First experience for internal dosimetry calculation of red marrow in Thai-neuroendocrine tumor patients treated with 131I-MIBG

Objectives Bone marrow suppression, especially thrombocytopenia and leucopenia, is a serious side effect in patients treated with Iodine-131 labeled Metaiodobenzylguanidine (¹³¹I-MIBG). The aim of this study was to calculate internal radiation absorbed dose to the red marrow by measuring retention activity in the peripheral blood and whole body, based on MIRD schema for avoiding this side effect.

Methods 37 MBq of ¹³¹I-MIBG was injected in 14 neuroendocrine tumor patients (22 samples). Blood and whole body activity were measured to obtain the effective half-life (T_eff) and the residence time and calculate the internal absorbed dose to red marrow using the MIRD equation.

Results The T_eff (mean±S.D.) was 23.5±5.4 and 16.6±4.8 hours for whole body and blood, respectively. The residence time was 33.9±7.5 hours. The internal absorbed dose to red marrow was 0.25±0.19 mGy/MBq (22 samples). After treatment, there were thrombocytopenia in 4 patients (29%) with a mean time to nadir of 22 and leukopenia in 5 patients (36%), with a mean time to nadir of 48 days.

Conclusions The red marrow dosimety calculated from patient-specific data with a diagnostic dose of ¹³¹I-MIBG was a good predictor for bone marrow suppression. Therefore, this method can be used for ¹³¹I-MIBG treatment planning to maximize the treatment dose with reducing risk of thrombocytopenia and leukopenia.