Patient dosimetry for ¹³¹I-MIBG therapy for neuroendocrine tumours based on ¹²³I-MIBG scans

Abstract.

Pre-therapeutic metaiodobenzylguanidine (MIBG) scans can be performed using labelling with either iodine-123 or iodine-131. ¹²³I-MIBG scans provide better image quality and count statistics, while ¹³¹I-MIBG allows registration of tracer kinetics over a longer period. The aim of this study was to determine how much information about the ¹³¹I-MIBG therapy total body dose according to the MIRD formalism can be gathered from ¹²³I-MIBG pre-therapy scans. Thirty-eight ¹³¹I-MIBG therapies administered to a total of 15 patients suffering from neuroblastoma (n=6), carcinoid tumours (n=5), phaeochromocytoma (n=3) and medullary thyroid carcinoma (n=1) were included. The mean administered activity was 5.3 GBq (SD 2.4 GBq). Three biplanar ¹²³I-MIBG total body scans were taken only once before a series of therapies while three biplanar ¹³¹I-MIBG scans were taken after each therapy. Attenuation correction was performed taking into account the difference in attenuation between ¹²³I and ¹³¹I. Using the MIRD formalism, the total body dose to the patient was calculated on the basis of: (1) a single exponential fit drawn through the data from the ¹²³I-MIBG pre-therapy scans, (2) a bi-exponential fit through the combined data of ¹²³I-MIBG pre-therapy and ¹³¹I-MIBG post-therapy scans. The mean total body dose calculated in our study was significantly higher for patients suffering from neuroblastoma (mean±SD 0.37±0.21 mGy/MBq) than for patients suffering from phaeochromocytoma (0.08±0.02 mGy/MBq), carcinoid tumours (0.07±0.01 mGy/MBq) and medullary thyroid carcinoma (0.09 mGy/MBq). The correlation coefficient between the dose calculated on the basis of the ¹²³I-MIBG pre-therapy scans and the subsequent ¹³¹I-MIBG therapy was 0.93 when a correction factor of 1.26 was taken into account. When considering all following therapies, the correlation was 0.85 and the correction factor, 1.20. Our results show that it is feasible to use data from pre-therapy ¹²³I-MIBG scans to calculate the total body dose of the subsequent ¹³¹I-MIBG therapy.