Scintillation and radioluminescence mechanism in β-Ga2O3 semiconducting single crystals

In this paper, we present the results of experiments on samples of β-Ga2O3 single crystals under a project aimed at assessing and improving the scintillation performance of this material by studying scintillation and radioluminescence mechanism and its limitations. In addition to standard experiments, such as scintillation light yields and time profiles, radio-, and thermoluminescence, we developed and tested a new and promising two-beam experiment, in which a sample is excited by an X-ray beam and additionally stimulated by an IR laser diode. Fe and Mg doping compensate for the inherent n-type conductivity of β-Ga2O3 to obtain semi-insulating single crystals for large-area substrates and wafers. At the same time, residual Fe and Ir are ubiquitous uncontrolled impurities leached from the Ir crucibles used to grow large bulk crystals by the Czochralski method. For these experiments, we selected four samples cut from the Czochralski grown 2-cm diameter β-Ga2O3 single crystal boules; one with a reduced Fe content, two unintentionally Fe- and Ir-doped (UID) with lower and higher Fe content, and one doped with Mg. We find that steady-state radioluminescence spectra measured at temperatures between 10 and 350 K are dominated by the UV emission peaking at about 350-370 nm. Unfortunately, even for the best sample with a reduced Fe-content, the intensity of this emission drops precipitously with the temperature down to about 10 % at 300 K. From the two-beam experiments, we conclude that recombination via inadvertent Fe impurity involving three charge states (2+, 3+, and 4+) may reduce a steady-state UV emission of β-Ga2O3 under X-ray excitation by as much as 60-70 %, one-third to one-half of which is due to the recombination (specific for Fe-doped β-Ga2O3) involving the 4+ and 3+ charge states of Fe and the remaining 50-70 % being due to a more familiar route typical of other oxides, involving the 2+ and 3+ charge states of Fe. These losses are at higher temperatures enhanced by a thermally activated redistribution of self-trapped holes (STHs). In addition, the trapping of electrons by Fe and holes by Mg, Fe, and Ir may be responsible for scintillation light loss and reduction of the zero-time amplitude essential for the fast timing scintillation applications. Despite indirect evidence of competitive recombination in β-Ga2O3 involving a deep Ir3+/4+ donor level, we could not quantitatively assess losses of the UV steady state radioluminescence light due to the inadvertent Ir impurity.

Does the Parathyroidectomy Endanger the Transplanted Kidney?

BACKGROUND

Some investigators maintain that a parathyroidectomy (PTX) performed for tertiary hyperparathyroidism may potentially cause graft malfunction or even loss of the transplanted kidney after the operation. The goal of this study was to determine if parathyroidectomy affects transplanted kidney function.

METHODS

The study group consisted of 48 renal graft recipients who underwent operation due to tertiary hyperparathyroidism. Thirty-nine subtotal parathyroidectomies and 9 more selective, less than subtotal parathyroidectomies were performed. The estimated glomerular filtration rate (eGFR) was calculated retrospectively on days 2 to 3 and 4 to 5 and at 1, 3, 6, 12, 24, and 36 months after PTX; these findings were compared with preoperative values. The cumulative graft survival rate in the postoperative period was assessed.

RESULTS

In the follow-up period, 4 of 48 patients returned to hemodialysis (after 1, 7, 22, and 57 months after PTX). In the first case, the patient had stopped taking the immunosuppressive drugs 1 month after PTX. Cumulative graft survival rate after PTX was 98.0% after 6 months, 96% after 12 months, and 93% after 2 and 3 years. The mean preoperative eGFR was 52 ± 17.15 mL/min/1.73 m(2), and the median was 48.28 mL/min/1.73 m(2). Overall and in the subtotal parathyroidectomy group, eGFR was significantly lower (P < .001) only on days 2 to 3. There were no differences between preoperative and postoperative eGFR values in the other follow-up periods. In the more selective, less than subtotal parathyroidectomy group, the decrease in eGFR values was nonsignificant compared with preoperative findings in the early postoperative period as well as in all follow-up periods.

CONCLUSIONS

In this study, PTX did not significantly impair transplanted kidney function, but in the early postoperative period, transient reductions in graft function did occur.

