Jérôme Clerc

SIMS MICROSCOPY : METHODOLOGY, PROBLEMS AND PERSPECTIVES IN MAPPING DRUGS AND NUCLEAR MEDICINE COMPOUNDS

Secondary ion mass spectrometry (SIMS) microscopy, a mass spectrometry method designed in the 1960s, offers new analytical capabilities, high sensitivity (ppm to ppb region), high specificity and improved lateral resolution, thus facilitating insight into many physiological and biomedical questions. Apart from the sample preparation and the physical characteristics of the detection, the biological model must also be considered.

SIMS microscopy in the biomedical field

We attempted to indicate the requirements for biomedical applications of SIMS microscopy. Sample preparation methodology should preserve both the structural and the chemical integrity of the tissue. Furthermore, it is often necessary to correlate ionic and light microscope images. This implies a common methodological approach to sample preparation for both microscopes. The use of low or high mass resolution depends on the elements studied and their concentrations. To improve the acquisition and processing of images, digital imaging systems have to be designed and require both ionic and optical image superimposition. However, the images do not accurately reflect element concentration; a relative quantitative approach is possible by measuring secondary ion beam intensity. Using an internal reference element (carbon) and standard curves the results are expressed in μg/mg of tissue. Despite their limited lateral resolution (0.5 μm) the actual SIMS microscopes are very suitable for the resolution of biomedical problems posed by action modes and drug localization in human pathology. SIMS microscopy should provide a new tool for metabolic radiotherapy. by facilitating dose evaluation. The advent of high lateral resolution SIMS imaging (< 0.1 μm) should open up new fields in biomedical investigation.

Recent developments in medical applications of SIMS microscopy

The purpose of this review is to present the recent developments in the medical applications of SIMS microscopy. This technique is one of the microanalytical mass spectrometry methods which allow in theory the detection of all the elements of the Mendeleiev table as well as the separation of stable and radioactive isotopes. It is based on a phenomenon whereby a biological sample surface is sputtered by bombardment with an energetic primary ion beam. Part of the sputtered matter is ionized and the resulting secondary ions are characteristic of the atomic composition of the analyzed area. These secondary positive or negative ions are collected and separated in a mass spectrometer at low or high mass resolution, which is dependent on both the element studied and its concentration. An analytic image which conserves the tissue distribution of the selected element is displayed on a fluorescent screen linked to an image processing system. Local elemental concentration can also be measured. Results are highly dependent on the techniques used for sample preparation which should preserve both the chemical and the structural integrity of the tissue. Further, the ionic images must be correlated with corresponding images of the same areas of the serial sections observed in a photonic microscope. With our SIMS microscope (lateral resolution approximately 0.5 microns, and mass resolution 300 to 12,000) we have demonstrated that this microscopic imaging technique is suitable for physiopathological studies. We revisited thyroid iodine metabolism by mapping chemical elements such as 32S and 127I, characteristic of hormonal physiology. Newly organified iodine (radioiodine) can be evaluated in relation to previously stored iodine (127I) in a given follicle, thus allowing an appraisal of glandular adaptation to aging and iodine overload. Another area in which SIMS can be used in medicine, is for the localization of drug markers in tumor tissue (e.g. fluorine-5-fluouracil, iodine in iododeoxyrubicin). This could facilitate the evaluation of the intratumor drug concentration at the onset of the treatment. Likewise, SIMS can be used to localize radiopharmaceuticals used in diagnosis (e.g. technetium) and therapy (131I of metaiodobenzylguanidine). This would permit a better evaluation of the radiation dose delivered to tissue. Further prospects are within reach with the imminent advent of higher lateral resolution (0.05 microns) SIMS microscopes.

Contribution of Mass Resolution to Secondary Ion Mass Spectrometry Microscopy Imaging in Biological Microanalysis

This paper presents recent developments in nuclear medical applications of secondary ion mass spectrometry (SIMS) microscopy. This technique is the only method (with laser microprobe mass spectrometry) potentially able to map all the elements of the periodic table, including stable and radioactive isotopes. We have demonstrated with our microscope (lateral resolution 0.5 µm; mass resolution 300–10 000) that this imaging technique can localize radioligand biodistributions on tissue sections if a mass resolution above 2000 is used. We chose three radioactive isotopes: technetium-99 m (99mTc), strontium-89 (89Sr) and bromine-76 (76Br). The 99mTc is introduced into the cell by a lipophilic ligand. The 89Sr is metabolised and integrated into bone matrix because it is a calcium analog. The 76Br targets adrenals because it is bound to a ligand which has chemical analogies with endogenous bioamines. Our data confirm and extend to other models the suggestion that SIMS should constitute a pertinent approach to cellular dosimetries.

