Does delayed excretion of I-131 mIBG interfere with an I-123 mIBG diagnostic scan 6 weeks after the therapy?

(131)I-metaiodobenzylguanidine ((131)I-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 (131)I-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 muSv/h (0.03 mR/h) on contact with spinal regions that had shown increased uptake on the scan. A planar image obtained using the (131)I setting and a high-energy collimator did not demonstrate any focal uptake. (123)I-MIBG was administered, and the 24-h scan was of diagnostic quality, without degradation from the remaining (131)I-MIBG. Additional study is needed on whether the Curie score affects elimination of (131)I-MIBG and on whether the period of hospitalized radiation isolation needs to be extended.