Evaluation of Al2O3:C optically stimulated luminescence (OSL) dosimeters for passive dosimetry of high-energy photon and electron beams in radiotherapy.

This article investigates the performance of Al2O3: C optically stimulated luminescence dosimeters (OSLDs) for application in radiotherapy. Central-axis depth dose curves and optically stimulated luminescence (OSL) responses were obtained in a water phantom for 6 and 18 MV photons, and for 6, 9, 12, 16, and 20 MeV electron beams from a Varian 21EX linear accelerator. Single OSL measurements could be repeated with a precision of 0.7% (one standard deviation) and the differences between absorbed doses measured with OSLDs and an ionization chamber were within +/- 1% for photon beams. Similar results were obtained for electron beams in the low-gradient region after correction for a 1.9% photon-to-electron bias. The distance-to-agreement values were of the order of 0.5-1.0 mm for electrons in high dose gradient regions. Additional investigations also demonstrated that the OSL response dependence on dose rate, field size, and irradiation temperature is less than 1% in the conditions of the present study. Regarding the beam energy/quality dependence, the relative response of the OSLD for 18 MV was (0.51 +/- 0.48)% of the response for the 6 MV photon beam. The OSLD response for the electron beams relative to the 6 MV photon beam. The OSLD response for the electron beams relative to the 6 MV photon beam was in average 1.9% higher, but this result requires further confirmation. The relative response did not seem to vary with electron energy at dmax within the experimental uncertainties (0.5% in average) and, therefore, a fixed correction factor of 1.9% eliminated the energy dependence in our experimental conditions.

High-precision dosimetry for radiotherapy using the optically stimulated luminescence technique and thin Al2O3:C dosimeters.

The potential of using the optically stimulated luminescence (OSL) technique with aluminium oxide (Al(2)O(3):C) dosimeters for a precise and accurate estimation of absorbed doses delivered by high-energy photon beams was investigated. This study demonstrates the high reproducibility of the OSL measurements and presents a preliminary determination of the depth-dose curve in water for a 6 MV photon beam from a linear accelerator. The uncertainty of a single OSL measurement, estimated from the variance of a large sample of dosimeters irradiated with the same dose, was 0.7%. In the depth-dose curve obtained using the OSL technique, the difference between the measured and expected doses was < or =0.7% for depths between 1.5 and 10 cm, and 1.1% for a depth of 15 cm. The readout procedure includes a normalization of the response of the dosimeter with respect to a reference dose in order to eliminate variations in the dosimeter mass, dosimeter sensitivity, and the reader's sensitivity. This may be relevant for quality assurance programmes, since it simplifies the requirements in terms of personnel training to achieve the precision and accuracy necessary for radiotherapy applications. We concluded that the OSL technique has the potential to be reliably incorporated in quality assurance programmes and dose verification.

Fracture toughness of posterior resin composites.

OBJECTIVES: The purpose of this study was to evaluate and compare the resistance to crack propagation, as measured by the fracture toughness, of some packable posterior resin composites with other posterior resin composite materials. METHOD AND MATERIALS: Fracture toughness determinations were made for the 5 packable and the other posterior resin composites using 3-point flexure of beams with a standardized central single-edge notch. Ten beams of each material were tested on an Instron test machine. The maximum loads were determined, from which the fracture toughness values (KIC) were calculated. The data were analyzed statistically using ANOVA and t tests. RESULTS: The resin composites tested demonstrated a range of KIC values. The materials were separated according to the mean KIC values into 5 groups that were statistically different. While 2 packable resin composites had KIC values that were among the highest, the other packables were substantially lower than nonpackables. The intermediate value group consisted of 3 of the conventional posterior resin composites. CONCLUSION: There was a very weak correlation between fracture toughness and reported volume concentration of particulate reinforcing elements. The posterior resin composites tested exhibited a spectrum of KIC values. The packable resin composites were distributed along this spectrum, with 2 products exhibiting high potential for resistance to crack propagation.

Fracture toughness of various core build-up materials.