Equivalent dose rate at 1m of patients with known or suspected neuroendocrine tumor exiting a nuclear medicine department after 68Ga-DOTATOC, 18F-FDOPA or 18F-FDG PET/CT, or 111In-pentetreotide or 123I-mIBG SPECT/CT

123I-metaiodobenzylguanidine (MIBG) and 111In-pentetrotide SPECT have been used for functional imaging of neuroendocrine tumors (NETs) for the last 2 decades. More recently, PET/CT imaging with 18F-FDG, 18F-fluorodihydroxyphenylalanine (FDOPA), and 68Ga somatostatin-receptor ligands in NETs has been expanding. A literature search could find no direct measurements of the dose rate from NET patients exiting the nuclear medicine department after undergoing PET/CT with 18F-FDOPA or 68Ga-DOTATOC, a somatostatin analog. Methods: We measured the dose rates from 93 NET patients on leaving the department after undergoing PET/CT or SPECT/CT in our centers. In total, 103 paired measurements of equivalent dose rate at 1 m (EDR-1m) from the sternum and urinary bladder were obtained. The detector faced the sternum or bladder and was 1 m away from and directly in front of the patient. The practice for exiting the department differed according to whether the patient had been referred for PET/CT or for SPECT/CT. PET/CT patients were discharged after imaging, whereas SPECT/CT patients left the department earlier, just after radiopharmaceutical injection. Results: The median administered activity was 122 MBq in 53 68Ga-DOTATOC PET/CT studies, 198 MBq in 15 18F-FDOPA PET/CT studies, and 176 MBq in 13 18F-FDG PET/CT studies. The corresponding median EDR-1m was 4.8, 9.5, and 8.8 μSv/h, respectively, facing the sternum, and 5.1, 10.1, and 9.5 μSv/h, respectively, facing the bladder. The median administered activity was 170 MBq in 12 111In-pentetreotide SPECT/CT studies and 186 MBq in 10 123I-MIBG SPECT/CT studies. The corresponding median EDR-1m was 9.4, and 4.9 μSv/h, respectively, at the level of the sternum, and 9.3 and 4.7 μSv/h, respectively, at the level of the bladder. The EDR-1m was less than 20 μSv/h in all patients. Thus, when exiting the nuclear medicine department, the NET patients injected with 68Ga-DOTATOC or 123I MIBG emitted an average EDR-1m roughly half that of patients injected with other radiopharmaceuticals. This finding is a complementary argument for replacing SPECT by PET somatostatin-receptor imaging. Conclusion: Our current practice of allowing patients to exit after PET/CT imaging or just after SPECT radiopharmaceutical injection appears to be safe from a radiation protection point of view. Restrictive advice is unnecessary for NET patients being discharged from the department.

Prognostic value of suppressed thyrotropin level and positive thyrotropin-receptor antibody activity in Graves' disease with long-lasting clinical remission.

OBJECTIVEnTo determine the prognostic value of suppressed thyrotropin (TSH) level and positive TSH-receptor antibodies (TSH-R Ab) in patients with Graves disease who have long-lasting clinical remission.nnnMETHODSnWe retrospectively studied patients with Graves disease who underwent follow-up for a mean of 55 months after the withdrawal of antithyroid drug treatment. Study patients were 84 consecutive subjects in clinical remission, with normal serum free thyroxine (FT(4)) and free triiodothyronine (FT(3)) levels, regardless of serum TSH levels, a mean of 35 months (range, 6 to 135) after discontinuation of carbimazole therapy. Eighty-seven euthyroid subjects were used as control study participants. All subjects had serum determinations of FT(4) and FT(3) (radioimmunoassay), TSH (highly sensitive immunoradiometric method), TSH-R Ab (radioreceptor assay), and microsomal antibodies (M Ab, passive hemagglutination method).nnnRESULTSnIn the study patients, serum TSH was suppressed (</=0.10 mU/L) in 13 cases (15%), TSH-R Ab were positive (>/=15%) in 11 cases (13%), and M Ab were positive (>/=1:100) in 54 cases (64%). Simultaneous suppressed TSH and positive TSH-R Ab levels were present in six patients. During the follow-up, 11 patients had a relapse, demonstrated by above-normal values for serum FT(4) and FT(3) in association with clinical symptoms of hyperthyroidism. Five of them had a previously suppressed TSH level, three had a positive TSH-R Ab level, and six had a positive M Ab titer. Relapse was significantly more likely in patients with a previously suppressed TSH level (P<0.02) but not in patients with a previously positive TSH-R Ab level or positive M Ab titer.nnnCONCLUSIONnPatients with Graves disease and long-lasting clinical remission after discontinuation of carbimazole therapy may have a suppressed TSH level, a positive TSH-R Ab level, or a positive M Ab titer (or some combination of these findings). Although positive TSH-R Ab and M Ab have no significant prognostic value, a suppressed TSH level is indicative of subclinical hyperthyroidism and higher risk of relapse.