[Biological aspects of radiation in nuclear medicine]

Radiotherapy with unsealed radionuclides differs from external radiotherapy with regard to the radiation quality and energy range, the regional dose uniformity and the time course of irradiation regimen. External radiotherapy is planned precisely and can be applied to a target volume independently from blood flow during a course of irradiation fractions. In contrary, administered radiopharmaceuticals distribute according to their pharmacokinetic properties and generate a continuous irradiation corresponding to the effective halflife. The resulting dose rates are approximately 1Gy/min and 1Gy/h, respectively. The biokinetics of radiopharmaceuticals involves cellular accumulation and retention with highly variable affinity to specific organs that can be modulated as well. A remarkable dose gradient is found at the edge of volumes with enhanced uptake. The biological effect of an irradiation with decreasing intensity can be compared with the radiation effect caused by conventional fractionation with 2 Gy a day in external beam therapy by means of the linear-quadratic model. However, the experimental validation of this translation is still under investigation. Radionuclide therapy is usually performed in several cycles some month apart. This procedure fails to meet external radiotherapy. The vision of a combined external-internal radiotherapy requires efforts for a common dosimetry approach both in vitro and in vivo with a physical and biological verification of the results.

Strahlenbiologie

Radiotherapy with unsealed radionuclides differs from external radiotherapy with regard to the radiation quality and energy range, the regional dose uniformity and the time course of irradiation regimen. External radiotherapy is planned precisely and can be applied to a target volume independently from blood flow during a course of irradiation fractions. In contrary, administered radiopharmaceuticals distribute according to their pharmacokinetic properties and generate a continuous irradiation corresponding to the effective halflife. The resulting dose rates are approximately 1 Gy/min and 1 Gy/h, respectively. The bio – kinetics of radiopharmaceuticals involves cellular accumulation and retention with highly variable affinity to specific organs that can be modulated as well. A remarkable dose gradient is found at the edge of volumes with enhanced uptake. The biological effect of an irradiation with decreasing intensity can be compared with the radiation effect caused by conventional fractionation with 2 Gy a day in external beam therapy by means of the linear-quadratic model. However, the experimental validation of this translation is still under investigation. Radionuclide therapy is usually performed in several cycles some month apart. This procedure fails to meet external radiotherapy. The vision of a combined external-internal radiotherapy requires efforts for a common dosimetry approach both in vitro and in vivo with a physical and biological verification of the results.

TSH receptor antibody (TRAb) assays based on the human monoclonal autoantibody M22 are more sensitive than bovine TSH based assays

Measurements of TSH receptor autoantibodies (TRAb) using assays based on the human monoclonal TSH receptor autoantibody M22 or bovine TSH have been compared in 136 adult patients. They suffered from Graves' disease (GD, n=62), Hashimoto's thyroiditis (HT, n=26), or non-autoimmune hyperthyroidism (NAH, n=48) and were selected on the basis of undetectable, borderline or low TRAb levels (0.6-3 IU/l) as measured by TSH based TRAb assay (Dynotest TRAKhuman from BRAHMS). The time interval between initial diagnosis of GD and TRAb determination was high and ranged from 1 month to 3.5 years (median: 2.3 years). Using the kit manufacturer's cutoff values, 53/62 (85.5%) of the selected group of GD patients were TRAb positive (>0.4 IU/l) by M22 based TRAb ELISA (Medizym TRAb clone, Medipan) and 45/62 (72.6%) were TRAb positive (>1.5 IU/l) by TSH based TRAb assay. In the HT group, 9/26 (34.6%) sera were positive in the M22 based ELISA and all but one of these 9 were positive or borderline in the TSH based assay. ROC plot analysis of the GD group using the NAH group as reference showed that at 95% specificity, the bovine TSH based TRAb assay had a sensitivity of 62.9% (cutoff for positivity=1.64 IU/l) and the M22 based TRAb ELISA a sensitivity of 90.3% (cutoff for positivity=0.32 IU/l). Overall therefore, the M22 based Medizym TRAb clone assay is more sensitive than the bovine TSH based Dynotest TRAK human assay.

Chromatographic determination of radiochemical purity

Thin layer chromatography is well established for quality control of radiopharmaceuticals. A convenient and widely used stationary phase are ITLC SG strips. However, the Pall Corporation stopped manufacturing of the silica gel impregnated glass fibre strips (ITLC SG). Material, Methode: As a replacement we tested silicic acid impregnated glass fibre strips from Varian (ITLC SA) and sufficient mobile phases. Results: The chromatography with these strips takes two to three times longer than with ITLC SG, but it is in an acceptable range. Only three mobile phases are necessary to test most of the common in-house made radiopharmaceuticals. Conclusion: The proposed method is suitable for routinely measuring the radiochemical purity of radiophamaceuticals.

[BeO-OSL detectors for dose measurements in cell cultures]

AIM

The absorbed dose is an important parameter in experiments involving irradiation of cells in vitro with unsealed radionuclides. Typically, this is estimated with a model calculation, although the results thus obtained cannot be verified. Generally used real-time measurement methods are not applicable in this setting. A new detector material with in vitro suitability is the subject of this work.