PURPOSE: The purpose of this investigation was to compare the fracture toughness of several core materials. MATERIALS AND METHODS: Five core build-up materials were tested: (1) glass ionomer, (2) resin-modified glass ionomer, (3) titanium-reinforced composite, (4) composite resin with fluoride, and (5) amalgam. Fracture toughness determinations were made using 3-point flexure of beams with a central single-edge notch. The notch was standardized by the use of a special mold into which each of the materials was condensed. Ten beams of each material were tested on an Instron test machine (Instron Corp, Canton, MA) at a crosshead speed of 1.25 mm/min. The maximum loads were determined from which the fracture toughness values (KIC) were calculated. The data were analyzed statistically using analysis of variance and t tests. RESULTS: The mean fracture toughness value in MN.m-3/2 for each of them is as follows: (1) glass ionomer, 0.717 +/- 0.071; (2) resin-modified glass ionomer, 0.747 +/- 0.061; (3) titanium-reinforced composite, 1.409 +/- 0.051; (4) composite resin with fluoride, 1.660 +/- 0.086; and (5) amalgam, 1.521 +/- 0.081. There was no significant difference in the KIC values for the glass ionomer-based materials, and both of these were significantly lower than amalgam, titanium-reinforced composite resin, and composite resin with fluoride (p < .001). CONCLUSION: The titanium-reinforced composite resin, the composite resin with fluoride, and amalgam materials showed fracture toughness most likely to withstand the stresses generated during mastication.

A new rectal model for dosimetry applications.

UNLABELLED: A revised geometric representative model of the lower part of the colon, including the rectum, the urinary bladder and prostate, is proposed for use in the calculation of absorbed dose from injected radiopharmaceuticals. The lower segment of the sigmoid colon as described in the 1987 Oak Ridge National Laboratory mathematical phantoms does not accurately represent the combined urinary bladder/rectal/prostate geometry. In the revised model in this study, the lower part of the abdomen includes an explicitly defined rectum. The shape of sigmoid colon is more anatomically structured, and the diameters of the descending colon are modified to better approximate their true anatomic dimensions. To avoid organ overlap and for more accurate representation of the urinary bladder and the prostate gland (in the male), these organs are shifted from their originally defined positions. The insertion of the rectum and the shifting of the urinary bladder will not overlap with or displace the female phantom's ovaries or the uterus. In the adult male phantom, the prostatic urethra and seminal duct are also included explicitly in the model. The relevant structures are defined for the newborn and 1-, 5-, 10- and 15-y-old (adult female) and adult male phantoms. METHODS: Values of the specific absorbed fractions and radionuclide S values were calculated with the SIMDOS dosimetry package. Results for 99mTc and other radionuclides are compared with previously reported values. RESULTS: The new model was used to calculate S values that may be crucial to calculations of the effective dose equivalent. For 131I, the S (prostate<--urinary bladder contents) and S (lower large intestine [LLI] wall<--urinary bladder contents) are 6.7 x 10(-6) and 3.41 x 10(-6) mGy/MBq x s, respectively. Corresponding values given by the MIRDOSE3 computer program are 6.23 x 10(-6) and 1.53 x 10(-6) mGy/MBq x s, respectively. The value of S (rectum wall<--urinary bladder contents) is 4.84 x 10(-5) mGy/MBq x s. For 99mTc, we report S (testes<--prostate) and S (LLI wall<--prostate) of 9.41 x 10(-7) and 1.53 x 10(-7) mGy/MBq x s versus 1.33 x 10(-6) and 7.57 x 10(-6) mGy/MBq x s given by MIRDOSE3, respectively. The value of S (rectum wall<--prostate) for 99mTc is given as 4.05 x 10(-6) mGy/MBq x s in the present model. CONCLUSION: The new revised rectal model describes an anatomically realistic lower abdomen region, thus giving improved estimates of absorbed dose. Due to shifting the prostate gland, a 30%-45% reduction in the testes dose and the insertion of the rectum leads to 48%-55% increase in the LLI wall dose when the prostate is the source organ.

Community-based dental programs: University of Medicine and Dentistry of New Jersey-New Jersey Dental School.

The dental school plans to incorporate CODE into the curriculum so that more students have community-based dental educational experiences. Future plans also include increasing standardization of reports, clinical and administrative procedures, resources, and processes across the sites in order to lower managerial overhead. This process will be aided by further enhancement of computerized information systems and electronic links. The major lesson learned is that new extramural programs can be created and sustained by pooling school resources with those from the private and public sectors. Funding sources and opportunities available to one party alone are insufficient. While one-time funding was used to build and furnish the NJDS extramural sites, the clinics were established only after business plans demonstrated the availability of funds to sustain their operations. The Statewide Network of Community Oral Health Care and CODE models are still evolving, but they are replicable not only in dental education but in other types of health services. The details of the partnerships and funding streams will vary from site to site, but through outreach and careful negotiation with potential partners and detailed contracts, the community service and educational missions of a health professions school can have a successful outcome.