Melt growth and properties of bulk BaSnO3 single crystals

We present the first-time growth of bulk BaSnO₃ single crystals from the melt by direct solidification, their basic electrical and optical properties as well as their structural quality. Our measurement of the melting point (MP) of BaSnO₃ amounts to 1855 °C  ±  25 K. At this temperature an intensive decomposition and non-stoichiometric evaporation takes place as the partial pressure of SnO(g) is about 90 times higher than that of BaO(g). X ray powder diffraction identified only the BaSnO₃ perovskite phase, while narrow rocking curves having a full width at half maximum of 26 arcsec and etch pit densities below 10⁶ cm^-2 confirm a high degree of structural perfection of the single crystals. In this respect they surpass the structural properties of those single crystals that were reported in the literature. The electrical conductivity of nominally undoped crystals depends on the growth conditions and ranges from insulating to medium n-type conductivity. After post-growth annealing in an oxidizing atmosphere undoped crystals are generally insulating. Doping the crystals with lanthanum during growth results in a high n-type conductivity. For a La doping concentration of 0.123 wt.% we measured an electron concentration of 3.3  ×  10¹⁹ cm^-3 and an electron mobility of 219 cm² V^-1 s^-1. Based on optical absorption measurements we determined an energy of 3.17  ±  0.04 eV at 5 K and of 2.99  ±  0.04 eV at 297 K for the indirect band gap of BaSnO₃.

Tolerance of enteric-coated mycophenolate sodium in combination with calcineurin inhibitor in kidney transplant recipients: Polish experience

INTRODUCTION

Enteric-coated mycophenolate sodium (EC-MPS) was developed to reduce the incidence of gastrointestinal adverse effects. This multicenter observational study was designed to evaluate the safety profile and drug tolerance in kidney transplant recipients.

METHODS

Three hundred adult kidney recipients (median age 48 years) were enrolled over 3 years to receive EC-MPS de novo (n=175), as a switch from azathioprine (n=62) or mycophenolate mofetil MMF (n=63); in combination with calcineurin inhibitor. Drug doses, serum creatinine, estimated glomerular filtration rate (eGFR), as well as drug tolerance, patient and physician evaluation of therapy (on a 4-point scale) were recorded at enrollment and followed over 28 weeks. We modeled the probability of the highest level (ie, best result) of the categorical outcome variable.

RESULTS

Two hundred seventy-three patients completed the study (91%). In the pooled study group (1) best drug tolerance was expected more frequently with tacrolimus versus cyclosporine (odds ratio [OR] 2.12, P<.05); (2) best physician evaluation, with earlier EC-MPS introduction (OR for 4-week delay: 0.99, P<.03) and higher eGFR (OR for 5 mL/min increase: 1.21, P<.01). Among the EC-MPS de novo administrations group: (1) best drug tolerance was expected more frequently with coadministered tacrolimus versus cyclosporine (OR 3.14, P<.02); (2) best patient evaluation, with higher eGFR (OR for 1 mL/min increase: 1.04, P<.04); and (3) best physician evaluation, with higher eGFR (OR for 1 mL/min increase: 1.04, P<.001) and earlier EC-MPS introduction (OR for 4-week delay: 0.99, P<.03). In the conversion from MMF to EC-MPS group: (1) best drug tolerance was expected less frequently with coadministered cyclosporine versus tacrolimus (OR 0.05, P<.04) and more frequently with younger recipients (OR .001, P<.04); (2) best physician evaluation was expected more frequently with lower EC-MPS dose (OR for 360-mg dose increase: 0.4, P<.01) and with higher eGFR (OR for 5 mL/min increase: 1.42, P<.002). Adverse events were reported among 49/300 patients (16 serious adverse events).

CONCLUSIONS

EC-MPS was tolerated better by younger kidney recipients, when combined with tacrolimus versus cyclosporine, and when introduced earlier after transplantation. EC-MPS tolerance decreased gradually with renal function deterioration.

Transplant glomerulopathy: clinical and pathological correlations

OBJECTIVE

Chronic transplant glomerulopathy (TG) is one of the leading causes of severe posttransplantation proteinuria and graft loss. Our current knowledge about risk factors for the development of TG, as well as factors that affect its dynamics and prognosis, is poor. We sought to describe the pathological and clinical risk factors and correlations of TG as well as parameters that influenced the survival of grafts with that pathology.

