316. I-123 based pre-therapy dosimetry for I-131 thyroid cancer therapy in paediatric patient

Abstract

Purpose The use of I-123-NaI has several possible advantages for pre-therapeutic red marrow (RM) dosimetry in I-131 differentiated thyroid cancer (DTC) therapy: it reduces radiation exposure and provides high-sensitivity diagnostic scans in paediatric patients. Unfortunately, the short physical half-time of I-123 could be inadequate for dosimetric accuracy. The aim of this study was to investigate the possibility of using I-123 for pre-therapeutic RM dosimetry of DTC paediatric patients comparing whole-body (WB) and blood residence time (τ) calculated during pre-treatment (PT) and in-treatment (IT) dosimetry. Methods Fourteen DTC patients were analysed. I-123 biokinetic was studied collecting blood samples and WB images with a time window ranging from 0.5 to 54-h. Residence times of blood and WB were determined according to EANM guidelines. The time window for IT RM dosimetry was [0.5;144] h and WB activities were estimated using a probe at 2 m from the patient. To evaluate the impact of a different time sampling, τ values were compared with Wilcoxon paired-samples test and its percentage differences between PT and IT were evaluated using I-131 data both in a restricted time window [0;54] h and entire time window. Results Using the restricted time window, τ blood mean[range] was 0.00048[0.00031;0.00078] h/g and 0.00055[0.00038;0.0010] h/g for PT and IT respectively with a mean percentage difference of −9 ± 39% (p-value = 0.15). Analogously, τ WB was 20.7[13.4;29.0] h and 19.9[12.3;31.2] h for PT and IT respectively with a percent difference of 5.8 ± 36% (p-value = 0.30). Using all data-set, the differences between PT and IT residence times increase to −14.9 ± 48% (p-value Conclusions The resulted p-values did not show statistical significant differences when the same time window is used both for blood and for WB. Using all data-set, the differences between PT and IT increase and only blood p-value shows a statistical difference.