Monitoring the efficacy of iodine-131-MIBG therapy using fluorine-18-FDG-PET

Background: The purpose of this study was to assess the potential of fluorine-18 fluoro-2-deoxy-D-glucose positron emission tomography (F-18-FDG-PET) for monitoring the efficacy of iodine-131 metaiodobenzylguanidine (I-131-MIBG) therapy in neuroendocrine tumours. Methods: A total of seven I-131-MIBG therapies with 3.7 to 10.2 GBq were carried out in three patients suffering respectively from a phaeochromocytoma, a paraganglioma and a metastatic neuroendocrine tumour of an unknown primary. The post-therapeutic whole-body scintigrams were compared with the results of six F-18-FDG-PET studies performed at the time of therapies. ONe patient received three PET scans prior to each one of the MIBG therapies, and one patient was studied twice. Results: F-18-FDG uptake in tumour sites seemed to correlate well with tumour differentiation, showing no uptake in one patient with a highly differentiated neuroendocrine tumour, and moderate-to-intense uptake in the two other patients with metastatic disease. Those tumour sites that had a simultaneous positive uptake in both the MIBG scintigram and the PET scan showed response to therapy as a continous reduction in MIBG update over time. They also showed a qualitative decrease in FDG accumulation during the follow-up. This was associated with a decrease in the mean and maximum standard uptake values of more than 50% in some metastases, while the X-ray computed tomography showed no decrease in tumour volume. Two patients revealed additional metastases that were unknown on the basis of prior diagnosis or therapeutic PET scans and radiological follow-up. Conclusion: It may be concluded from these cases that F-18-FDG-PET is valuable tool for an initial metabolic staging of neuroendocrine tumours prior to I-131-MIBG therapy, as it can reveal tumour sites beyond the reach of radioisotope therapy. It may also be of importance in assessing therapeutic potential in those sites that do show positive MIBG uptake.