Neuroimaging and Radiopharmaceutical Therapy
Brain imaging and radionuclide therapy
Brain death scintigraphy uses a perfusion tracer; absent cerebral perfusion with preserved facial and scalp activity is consistent with brain death. I-123 ioflupane (DaTscan) images presynaptic dopamine transporters and is abnormal in Parkinson disease but normal in essential tremor and drug-induced parkinsonism. Perfusion SPECT (HMPAO/ECD) shows ictal hyperperfusion and interictal hypoperfusion in epilepsy. FDG PET shows temporoparietal hypometabolism in Alzheimer disease, frontal and anterior temporal in frontotemporal dementia, and occipital in dementia with Lewy bodies; amyloid PET is sensitive but less specific, so a negative scan is most useful for exclusion.
Therapy requires dosimetric thinking. I-131 for Graves disease can be calculated by formula; for differentiated thyroid cancer, remnant ablation follows a low-iodine diet, thyroid hormone withdrawal or rhTSH, and pregnancy testing. Lu-177 DOTATATE PRRT is given as four cycles of 7.4 GBq with amino-acid renal protection and antiemetics. Y-90 microsphere radioembolization requires a pretreatment MAA lung-shunt fraction, generally contraindicated above 20%. Ra-223 is six injections of 55 kBq/kg every four weeks for bone-metastatic castration-resistant prostate cancer.
Traps: diagnostic I-131 can cause stunning; iodine-containing contrast delays therapy; marrow suppression after PRRT must be monitored; and Ra-223 requires adequate blood counts before each dose.
Worked example (rule of thumb): I-131 activity (µCi) = (desired µCi/g × gland mass in g × 100) / 24-h uptake (%). For 150 µCi/g, a 30-g gland, and 50% uptake: (150 × 30 × 100)/50 = 9,000 µCi, or 9 mCi.