Preparation and use of NHS-MAG3 for technetium-99m labeling of DNA.

The chelator mercaptoacetylglycylglycylglycine (MAG3) is on of several amidothiols that have been used successfully to radiolabeled proteins and other molecules with 99mTc. Prior to radiolabeling, the sulfur in these amidothiols is usually protected by a benzoyl group (i.e. S-benzoyl MAG3) which requires extreme alkaline pH or boiling water temperatures for rapid deprotection. As a result, the benzoyl-protected chelator is radiolabeled prior to conjugation (i.e. preconjugation labeling) in the case of carriers such as proteins or polypeptides which cannot withstand harsh conditions. We have employed a simple, two-step, synthesis of the N-hydroxysuccinimide ester of MAG3 in which the sulfur is protected with an acetyl group (i.e. S-acetyl NHS-MAG3). A single-stranded amine-derivitized DNA was coupled with NHS-S-acetyl MAG3. Radiolabeling was accomplished at room temperature and neutral pH by transchelation from 99mTc-tartrate. In comparison to labeled SHNH-DNA, the labeled MAG3-DNA was unstable to cysteine transchelation, however, in contrast to SHNH-DNA, no evidence for serum protein binding of the labeled MAG3-DNA was observed. We conclude that the S-acetyl NHS MAG3 bifunctional chelator may prove to be an attractive alternative method of radiolabeling DNA and other biologically important molecules with 99mTc.

In vivo hybridization of technetium-99m-labeled peptide nucleic acid (PNA).

UNLABELLED: Hybridization of a radiolabeled single-stranded DNA oligonucleotide with its single-stranded complement in vivo has not yet been convincingly demonstrated. A contributing factor may be unfavorable in vivo properties of the phosphodiester and phosphorothioate DNAs. Peptide nucleic acid (PNA) oligomers have been reported to possess in vivo properties more suitable for radiopharmaceutical applications. METHODS: We have radiolabeled an amine-derivatized 15-base PNA oligomer with 99mTc through a modified MAG3 chelator. RESULTS: The ability of the PNA to hybridize in vitro with its complement appeared to be unimpaired after conjugation and radiolabeling. Size-exclusion, high-performance liquid chromatography (HPLC) analysis of 37 degrees C serum after 24 hr of incubation showed the radiolabel to be present predominately as labeled PNA with indications of labeled serum proteins and a low molecular weight catabolite. Whole-body clearance in mice was rapid, with 50% of the label eliminated in about 2 hr. After 2.5 hr, the highest uptake (kidneys) was only 1.5% of the injected dose/g; less than 0.07%/g was present in all sampled tissues at 24 hr. To evaluate in vivo hybridization, beads were implanted subcutaneously in both thighs of normal mice. In the left thigh only, the beads were conjugated with complementary single-stranded PNA. At 23 hr following intraperitoneal administration of the labeled PNA, the left/right thigh radioactivity ratio was 6:1. Whole-body images at this time showed only bladder, kidneys and the left thigh. CONCLUSION: Unlike the radiolabeled DNAs investigated in this laboratory, 99mTc-PNA displays stability and pharmacokinetic properties suitable for eventual use as radiopharmaceuticals.

Technetium-99m antibodies labeled with MAG3 and SHNH: an in vitro and animal in vivo comparison.

The in vitro stability and animal pharmacokinetics of 99mTc bound to Sandoz and C110 IgG antibodies via a modified MAG3 has been compared with the hydrazino nicotinamide (SHNH) moiety as standard. For both antibodies, the stabilities of the label to challenge at up to 50:1 cysteine: IgG molar ratio were comparable, but at higher molar ratios, MAG3 showed greater instabilities. For the Sandoz antibody, size-exclusion HPLC analysis of 37 degrees C serum incubates and plasma samples from injected mice showed no clearly distinguishable differences. In the C110 case, some increased high molecular weight radioactivity was apparent with MAG3. Biodistributions in normal mice showed significant differences only in liver (Sandoz) and liver, spleen, intestines, stomach, and blood (C110), with SHNH usually providing higher levels. Thus, for two different antibodies and under the conditions of this study, the MAG3 chelator provided a 99mTc label with properties similar to that of SHNH moiety.

