BRAIN UPTAKE OF I-131 METAIODOBENZYLGUANIDINE FOLLOWING THERAPY OF MALIGNANT PHEOCHROMOCYTOMA

Intracranial metaiodobenzylguanidine (MIBG) uptake is occasionally and only faintly visualized on diagnostic studies. Recently, intense norm al cerebellar uptake was described on posttherapy MIBG images. Experie nce at the University of Michigan with posttherapy MIBG scintigraphy o f pheochromocytoma was reviewed. The patterns and correlates of intrac ranial uptake after therapeutic I-131 MIBG in 25 patients (61 patient treatment encounters) were evaluated by review of records and blinded consensus interpretation of diagnostic and posttherapeutic MIBG scans. Thirty-nine (64%) patient treatment encounters demonstrated at least faint (grade 1) MIBG uptake in one or more brain sites; the most commo n site was the cerebellum. There was a statistically significant relat ion between intracranial uptake and 1) size of therapeutic dose and 2) patient age, but no relation between intracranial uptake and gender, body mass index, plasma epinephrine level, plasma norepinephrine level , urine metanephrine level, or the therapy-to-imaging interval. Althou gh the influence of age on the pattern and intensity of intracranial u ptake is unexplained, the relation to therapy dose may be explained by the possible generation of MIBG metabolites that can cross the blood- brain barrier thigh activity administered and the delay until imaging) . Further studies are needed to define mechanisms of intracranial upta ke and relation to responses and toxicity after MIBG therapy of neuroe ndocrine tumors.