MATERIALS AND METHODS

We retrospectively reevaluated 86 kidney transplant cases with TG that have been recognized on the basis of an indication biopsy since 1997. All TG as well as all pre-TG (previous) biopsies were characterized for the presence of C4d deposits in the graft.

RESULTS

Younger recipient age and minimal immunosuppression due to drug withdrawal or suboptimal drug doses/blood levels within 3 to 6 months preceding the biopsy were associated with C4d deposition in peritubular capillaries (PTC; P = .0053 and P = .0365, respectively). Diffuse PTC-itis (P = .029, RR [95% confidence interval] = 3.349 [1.131-9.919]) and total interstitial inflammation score (P = .015, RR [95% confidence interval] = 9.662 [1.784-52.329]) were observed to show a negative impact on graft survival. C4d deposition in PTC and glomeruli, the level of pretransplantation sensitization, episodes of acute rejection, and C4d in previous (pre-TG) biopsies did not influence the survival of grafts with TG.

CONCLUSIONS

Younger recipient age and minimal immunosuppression were associated with C4d positivity in grafts with TG. The survival of kidney grafts with TG was significantly affected by the magnitude of inflammation in the interstitium and PTC, but not by C4d positivity in PTC and glomeruli.

Duodenal Patch and Sphincterotomy: Modification of an Old Technique to Prevent Graft Pancreatitis

The aim of this study was to preliminarily evaluate the duodenal patch technique combined with open sphincterotomy in terms of prevention of graft pancreatitis. From April 2003 to March 2005, 17 simultaneous pancreas and kidney transplantations were performed using this technique. All recipients are alive with good renal transplant function. Directly after sphincterotomy in 16 pancreatic grafts a good outflow of clear pancreatic juice and a lessening of graft tenseness were observed during surgery. In two transplants an additional sphincterotomy of the Santorini duct sphincter was necessary. In one recipient no pancreatic juice secretion was observed and insulin independence was not obtained. This graft was explanted shortly afterward. In 13 recipients no graft pancreatic or peripancreatic fluid collection requiring intervention was observed. Of the three recipients who developed graft pancreatitis, two required graft pancreatectomy. In conclusion, Sphincterotomy facilitates pancreatic juice outflow by reducing intraoperative graft edema, which could lead to subsequent inflammation. Further studies on the factors inducing graft pancreatitis are necessary to eliminate this severe complication.

Posttransplantation diabetus mellitus under calcineurin inhibitor

BACKGROUND

The development of postransplantation diabetes mellitus (PTDM) is a serious complication of kidney transplantation. PTDM has a major impact on quality of life decreasing rates of patient and graft survival. It is well known that some currently used immunosuppressants are diabetogenic. Greater diabetogenicity of FK-506 has been reported in multicenter trials. We initiated a study of conversion from tacrolimus (FK-506) to cyclosporine (CsA) among kidney allograft recipients presenting with PTDM to evaluate whether this maneuver would ameliorate a diabetic state.

METHODS

This analysis of 20 adult, renal allograft recipients presenting with PTDM assumed the need for insulin therapy or oral hypoglycemics before and after conversion of the immunosuppressive regimen. The criteria for evaluating the outcome were as follows: dose reduction of insulin or oral hypoglycemic agents, adequacy of glucose control, C-peptide levels, and insulin concentration.

RESULTS

During the follow-up, we observed an improvement in the control of blood glucose in the converted group. In 13 patients, satisfactory glucose control was obtained without insulin or any other agent. In 3 patients a significant dose reduction of required insulin was possible. In another 2 patients who were insulin-dependent, the switch to oral hypoglycemic treatment was clinically possible after conversion. After conversion we observed significantly lowered fasting blood glucose levels and increased C-peptide levels.

CONCLUSIONS

The conversion from a tacrolimus to a CsA-based immunosuppressive regimen resulted in better glucose metabolism. We demonstrated a positive effect of conversion on the diabetic state of patients with PTDM.