Radiation absorbed dose from indium-111-CYT-356.

UNLABELLED: Indium-CYT-356 is an agent developed by CYTOGEN Inc. (CYT) (Princeton, NJ) for the use in staging patients with prostate cancer. This investigation was performed to provide the human dosimetry needed for Food and Drug Administration approval for routine use in patients. METHODS: Biodistribution data collected from three sites were obtained from prostate cancer patients who received diagnostic doses of 111In-CYT-356. Data included blood and urine collections, and the organ uptake value was measured from sequential conjugate whole-body and planar images over a 7-10 day period. Dose contributions from radioactivity in transit through the GI tract were estimated using a compartmental model. The calculations used the MIRD methodology and MIRDOSE 3. RESULTS: The total-body dose observed was 0.14 mGy/MBq, and the effective dose was found to be 0.25-0.29 mSv/MBq. The largest organ doses were found for the liver (1.0 mGy/MBq), kidneys (0.67 mGy/MBq) and spleen (0.88 mGy/MBq). CONCLUSION: The radiation dose to the patient from a typical 185 MBq administration of 111In-CYT-356 is comparable to the dose from other 111In-labeled whole antibodies used in the diagnosis and management of cancer patients. The inclusion of the GI tract as a source organ increases the effective dose by 18%.

Comparative properties of a technetium-99m-labeled single-stranded natural DNA and a phosphorothioate derivative in vitro and in mice.

Oligonucleotides, particularly single stranded, may ultimately be of considerable use as radiopharmaceuticals. We have compared a synthetic 22-base single-stranded phosphodiester DNA with its phosphorothioate analog after both were radiolabeled with 99mTc via the hydrazino nicotinamide chelator. Whole body clearance of the label in mice was much slower when introduced on the phosphorothioate (30% vs. 75% clearance at 6 hr) because of immediate and persistent accumulation in liver (47% vs. 2% injected dose/g at 4 hr). The label in both cases was present in urine primarily on low molecular weight catabolites. High-performance liquid chromatography analysis of 37 degrees C serum incubates showed serum protein binding of 99mTc in both cases (about 100% bound at 24 hr) but to different proteins. Different behavior with respect to protein binding was also observed in the analysis of liver and kidney homogenates: the phosphodiester label was almost quantitatively converted to lower molecular weight catabolites after only 15 min, whereas the phosphorothioate label was primarily on proteins. The rapid digestion of the phosphodiester by nucleases was not observed, probably because protein binding of the labeled oligonucleotides stabilized against degradation. Thus the phosphodiester DNA may be the preferred 99mTc-labeled oligonucleotide in certain circumstances to avoid the high and persistent liver uptake observed with the phosphorothioate DNA.

Protein labelling via deoxyribonucleic acid hybridization.

A novel method of radiolabelling antibodies and other proteins is described. A small single-stranded DNA was covalently conjugated to an antibody and labelled by hybridization following the addition of the complementary single-stranded DNA labelled with technetium-99m (99Tc(m)) or indium-111 (111In). Antibody labelling efficiencies were 100% in about 1 h at room temperature with specific activities of up to 30 microCi micrograms-1 of IgG for 99Tc(m). Both diester and thioate DNAs were used. Both the diester- and thioate-labelled antibodies showed complete label stability in 37 degrees C saline. After 24 h in 37 degrees C serum, however, about 40% of the label in the case of the diester antibody was on low molecular weight species--probably labelled catabolites from nuclease degradation of the phosphodiester DNA. In contrast, the 99Tc(m) label on the thioate antibody was immediately and quantitatively bound to serum proteins--probably due to non-specific binding through the sulphur groups. Biodistribution studies in normal mice reflect these in vitro observations: 99Tc(m) on the diester antibody was rapidly cleared through the kidneys, probably as low molecular weight catabolite, while on the thioate antibody, the 99Tc(m) label was predominately deposited in the liver. In conclusion, by modifying with a single-stranded DNA, proteins may be readily labelled with a variety of radionuclides by DNA hybridization. The properties of the radiolabel are strongly influenced by the nature of the DNA.

Improved tumor localization with (strept)avidin and labeled biotin as a substitute for antibody.

