Patient external dose rate after 177Lu-DOTATATE therapy: factors affecting its decrease and predictive value.

Rationale: Peptide receptor radionuclide therapy (PRRT) with 177Lu-DOTATATE (oxodotreotide) results in external radiation exposure from the patient. In the PREELU observational prospective study, we determined the equivalent dose rate at 1 m of the patient (EDR-1m) for a period following PRRT. The main objective was to predict which patients could be discharged from the hospital at approximately 3 h after the administration of 177Lu-DOTATATE, i.e. at the end of the infusion of amino-acids according to our PRRT protocol. As presenting no undue risk of radiation exposure for the public, those patients could be treated as outpatients or day patients, rather than inpatients. Methods: We sequentially measured EDR-1m facing the sternum and then the pelvis during 50 PRRT in 24 patients with metastatic neuroendocrine tumours, each 30 minutes after ending administration of Lutathera, over at least 180 minutes. Results: 180 minutes after the administration of ca. 7400 MBq of Lutathera, EDR-1m was <40 µSv/h in all cases, and <25 µSv/h in 32 cases (64%). After an overnight hospital stay, EDR-1m was <25 µSv/h in all cases. The EDR-1m value measured facing the sternum was the greatest in about one-fourth of paired measurements. In patients whose creatinine clearance was >96 mL/min/1.73m2, the EDR-1m was most likely (predictive value=90%) to drop below 25 µSv/h within 180 minutes after the administration of Lutathera. In 16 patients who benefited from several PRRT cycles, the creatinine clearance did not decrease significantly from one cycle to the next, probably due to the kidney protection by the amino-acid infusion. The patients whose EDR-1m dropped below 25 µSv/h at 180 minutes during their first PRRT cycle were unlikely (predictive value= 88%) to decease during the following two years. Conclusion: All patients could have been discharged 3 h after administration according to the criterion EDR-1m <40 µSv/h. Using EDR-1m <25 µSv/h as criterion, an extended hospital stay beyond 3 h would have been necessary in around one-third of the PRRT treatments and could be anticipated based on creatinine clearance ≤96 mL/min/1.73m2. EDR-1m <25 µSv/h at 180 min during the first PRRT yielded a strong predictive value on the patient's survival at two years, a finding that should be confirmed in future studies.

Equivalent Dose Rate 1 Meter from Neuroendocrine Tumor Patients Exiting the Nuclear Medicine Department After Undergoing Imaging.

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.

Which thyroid cancer patients need periodic stimulation tests?

PURPOSE: Recurrences are frequent in thyroid cancer patients and long-term follow-up is therefore necessary. We evaluated the yield of rhTSH stimulation in three groups of patients, classified according to the UICC/TNM risk stratification and the results of first follow-up testing. METHODS: The study population comprised 129 patients referred for rhTSH testing. All had undergone first follow-up testing after thyroid hormone withdrawal (off-T4) within 1 year of 131I ablation. Negative first follow-up testing was defined as Tg <2 ng/ml and no neck uptake on 131I diagnostic whole-body scan. Seventy-five patients had stage I thyroid cancer and negative first follow-up testing (group A), 19 had stage I disease and positive first follow-up testing (group B), and 35 had stage II-IV disease (group C). RhTSH stimulation was performed an average of 6 years after first follow-up testing. RESULTS: 131I diagnostic scanning after rhTSH was negative in all 75 group A patients. Only one group A patient had detectable Tg after rhTSH injection (1.5 ng/ml), but Tg had also been detected at baseline in this patient (1.45 ng/ml). Given the absence of a response to stimulation, suggesting an interference, Tg was reassessed with a different technique and proved to be undetectable (<0.1 ng/ml). Stimulation with rhTSH in group B showed residual Tg in seven patients and residual 131I uptake in the thyroid bed in two patients, but none of these patients had signs of disease progression. Five group C patients (14%) had a positive rhTSH test result, and this was suggestive of disease progression in at least two cases. CONCLUSION: The first follow-up testing is essential for prognostic classification after 131I ablation of thyroid cancer. In stage I patients, undetectable Tg and negative 131I scan 1 year after ablation define a large population of subjects who have a very low risk of recurrence and who do not require further stimulation tests. In contrast, periodic rhTSH stimulation tests appear useful in higher-risk patients.