACKGROUND: The regulation of the unusually leaky paracellular pathway of the proximal tubule is poorly understood partially because of the lack of an appropriate in vitro cell model. In this study, we determined whether the paracellular permeability of opossum kidney (OK) cells would resemble that of the in vivo proximal tubule epithelium. METHODS: The parental and subclonal OK cells and, for comparison, LLC-PK1 cells were cultured on permeable Transwell supports. The apparent paracellular permeability coefficient (Papp) for the extracellular marker 3H-mannitol was determined. RESULTS: The Papp of OK cell sheets (12.17 x10-6 cm/sec) was remarkably close to the previously reported Papp of rat proximal tubules. The Papp of LLC-PK1 cells, another proximal tubule cell line, however, was approximately 20-fold lower than that of both OK cells and the in vivo proximal tubule. Phorbol 12-myristate 13-acetate, a protein kinase C activator, enhanced the Papp of OK cell sheets. The characteristic response of paracellular permeability to Ca2+ switch was demonstrated in OK cell sheets. Slight variations of Papp among several OK subclones were observed. Basal to apical Papp was uniformly higher than apical to basal Papp, independent of cell subtype. This rectification was attenuated by inhibition of active transport. CONCLUSIONS: OK cell sheets cultured on Transwell supports possess a leaky paracellular pathway resembling that of the proximal tubule epithelium in vivo.

A comparison of beam characteristics for gated and nongated clinical x-ray beams.

Respiratory gating has only recently been applied to conventional external beam radiotherapy. In order for respiratory gating to be used clinically, an evaluation of the dosimetric effects of small units of delivered dose must be performed. The purpose of this study is to systematically evaluate the effect of various gating sequences on x-ray central axis output, ionization ratios (nominal accelerating potential), beam flatness, and beam symmetry. Measurements were taken for 6 and 18 MV photons on a linear accelerator that generates the gate by using a gridded electron gun to stop the electron flow to the wave-guide. The beam output, energy, flatness, and symmetry did not vary by more than 0.8 percent in most of the gating sequences. The maximum output deviations (0.8 percent), flatness deviations (1.9 percent), and symmetry deviations (0.8 percent) occurred when a low number of monitor units (<5 MU) were delivered in the gating window. Although these deviations are not clinically significant, each linear accelerator should be evaluated carefully before clinical implementation.

Clinical application of digitally-reconstructed radiographs generated from magnetic resonance imaging for intracranial lesions.

PURPOSE: The purpose of this work is to demonstrate the clinical utility of magnetic resonance (MR) imaging-based digitally reconstructed radiographs (DRRs) for the setup and verification of patients with intracranial lesions. METHODS AND MATERIALS: MR images of 16 patients with various intracranial lesions were obtained for treatment planning and virtual simulation. Five-millimeter-thick contiguous T1-weighted postcontrast transverse slices were obtained using a standard head coil in a General Electric Signa 1.5T MR scanner. MR-DRRs were generated using the "pseudo density" technique on an existing treatment planning computer without any special modifications. Anterior and lateral verification films were taken for each patient for visual comparison with MR-based DRRs. RESULTS: Visual alignment with bony landmarks, including the orbits, frontal sinus, sphenoid sinus, auditory meatus, nasal bone, vomer bone, mastoid process, and the cranium were used by physicians, physicists, and therapists to verify patient positioning. Misalignments from 3 to 10 mm were visually identified and corrected using this technique. CONCLUSION: A method for visually utilizing MR-based DRRs during simulation has been developed and clinically implemented. The quality of MR-DRRs generated using this technique is such that physicians, physicists, and therapists can easily and routinely compare MR-DRRs side-by-side with simulation films.

Dosimetric verification of two commercially available three-dimensional treatment planning systems using the TG 23 test package.

