Malignant Pheochromocytoma and Metastasis. (*)

            (*) As long as a diagnostic MIBG procedure demonstrates significant increased uptake and retention in lesions.

            Treatment with MIBG is palliative. Chemotherapy lacks of  tumor specificity and toxicity is significantly greater than that of MIBG.  MIBG treatment could be used as first option in patients who depict great tumor uptake and retention of the tracer. Chemotherapy would be reserved for those patients MIBG (-) or with no response to treatment with MIBG. It has been observed that patients MIBG (-) have become MIBG (+) after such treatment. Therefore, both therapies should be taken into account when evaluating alternatives in the treatment of malignant pheochromocytoma. 

            It has been used various methods to assess the dose to be administered, but these measurements  are complex and inaccurate due to the heterogeneous distribution of MIBG in tumor mass, differences in the kinetics of "tracer" dose/therapeutic dose and difficulty in knowing the exact volume of tumor mass. Most groups tend  to use a fixed-dose ranging from 100-300 mCi with a high specific activity (1,486 Bq / mg). The maximum acceptable dose to bone marrow is of 2 Gy. Different side effects after administration of therapeutic doses of MIBG have been described, however, toxicity is almost completely confined to the blood system where there is reduced circulating leukocytes and platelets after each treatment. The lowest values occurring at approximately the third to fourth week after treatment and recover slowly in coming weeks. Platelets diminution is usually greater than that of leukocytes.

            Treatment with MIBG can be evaluated in terms of hormonal, biochemical and tumor response (mass). Tumor response is considered: 1) complete, if there is regression of all clinical evidence of tumor, including radiological findings and negative MIBG scan; 2) partial response when there is a reduction in tumor mass equal to or greater than 50 % and recalcification of lytic bone lesions, without appearance of new lesions; 3) unchanged; 4) progression of the disease, when new lesions or increase of existing ones by 25% or more is observed. The hormonal response is classified as: 1) complete response, when normalization of  all determinations in urine is proved; 2) partial response, reduction of  level by 50% or more; 3) unchanged; 4) progression, 25% or more of increased hormone levels in urine.

            In a review by Hoefnagel of the experience in the literature with MIBG treatment of 99 patients with malignant pheochromocytoma, it was found  only one patient in whom complete remission was obtained. No patient had tumor response without hormonal response. Metastases in soft tissues are more sensitive to MIBG treatment than those of bone. Hormonal response did not occur before administering the third dose of MIBG. Some groups carry out control of the response to treatment after three months of its administration, giving a new dose based on the results, considering that the treatment should not be abandoned if there is not an early response. Others assess the response after the third or fourth dose.

            Clinical improvement is associated with hormonal response, and this occurs in approximately 79% of cases, so treatment with MIBG is undoubtedly an effective palliative treatment. The duration of improvement varies from 5 to 54 months. Some patients present with recurrence and rapid progression, suggesting the appearance of more aggressive cell clones with increased resistance to radiation. There are few references in the literature regarding the treatment of malignant paragangliomas with MIBG, however, the results are somewhat encouraging.

            In conclusion, the 131-I-MIBG is an effective palliative treatment in malignant pheochromocytomas.

References:

1    Krempf M, Lumbroso R, Mornex R et al. Use of m-131-I-Iodobenzylguanidine in the treatment of malignant pheochromocytoma. J Clin Endocrinol Metab 1991; 72: 455-461.

2    Hoefnagel CA, Lewington VJ. “MIBG therapy” in Nuclear Medicine in clinical diagnosis and treatment. IPC Murray and PJ Ell. Eds. Churchill Livingston. 1994; 2: 851-864.

3    Castel V, Canete A, Melero C et al. Results of the cooperative protocol (N-III-95) for metastatic relapse and refractary neuroblastoma. Med Pediatr Oncol 2000; 35(6): 724-726.

4    Diez L, Mitjavila M, Tumores Neuroendocrinos: estudios con meta-yodo-bencil-guanidina, p: 549-561 in: Medicina Nuclear Aplicaciones Clínicas. Eds: I. Carrio - P. González. Editorial Masson, Barcelona España, 2003.

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