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Benzylguanidine is a chemical selectively taken up by cells of the sympathetic nervous system. This guanethidine derivative can be labelled with radioactive iodine, and both the 131-I and 123-I radioisotopes of iodine have been used in scintigraphic studies. In such studies, the selective uptake of the radioactive iodobenzylguanidine can be visualized using scintillation detectors. This technique has been used to image tumors such as pheochromocytoma, medullary thyroid carcinoma, paraganglioma, and neuroblastoma. The same selective uptake might be exploited in a therapeutic capacity as well. If the tumors take up a small dose of radioactive tracer for imaging, they might take up a larger dose of radioactive compound which might serve as a direct source of radiotherapy within the cancer itself. This technique was evaluated in the treatment of 31 children with advance neuroblastoma. All 31 patients had tumors that were visible on scintigraphy, indicating that their tumors took up the radioactive compound. All 31 patients had either failed to respond to conventional therapy or relapsed after conventional therapy. The children were given doses of radioactive I-131 meta-iodobenzylguanidine ranging from 2.8 to 6.0 gigaBecquerels (a gigaBecquerel is radioactivity at a level of one billion radioactive disintegrations per second). Two complete and four partial response were observed. These patients relapsed and all 31 patients are either dead or alive with disease. (Consumer Summary produced by Reliance Medical Information, Inc.)

¹³¹I-metaiodobenzylguanidine (¹³¹I-MIBG) is selectively taken up by cells of neural crest origin, allowing targeted radiotherapy of tumors such as neuroblastoma (NB) and pheochromocytoma. Radiotherapy may provide additional benefits in the treatment of NB, with moderate side effects such as hematologic and thyroid toxicity. However, with longer follow-up, other complications might occur. We describe our experience with second cancers occurring in children treated with ¹³¹I-MIBG and chemotherapy. The clinical records of 119 consecutive NB cases treated with ¹³¹I-MIBG at a single institution between 1984 and 2001 were reviewed for the occurrence of a second malignant neoplasm (SMN). Overall, five cases of SMN occurred in the study patients. In particular, two cases of myeloid leukemia, one of angiomatous fibrous histiocytoma, one of malignant schwannoma, and one case of rhabdomyosarcoma were detected. The schwannoma and the rhabdomyosarcoma developed within the residual neuroblastic mass after first-line therapy. Should ¹³¹I-MIBG treatment become more broadly employed in the therapeutic strategy for neuroblastoma, the risk of second cancer will have to be taken into consideration. The organization of an international registry of subjects treated with ¹³¹I-MIBG might better define the frequency and features of second malignancies following this radiometabolic approach. Cancer 2003;97:1332–38. © 2003 American Cancer Society. DOI 10.1002/cncr.11167