The Task Group 23 (TG-23) radiation treatment planning dosimetry verification package was used to evaluate the dosimetric accuracy of two commercially available treatment planning systems. The TG-23 test package contains experimentally measured beam data for two x-ray beams (4 and 18 MV) that can be used as input for 3D-RTP (three-dimensional radiation treatment planning) systems. Once the beam data is entered and modeled, a series of test cases are performed that isolate different aspects of the dose computational process. The computed values from the 3D-RTP system are compared against the measured dosimetry data, included in the package, for a set of comparison points within each test case. Both of the treatment planning systems that were studied provided excellent agreement between computed and measured doses. The cumulative 4 and 18 MV TG-23 test results for the convolution/superposition based planning system indicates that 96% of the dosimetric test points are within +/-2%, and 98% are within +/-3% of the tabulated TG-23 values. The dosimetric TG-23 test results for the pencil beam kernel based planning system are similar, with 96% of the test points falling within +/-2%, and 99% falling within +/-3% of the TG-23 measurements.

Clinical efficacy of respiratory gated conformal radiation therapy.

One major limitation of three-dimensional conformal radiation therapy that has not been adequately addressed is respiration-induced organ motion. During respiration, tumors in the abdomen can typically move from 1 to 3 centimeters. Because the size and shape of external radiation treatment fields do not change during treatment, the field size of the x-ray beam must be enlarged to encompass the tumor through the entire respiration cycle. Several manufacturers are developing respiratory gating systems. These systems allow the selective delivery of absorbed doses to moving target volumes in the abdomen during time intervals when the target volume is within the intended location. Before respiratory gated radiotherapy can be implemented clinically, the efficacy of the procedure must be justified. The magnitude of dosimetric and geometric uncertainties associated with respiratory motion must be identified to determine if gating can provide an advantage over conventional treatment techniques. In addition, clinical situations and specific types of cancer that could benefit from respiratory gating must also be identified.

Ionization chamber, electrometer, linear accelerator, field size, and energy dependence of the polarity effect in electron dosimetry.

Plane-parallel ionization chambers are the instrument of choice for use in electron calibration and dosimetry, but these chambers may exhibit large polarity effects. This study concentrates on measuring the dependence of the polarity error at various mean energies using different linear accelerator, field size, ion chamber, and electrometer combinations. The polarity error was shown to increase for all four ionization chambers as the mean energy at depth decreased, but was always less than one percent at d(max). Polarity error dependence was also observed for similar plane-parallel chambers, varying field sizes, and different linear accelerators, but no polarity error dependence was observed for similar cylindrical chambers and different electrometers. Measurements of the polarity error can be used to develop correction factors for future measurements that will help minimize the time spent performing electron dosimetry and calibrations. These correction factors can be used to calculate the correct reading without the need to reverse the chamber bias, thus reducing the number of measurements required.

Solar particle event dose and dose-rate distributions: parameterization of dose-time profiles, with subsequent dose-rate analysis.

Calculations of total dose and equivalent dose as functions of time, as well as dose-rate and equivalent dose rate since event start are presented for fifteen of the larger solar particle events that occurred during the period between November 1987 and August 1991. The doses, dose-equivalents, and rates presented are for exposures to the skin, ocular lens, and bone marrow behind a thickness of aluminum shielding which provides protection comparable to that of a thin spacecraft. The calculated dose vs time profiles are parameterized using a Weibull cumulative distribution as the fitting function. Parameters are determined using least-squares techniques. Fitted curves are then differentiated to produce smoothed dose-rate curves for each of the events. These results provide a useful starting point for the development of methods to predict the cumulative doses and times to reach various dose limits from a limited number of dosimeter measurements early in the evolution of a solar particle event.

Optical model methods of predicting nuclide production cross sections from heavy ion fragmentation.

Quantum mechanical optical potential methods for calculating inclusive isotope and element production cross sections from the fragmenting of heavy nuclei by intermediate- and high-energy protons and heavy ions are presented based upon a modified abrasion-ablation-FSI (frictional spectator interaction) collision model. The abrasion stage is treated as a quantum mechanical knockout process that leaves the residual prefragment in an excited state. Prefragment excitation energies are estimated using a combined liquid drop and FSI method. In ablation the prefragment deexcites by particle and photon emission to produce the final fragment. Contributions from electromagnetic dissociation to single nucleon removal cross sections are incorporated using a Weiszacker-Williams theory that includes electric dipole and electric quadrupole interactions. Estimates of elemental and isotopic production cross sections are in good agreement with published cross section measurements for a variety of projectile-target-beam energy combinations.

Magnetic resonance imaging based digitally reconstructed radiographs, virtual simulation, and three-dimensional treatment planning for brain neoplasms.

