Abstract:
Iodine-131 labeled metaiodobenzylguanidine (131I-MIBG) has been recognized as
an effective radiotherapeutic agent for treating neuroendocrine tumor. In the therapy,
bone marrow is the dose-limiting organ. The primary toxicity is myelosuppression,
especially thrombocytopenia and leucopenia. The aim of this study was to determine
internal radiation absorbed dose to red marrow by measuring the peripheral blood and
whole body retention activities. The calculation of the dose was based on the method
of Medical Internal Radiation Dose Committee (MIRD). After a diagnostic dose (37
MBq) of 131I-MIBG was administered intravenously to 14 patients with
neuroendocrine malignancy, blood and whole body activity concentrations were
measured with gamma counter and gamma probe system respectively. Effective halflife
(Te) and the residence time (τ) were obtained from the generated time-activity
curves. The averaged Te from whole body and blood retention curves were 23.5±5.4
and 16.6±4.8 hours, respectively. The estimated residence time was 33.9±7.5 hours.
An internal absorbed dose to the red marrow was 0.25±0.19 mGy/MBq. After
treatment, 29% (4 patients) developed grade 1 thrombocytopenia with a mean time to
nadir of 22 days, 36% (5 patients) developed grade 1 to 4 leukopenia with a mean time
to nadir of 48 days.
The red marrow dosimetry calculated from patient-specific data with a diagnostic
dose of 131I-MIBG was a good predictor for bone marrow suppression. The
pretreatment dosimetry results can be used to predict the individual degree of bone
marrow toxicity in the treatment planning to optimize 131I-MIBG therapy. The results
in a study of 14 patients with inoperable neuroendocrine tumors at Ramathibodi
Hospital demonstrated the substantial hematotoxicity associated with high-dose 131IMIBG
therapy. Although the 131I-MIBG does not cure, it allows the disease to be
controlled and, in combination with other treatments, bring these patients to a better
quality of life.