Low-dose (123)I-metaiodobenzylguanidine diagnostic scan is inferior to (131)I-metaiodobenzylguanidine posttreatment scan in detection of malignant pheochromocytoma and paraganglioma.

OBJECTIVE We assessed the lesion detectability of low-dose diagnostic (123)I-metaiodobenzylguanidine (MIBG) whole-body scans obtained at 6 and 24 h compared with posttreatment (131)I-MIBG whole-body scans in malignant pheochromocytoma and paraganglioma. METHODS Scintigrams obtained in 15 patients with malignant pheochromocytoma and paraganglioma were retrospectively analyzed. Diagnostic scans were performed with 111 MBq of (123)I-MIBG. Therapeutic doses of (131)I-MIBG (5.55-7.40 GBq) were administrated and whole-body scans were obtained at 2-5 days after (131)I-MIBG administrations. We compared the number of lesions and the lesion-to-referent count ratios at 6 and 24 h of (123)I-MIBG and at 2-5 days of (131)I-MIBG. RESULTS In comparison with the 6-h images of (123)I-MIBG, the 24-h images of (123)I-MIBG could detect more lesions in eight patients. Posttreatment (131)I-MIBG scans revealed new lesions in eight patients compared with the 24-h images of (123)I-MIBG. The lesion-to-referent count ratios at 6 and 24 h of (123)I-MIBG and at 3 days of (131)I-MIBG were increasing at later scanning time. There were significant differences in the lesion-to-referent count ratios between 6 and 24 h of (123)I-MIBG (P = 0.031), 6 h of (123)I-MIBG and 3 days of (131)I-MIBG (P = 0.020), and 24 h of (123)I-MIBG and 3 days of (131)I-MIBG (P = 0.018). CONCLUSION Low-dose diagnostic (123)I-MIBG whole-body scan is inferior to posttreatment (131)I-MIBG whole-body scan in malignant pheochromocytoma and paraganglioma. Considering the scan timing of (123)I-MIBG, 6-h images might have no superiority compared with 24-h images.