Currently, patients with brain neoplasms must undergo both computed tomography (CT) and magnetic resonance (MR) imaging to take advantage of CT's density information and MR's soft tissue imaging capabilities. A method has been developed that allows virtual simulation, digitally reconstructed radiographs (DRRs), and 3-D treatment planning of patients with brain neoplasms to be generated using only one T1-weighted MR data set. DRRs of an anthropomorphic RANDO head phantom were generated using MR and CT imaging. The MR based DRRs provided structural information equivalent to CT based DRRs. The spatial linearity of CT and MR image sets was evaluated by measuring the percent distortion and spatial error. There was no statistical difference in spatial linearity or accuracy between the CT and MR image sets. MR and CT based treatment planning were compared using a variety of different treatment accessories, field sizes, photon energies, and gantry positions. Doses at various points throughout the head phantom were used as comparison points between CT based heterogeneous, CT based homogenous, and MR based homogenous treatment planning of the head phantom. Lithium fluoride thermoluminescent dosimeters were used to verify the dosimetric accuracy of MR based treatment planning by taking measurements at these points. For treatment plans with fields that pass through large air cavities, such as the maxillary sinus, homogenous treatment planning produces unacceptable dosimetric error (2%-4%). For treatment plans with fields that pass through the skull, MR homogenous treatment planning can be used with a dosimetric accuracy of +/- 2%.

Effect of volume expansion on the paracellular flux of lanthanum in the proximal tubule.

Although studies of volume expansion (VE) in the Necturus suggest a major role for paracellular flux in reabsorption by the proximal tubule, results from morphologic or electrophysiologic studies of the effect of VE on the rat proximal tubule suggest only a minor role for paracellular transport. In the present study, during in vivo microperfusion, lanthanum was used as an extracellular marker to determine bidirectional paracellular flux in the rat proximal tubule before and during 10% VE. Lanthanum itself did not affect proximal tubule reabsorption (delta 0.7 +/- 3.3 nl/min, LaCl3 versus saline infusion, n = 7). When lanthanum was added to the luminal perfusate, paracellular lanthanum efflux from the lumen to the interstitium was 28.9 +/- 6.6 pg/min per mm, n = 7. Subsequent VE significantly decreased the paracellular lumen-to-interstitium efflux to 12.8 +/- 8.3 pg/min per mm concomitant with a 49% decrease in proximal fluid reabsorption (delta -2.6 +/- 0.9 nl/min per mm, P < 0.05). When lanthanum was infused interstitially, by means of a chronically implanted matrix, there was significant paracellular lanthanum influx from the interstitium into the lumen (143.9 +/- 18.6 pg/min per mm, n = 4). Subsequent VE significantly increased this interstitium-to-lumen influx to 212.1 +/- 29.2 pg/min per mm as proximal reabsorption was significantly decreased by 58% (delta -2.8 +/- 0.8 nl/min per mm, P < 0.05). Thus, VE affects bidirectional paracellular flux in a manner that would decrease proximal reabsorption; paracellular efflux from the lumen to the interstitium was decreased, whereas paracellular influx from the interstitium to the lumen was increased.

Solar particle event dose distributions: parameterization of dose-time profiles.

Calculations of total dose and dose equivalent as a function of time since the start of the event are presented for four of the major solar particle events that occurred during the period from August to December 1989. Results are presented for exposures to the skin, ocular lens and bone marrow shielded by a nominal thickness of aluminum shielding, comparable to that provided by a spacesuit. The calculated curves of organ dose and dose equivalent versus time are parameterized using a Weibull functional form for the fitting equation. The fitting parameters are determined using least squares regression techniques. These results provide a useful starting point for the development of methods to predict the cumulative doses and times to reach various dose limits from a limited number of dose measurements early in a solar particle event.

Optical model methods of predicting nuclide production from spallation reactions.

Quantum mechanical optical model methods for calculating isotope production cross sections from the spallation of heavy nuclei by high-energy protons are developed from a modified abrasion-ablation collision formalism. The abrasion step is treated quantum-mechanically as a knockout process which leaves the residual prefragment nucleus in an excited state. In ablation the prefragment deexcites to produce the final fragment. The excitation energies of the prefragments are estimated from a combination of liquid drop and frictional-spectator interaction considerations. Estimates of elemental and isotopic production cross sections are in good agreement with recently published cross section measurements.