The role of the protocol biopsies in renal allograft recipients

Subclinical rejection and long-term cyclosporine nephrotoxicity are well-known risk factors of chronic allograft nephropathy. In a prospective study 32 low-risk patients were randomized to either a reduced CsA dose (5 mg/kg/d) and daclizumab (group A, n = 16) for 7 months posttransplant with subsequent CsA tapering/withdrawal, or to a normal CsA dose (10 mg/kg/day) without daclizumab (group B, n = 16). Both groups received MMF and prednisone. Protocol biopsies were obtained at engraftment and 3 and 12 months after Tx. The number of rejection episodes was the primary endpoint. The secondary endpoints were: renal function, histological parameters related to CsA, and serum levels of TGF-beta and PDGF-BB. A low incidence of clinically suspected rejection episodes was observed (19% in group A and 12.4% in group B; P = NS). Although protocol biopsies showed 12 subclinical rejection episodes (six in group A, six in group B), serum creatinine levels were not different between the examined groups at 3 months. However, at 12 months, there was a statistically improved mean creatinine level in group A patients (1.2 mg/dL +/- 0.5 in group A vs 1.54 mg/dL in group B; P <.05). Chronic histopathologic changes were significant for biopsies at 3 and 12 months in both groups compared to the baseline findings for protocol biopsies (with no differences between groups, or between 3 and 12 months in both groups). Serum TGF-beta and PDGF-BB did not differ between the groups. Protocol biopsies may be useful to monitor safety and efficiency of new immunosuppressive protocols. Immunosuppressive regimens with low CsA doses followed by the drug's complete withdrawal seem to be efficient and safe in low-risk kidney allograft recipients.

Human herpesvirus-6 in renal transplant recipients: potential risk factors for the development of human herpesvirus-6 seroconversion

Herpesviruses, including human herpesvirus-6 (HHV-6), reactivate and have the potential to be pathogenic in immunocompromised patients. Little information is available regarding the correlation between immunosuppressive therapy and HHV-6 seroconversion after organ transplantation. Serum samples obtained from 120 kidney and kidney/pancreas transplant recipients were tested to explore the potential risk factors for developing HHV-6 infection including types of immunosuppression and induction/rejection therapy. Stored serum samples obtained prior to and at the 2nd, 4th, 12th and 48th weeks after transplantation were tested for anti-HHV-6 immunoglobulin (Ig)M antibodies using indirect immunofluorescence assay. Prior to transplantation and 48 weeks after transplantation the sera were additionally tested for anti-HHV-6 IgG using enzyme-linked immunoassay. Ninety-one percent of 120 recipients were HHV-6 IgG-positive before transplantation. One hundred seven of 120 patients were anti-HHV-6 IgM-negative before transplantation. Primary/secondary HHV-6 seroconversion occurred in sera of 46.6% of these 107 patients. HHV-6 seroconversion most frequently occurred 2 to 4 weeks after transplantation. There was no significant relationship between HHV-6 seroconversion and the treatment with methylprednisolone (MP). The incidence of HHV-6 seroconversion was significantly higher in subjects who were treated with the regimens including Daclizumab or Sirolimus as compared with those who were on other protocols. HHV-6 seropositivity in the Polish population of organ transplant recipients is very high. We demonstrated a trend toward association of HHV-6 seroconversion with type of immunosuppressive therapy.

Do high levels of serum triglycerides in pancreas graft recipients before transplantation promote graft pancreatitis?

OBJECTIVE

Graft pancreatitis is a serious complication following pancreas transplantation. The aim of this study was to evaluate the influence of pretransplant serum lipid levels on the development of graft pancreatitis among patients undergoing simultaneous pancreas and kidney transplantation (spkTx).

METHODS

We reviewed data from spkTx patients engrafted between 1999 and 2002. Group 1 consisted of 10 recipients with well-established pancreas and kidney graft function without postoperative pancreatitis; group 2 5 spkTx recipients who developed fatal graft pancreatitis in the first posttransplant month. The lipid parameters evaluated within 1 hour before transplantation and after hemodialysis included total cholesterol, HDL, LDL, VLDL, triglicerides and apoproteins A and B.

RESULTS

Triglycerides, apoprotein B and VLDL were significantly increased just before transplantation among patients who developed fatal pancreatitis compared to those patients with good graft function.

CONCLUSION

Recipient hypertriglyceridemia promotes graft pancreatitis in previously injured pancreatic graft.