METHODS

Optically-stimulated luminescence (OSL) dosimeters based on beryllium oxide (BeO) were used for dose measurement in cell cultures exposed to unsealed radionuclides. Their qualitative properties (e. g. energy-dependent count rate sensitivity, fading, contamination by radioactive liquids) were determined and compared to the results of a Monte Carlo simulation (using AMOS software). OSL dosimeters were tested in common cell culture setups with a known geometry.

RESULTS

Dose reproducibility of the OSL dosimeters was +/-1.5%. Fading at room temperature was 0.07% per day. Dose loss (optically-stimulated deletion) under ambient lighting conditions was 0.5% per minute. The Monte Carlo simulation for the relative sensitivity at different beta energies provided corresponding results to those obtained with the OSL dosimeters. Dose profile measurements using a 6 well plate and 14 ml PP tube showed that the geometry of the cell culture vessel has a marked influence on dose distribution with 188Re.

CONCLUSION

A new dosimeter system was calibrated with beta-emitters of different energy. It turned out as suitable for measuring dose in liquids. The dose profile measurements obtained are suitably precise to be used as a check against theoretical dose calculations.

Current progress and future challenges in the biochemical diagnosis and treatment of pheochromocytomas and paragangliomas

Findings from five independent studies - with close to 350 patients with pheochromocytoma and more than 2,500 in whom the tumor was excluded - indicate that measurements of plasma free metanephrines provide an overall diagnostic sensitivity of 98% and specificity of 92%. The recommendation that initial testing for the tumor should always include measurements of either plasma or urinary fractionated metanephrines results from recognition of the high diagnostic sensitivity of measurements of plasma metanephrines. The few patients with pheochromocytoma in whom the test may not yield a positive result include those with very small tumors or microscopic disease and others with tumors that do not produce norepinephrine and epinephrine. Such patients are typically normotensive and do not exhibit symptoms of catecholamine excess. Additional measurements of methoxytyramine can be useful for detecting those tumors that produce only dopamine. Suboptimal diagnostic specificity and difficulties in distinguishing true- from false-positive elevations of plasma metanephrines remain challenges for diagnosis. Improvements in analytical technology (e.g., liquid chromatography with tandem mass spectrometry) and new strategies for follow-up testing provide possible solutions to these problems. The single most important remaining clinical care challenge is the development of effective cures for patients with malignant disease. Current treatments, none of which are truly satisfactory, include chemotherapy and radiopharmaceutical therapy with (131)I-labelled M-iodobenzylguanidine or radioactive somatostatin analogues. Improvements in treatment may in the future come from several fronts, but proof of efficacy ideally will require well-coordinated multicenter prospective trials in larger numbers of patients than in previous studies.

[FDG-PET/CT in oncology. German Guideline]

FDG-PET/CT examinations combine metabolic and morphologic imaging within an integrated procedure. Over the past decade PET/CT imaging has gained wide clinical acceptance in the field of oncology. This FDG-PET/CT guideline focuses on indications, data acquisition and processing as well as documentation of FDG-PET/CT examinations in oncologic patients within a clinical and social context specific to Germany. Background information and definitions are followed by examples of clinical and research applications of FDG-PET/CT. Furthermore, protocols for CT scanning (low dose and contrast-enhanced CT) and PET emission imaging are discussed. Documentation and reporting of examinations are specified. Image interpretation criteria and sources of errors are discussed. Quality control for FDG and PET/CT-systems, qualification requirements of personnel as well as legal aspects are presented.

Radiation treatment planning in brain tumours: potential impact of 3-O-methyl-6-[(18)F]fluoro-L-DOPA and PET

AIM

Amino acid PET has become an important diagnostic tool for brain tumour imaging. In this data analysis, the potential impact of amino acid PET with 3-O-methyl-6-[(18)F]fluoro-L-DOPA ([(18)F]OMFD) on radiation treatment planning is addressed by the following questions: 1. Was tumour tissue identified with OMFD-PET which was not covered by the conventionally derived planning target volume (PTV)? 2. Would the PTV have been changed incorporating OMFD-PET?

PATIENTS, METHODS

OMFD-PET of 25 patients after subtotal resection of malignant glioma was evaluated. The region of elevated tracer uptake of PET and of contrast enhancing masses on MRI were outlined as separate gross tumour volumes (GTV(MRI) and GTV(OMFD)) and reconstructed in the planning CT for comparison with the conventionally drawn GTV(conv). A PTV(new) based on GTV(conv+MRI) was calculated. Pairwise differential volumes were calculated to estimate overlap and differential volumes delineation by each image modality and the PTV(conv) and PTV(new) respectively.

RESULTS

Differential volume analysis showed > 10 cm(3) of GTV(OMFD) outside GTV(conv) and GTV(MRI) in 5/25 patients respectively. From GTV(MRI) > 10 cm(3) were found outside GTV(OMFD) in 8/25 patients. Although all tumour areas indicated by [(18)F]OMFD were covered by the conventionally derived PTV, based on a GTV(OMFD+MRI), the PTV(new) would have been enlarged >20% in seven patients. In seven patients the PTV(new) would have been reduced.

CONCLUSION

OMFD-PET indicated tumour tissue outside the tumour region identified with MRI, adding valuable information for the delineation of the GTV in radiation treatment planning. OMFD-PET contains the potential to tailor the high dose radiation to the appropriate tumour volume, especially if dose escalation is desired.