Because of its short physical half life, the use of anti-tumor antibodies radiolabeled with 99mTc has necessitated early (i.e. 2-6 h post-administration) imaging. It is possible that at these early times localization of antibodies in certain tumors may be largely due to non-specific processes. If so, other proteins or agents may be preferred for early imaging of solid tumors. We have investigated tumor localization with labeled biotin administered subsequent to unlabeled and unconjugated streptavidin. Nude mice bearing anti-CEA tumors (LS174T) received 10 micrograms of 111In-labeled anti-CEA antibody (C110) or 111In-labeled streptavidin with sacrifice 5 h later. In an examination of pretargeting, other animals received 50 micrograms of unlabeled streptavidin followed 3 h later with 1 micrograms of 111In-labeled biotin (EB1) and sacrifice 2 h later. The biodistribution of labeled streptavidin was similar to that of labeled specific antibody except for lower blood and higher kidney levels. Tumor levels were also lower with labeled streptavidin but, because of still lower levels in liver and blood, the tumor/normal tissue ratios were improved. When unlabeled streptavidin was administered and followed by labeled biotin (pretargeting), tumor levels were further reduced modestly; however, normal tissue levels were greatly reduced such that the tumor/blood and tumor/liver ratios were 10.6 and 2.2 vs 1.5 and 0.5 for the specific antibody. Improvements were seen in all tissues sampled with the exception of kidney and muscle. A further control of labeled biotin alone (without the preinjection of streptavidin) showed minimal accumulations in all tissues with the exception of kidneys.(ABSTRACT TRUNCATED AT 250 WORDS)

The stability in liver homogenates of indium-111 and yttrium-90 attached to antibody via two popular chelators.

To investigate the influence of chelator on the stability in liver homogenates of 111In and 90Y-labeled antibodies, the C110 antibody was conjugated with the cyclic anhydride of DTPA (cDTPA) and with isocyanatobenzyl-DTPA (SCN-Bz-DTPA) and labeled with both radionuclides. After incubation in fresh liver homogenates at 37 degrees C for 1-2 days, the soluble fraction was analyzed by filtration, HPLC and TLC to determine the nature and extent of transchelation of the label and catabolism of the antibody. The loss of activity from antibody, as shown by passage through a low molecular weight (10 kDa cut-off) filter, was 3-5 times more pronounced for 90Y (51 and 68% at 1 and 2 days) than 111In (11 and 29%, respectively). No significant difference was observed between chelators. Furthermore, analysis of these low molecular weight species showed that even at 1 day, 90Y in contrast to 111In was present as one or more weak complexes and therefore no longer chelated to either DTPA or Bz-DTPA. Little evidence was observed for instability in liver of the thiourea bond whereby SCN-Bz-DTPA is attached to the antibody. By contrast the identification of 111In-DTPA in the homogenates demonstrates the instability of the amide bond generated by cDTPA conjugation. In conclusion, as expected, 90Y was shown to form less stable chelates than 111In, however, in this investigation the greater denticity of Bz-DTPA over DTPA did not improve stability with either radiolabel.

Directly and indirectly technetium-99m-labeled antibodies--a comparison of in vitro and animal in vivo properties.

To investigate the in vivo and in vitro properties of 99mTc when labeled to antibodies via one direct and one indirect method, the B72.3 and C110 IgG antibodies were radiolabeled directly via stannous ion reduction and indirectly via the hydrazino nicotinamide chelator and compared in vitro and in vivo. Antibody avidity (but not immunoreactive fraction) appeared to be independent of labeling methods for both antibodies. Following stannous ion reduction, antibodies were fragmented by denaturing SDS PAGE although only slight evidence of fragmentation was found in vivo. The direct label was instable to transchelation to cysteine and glutathione in vitro and in vivo. Following intravenous administration, urinary excretion of activity was threefold greater for the direct label and was almost exclusively labeled cysteine and glutathione. Significant differences in the biodistribution of 99mTc were also observed: liver levels were lower, kidney levels were higher and clearance of label from blood and tissues was faster for the direct label. At Day 1, tumor accumulation was threefold lower for the direct label although most normal tissues were also lower. In conclusion, when labeled to two antibodies by one direct method, 99mTc is unstable towards transchelation relative to one indirect method. These relative instabilities greatly influenced the biodistributions in mice and may influence the quality of images obtained in patients.

Pharmacokinetics of the FO23C5 anti-CEA antibody fragment labelled with 99Tcm and 111In: a comparison in patients.

