MIBG molecular imaging for evaluating response to chemotherapy in patients with malignant pheochromocytoma: Preliminary results

Malignant pheochromocytomas respond to chemotherapy with a reduction in tumor size and catecholamine secretion. We investigated the usefulness of molecular imaging with meta-iodobenzylguanidine (MIBG) for evaluating the effects of chemotherapy in patients with malignant pheochromocytoma. Six patients were studied before and after 6±4 months of combination chemotherapy with cyclophosphamide, vincristine, and dacarbazine. Urinary catecholamines, metanephrines, and vanillylmandelic acid (VMA) levels were measured before and after chemotherapy. [¹³¹I]MIBG uptake was calculated for each tumor lesion on images before and after chemotherapy. An intensity ratio (IR) of abnormal to normal tissue count density was used to evaluate the change in lesion activity with therapy. Urinary catecholamines, metanephrines, and VMA significantly decreased with chemotherapy. MIBG uptake decreased in most lesions and the reduction in overall IR correlated with the reduction in urinary VMA. However, the change in individual lesions was variable and MIBG IR did not change or increased in a number of lesions. In conclusion, MIBG imaging is useful in the evaluation of patients with malignant pheochromocytoma who are receiving chemotherapy. It can provide not only a measure of overall effectiveness of treatment but also allows a lesion-by-lesion evaluation of the heterogeneity of response to chemotherapy.