Blockade of cytochrome P-450 epoxygenase pathway attenuates the natriuresis of N(G)-monomethyl-L-arginine infusion in the spontaneously hypertensive rat.

Previous studies demonstrated that there is increased renal synthesis of cytochrome P-450-dependent arachidonic acid metabolites in the vasculature and tubules of the Okamoto spontaneously hypertensive rat (SHR). It has also been shown that the natriuretic response of the SHR to NG-monomethyl-L-arginine (L-NMMA) infusion is exaggerated compared with that of the normotensive Wistar-Kyoto rat. The purpose of this study was to determine the roles of cytochrome P-450 epoxygenase and cyclooxygenase pathways in the natriuresis that is observed with the systemic infusion of a high dose of L-NMMA to inhibit nitric oxide synthesis in the SHR and the Wistar-Kyoto rats. After a control clearance period of 20 minutes groups of adult SHR (n = 14) were given L-NMMA (15 mg/kg bolus followed by 500 microg/kg/min continuous infusion). In other groups of SHR either ketoconazole (0.5 mg/kg, n = 9) to inhibit the renal activity of cytochrome P-450 epoxygenase pathway or indomethacin (3 mg/kg, n = 7) to inhibit cyclooxygenase activity was administered intravenously 20 minutes before the control clearance period. After the control clearance period L-NMMA was infused as previously described. Infusion of L-NMMA in the control group of SHR resulted in a significant increase in fractional excretion of sodium (FE Na from 1.78% +/- 0.24% to 6.90% +/- 0.61%). In the ketoconazole-treated group of SHR, L-NMMA infusion resulted in a significant natriuresis (from 2.22% +/- 0.58% to 4.70% +/- 0.93%); however, the natriuretic response was significantly attenuated compared with that of the control group of SHR that received only L-NMMA (delta FE Na, 2.47% +/- 0.40% vs 5.24% +/- 0.55%). Indomethacin administration did not affect the natriuretic response to L-NMMA infusion in the SHR. In conclusion, the natriuretic response to L-NMMA infusion in the SHR is significantly attenuated by administration of ketoconazole but not indomethacin. This result suggests that the natriuretic effect of L-NMMA infusion in the SHR is mediated at least partly by cytochrome P-450 metabolites of the epoxygenase pathway.

Reversal of the antinatriuretic effect of prostaglandin E2 by verapamil in the rat.

Previous studies have demonstrated that prostaglandin E2 (PGE2) infusion increases intrarenal angiotensin-II (ANG-II) formation and decreases sodium excretion in the rat. PGE2 infusion may have direct tubular effects or indirect effects through increased intrarenal ANG-II. In the present study, the calcium channel blocker verapamil was used to determine whether it would reverse the PGE2-induced decrease in sodium excretion. To minimize any systemic and hemodynamic influences, verapamil and PGE2 were infused directly into the renal interstitium via a chronically implanted matrix. Fractional sodium excretion (FENa), glomerular filtration rate (GFR), mean arterial pressure (MAP), and plasma renin activity (PRA) were measured before and during renal interstitial infusion of PGE2 (10(-5) M) and/or verapamil (10(-3) M) in rats pretreated with indomethacin. The renal interstitial infusion of PGE2 alone significantly decreased FENa (delta-1.0 +/- 0.2%), whereas the addition of verapamil reversed the effect of PGE2 and significantly increased FENa (delta 2.6 +/- 0.3%, n = 9). The renal interstitial infusion of verapamil alone markedly increased FENa (delta 1.7 +/- 0.3%, n = 7), and this natriuresis was accompanied by a significant decrease in PRA (delta-0.6 +/- 0.1 ng/ml/h, p < 0.05). The addition of PGE2 to the interstitial infusion did not further affect FENa or PRA. There was a significant difference between the effect of interstitial PGE2 infusion and interstitial PGE2 infusion and interstitial verapamil infusion on PRa (delta 1.9 +/- 0.8 vs. delta -0.6 +/- 0.1 ng/ml/h, p < 0.05). GFR and MAP remained unchanged in response to the renal interstitial infusion of PGE2 and/or verapamil. In conclusion, verapamil reversed the PGE2-induced antinatriuresis in the rat.