The FO23C5 anti-carcinoembryonic antigen (CEA) F(ab')2 antibody was radiolabelled with 111In via diethylenetriaminepentaacetic acid (DTPA) and directly with 99Tcm by stannous ion and mercaptoethanol antibody reduction to compare the pharmacokinetics of these three agents. Four patients received 15 mCi 99Tcm-Fab' 1 week before receiving 1 mCi 111In-F(ab')2. Five additional patients received only the 99Tcm-Fab'. Radiochromatograms by high-performance liquid chromatographic (HPLC) analysis of serum and urine samples from patients receiving 111In were typical of those observed by us previously in connection with other antibodies. The identical analyses of samples from patients receiving 99Tcm showed no differences with the method of reduction and more complex radiochromatograms. In addition to peaks due to a mixture of labelled F(ab')2 and Fab' fragments and, occasionally, immune complexes, there were several peaks due to labelled cysteine and other small labelled species present in both serum and urine. The biodistribution of 99Tcm was as expected for a labelled Fab' fragment: relative to 111In, 99Tcm cleared rapidly from circulation and into kidneys and urine. Liver levels of 111In and 99Tcm were surprisingly similar at 1 day (12 versus 9% ID) although initial 111In levels were lower and increased while 99Tcm levels were higher and decreased. Spleen levels were also similar. In 4/9 patients receiving 99Tcm, hepatobiliary clearance was observed at levels which could confuse interpretation whereas this mode of clearance was observed in only 1/4 patients receiving 111In. Image quality was superior with 111In versus 99Tcm at 1 day postadministration as judged by counting rates and background activity whereas the opposite was true at 2-3 h postadministration.

Effect of cigarette smoking on salivary epidermal growth factor (EGF) and EGF receptor in human buccal mucosa.

The mouth acts as a primary target for cigarette smoke which is associated with several oral diseases and cancer. The present study investigated the effect of cigarette smoking on salivary EGF and the buccal EGF receptor. Samples of whole saliva and buccal biopsy were obtained from 15 healthy volunteers (10 smokers and 5 non-smokers). The smokers smoked 20 or more cigarettes/day for more than 5 years. Salivary cotinine (a major metabolite of nicotine) was determined by radioimmunoassay (RIA). The salivary cotinine level was consistent with the self-reported smoking status (smokers, 106-530 ng/ml saliva; non-smokers, < 2 ng/ml saliva). As compared to the non-smokers, the salivary EGF concentration (determined by RIA) was 32% lower in those smokers whose salivary cotinine level was 250 ng/ml or higher (non-smokers, 2.21 +/- 0.16; smokers, 1.57 +/- 0.09 ng/ml saliva; mean +/- S.E.M., P < 0.01). There was no significant difference in 125I-labeled EGF binding to the buccal receptor between the two groups. However, EGF stimulated the autophosphorylation of a 170-kDa protein band in the sample of non-smokers, but not in the smokers. The immunoblot analysis using anti-EGF receptor antibody indicated that the smoking-related deficiency in EGF receptor autophosphorylation was due to the functional alteration of the receptor proteins. In conclusion, cigarette smoking reduces the salivary EGF level and impairs the function of buccal EGF receptor, which may be associated with the pathology of smoking-related oral disease.

The stability of 99Tcm directly labelled to an Fab' antibody via stannous ion and mercaptoethanol reduction.

The anti-CEA FO23C5 F(ab')2 antibody was directly radiolabelled with 99mTcm by two methods (stannous ion and mercaptoethanol reduction) and compared in vitro and in vivo for label stability. By both methods, reduction of the F(ab')2 fragment produced primarily Fab' fragments. By both methods, the label was stable to 99Tcm-pertechnetate formation in vitro. Analysis by high performance liquid chromatography (HPLC) of serum, urine, kidney and liver homogenates from mice injected with 99Tcm-antibodies by both methods consistently showed a prominent radiolabelled peak with an estimated molecular weight of about 300 daltons. An identical peak was observed in the analysis of patient samples in a related investigation from this laboratory. Cysteine was radiolabelled with reduced 99Tcm and analysed by HPLC and thin layer chromatography (TLC); one of the 99Tcm-cysteine species so produced showed the same chromatographic behaviour as that of the 300 dalton species. In conclusion, the FO23C5 and other antibodies are stably labelled with 99Tcm via either stannous ion or mercaptoethanol reduction. In mice and in patients, the labelled proteins are either catabolized or, more likely, the 99Tcm label is transchelated such that the label is present on several low molecular weight species, the most prominent of which is postulated to be 99Tcm-cysteine.