Does Delayed Excretion of Therapeutic ¹³¹I-MIBG Interfere with a ¹²³I-MIBG Diagnostic Scan 6 Weeks After the Therapy?

¹³¹I-metaiodobenzylguanidine (¹³¹I-MIBG) is a theranostic agent useful for treatment of neuroendocrine malignancies. In this case, a child with a Curie score of 21 was administered 17.871 GBq (483 mCi) of ¹³¹I-MIBG. The elimination half-life progressively increased from 23 h to 77 h during the 11 d that the patient was hospitalized for radiation isolation. Six weeks after the posttherapy scan, a survey with an ion-chamber device yielded readings of 0.3 μSv/h (0.03 mR/h) on contact with spinal regions that had shown increased uptake on the scan. A planar image obtained using the ¹³¹I setting and a high-energy collimator did not demonstrate any focal uptake. ¹²³I-MIBG was administered, and the 24-h scan was of diagnostic quality, without degradation from the remaining ¹³¹I-MIBG. Additional study is needed on whether the Curie score affects elimination of ¹³¹I-MIBG and on whether the period of hospitalized radiation isolation needs to be extended.