Slice-based thresholding method for dosimetric evaluation of Ga-68 DONATOC

Full text of publication follows. Aim: the role of Gallium-68 (Ga⁶⁸) labeled somatostatin analogues in the management of neuroendocrine tumors (NETs) is well established. Though its clinical application has been studied extensively, studies reporting the radiation dosimetry aspects are few. Internal dosimetry is based on many assumptions, the most common being homogeneous radioactive uptake in organs. This assumption, however, does not hold true for most organs and doses are rather over-estimated. Herein, we have evaluated the dosimetry of Ga-68 DOTANOC in NETs patients based on slice-by-slice thresholding method. Methods: 5 patients (3 males; 2 females) with known NET were recruited in the study. Mean age of patients was 43 years (35-56 years). Patients were injected mean activity 60.01 MBq followed by whole body PET acquisition at 5 time points: 10, 20, 30, 70 and 120 minutes post-injection. Quantification (Bq/ml) for each organ (spleen, liver, kidneys, heart, urinary bladder and pituitary) was done using ROI (Region Of Interest) based method in each slice with automatic thresholding. Whole body counts were taken at respective time points for all the patients. Dosimetric calculations were performed using OLINDA/EXM 1.1 software. Results: the highest mean effective dose equivalent was observed in liver (8.18*10^-3 mSv/MBq) followed by Spleen (4.30*10^-3 mSv/MBq). The mean dose to kidneys, urinary bladder, heart and pituitary were 2.97*10^-3, 4.10*10^-3, 1.68*10^-3 and 5.23*10^-7 mSv/MBq, respectively. The mean whole body effective dose and effective dose equivalent were calculated to be 1.29*10^-2 and 2.28*10^-2 mSv/MBq. Conclusion: renal and bladder doses are attributed to the renal route of excretion of Ga-68-DOTANOC. The dose to other organs is owing to the physiological presence of somatostatin receptors on these organs. The evaluated doses in the present study are significantly lower than the ones reported in literature which is mainly because of the processing methodology. The dosimetric calculations in the already reported studies are based on the assumption of homogenous radioactive uptake in each organ. However, the present study employed more accurate thresholding of each organ derived from per slice and hence these values are considered to be more accurate. (authors)