68Ga-DOTA-peptides PET/CT is the gold standard for the evaluation of neuroendocrine tumors. This technique is currently performed in several nuclear medicine centers in Europe and thus subjected to EURATOM directives on medical exposure, requiring the establishment of diagnostic reference levels (DRL).
This study was performed on a sample of 50 patients undergoing 68Ga-DOTANOC PET/CT. Examinations were carried out using two Discovery STE and one Discovery 710 PET/CT (GE Healthcare). The CT protocol had the following parameters: 120 kV, 80 mA, 0.8 s/tube rotation, pitch 1.75, collimation 20 mm, and 3.75 mm slice thickness; at the moment no current modulation is adopted. The PET scan duration was 3 min for each bed position, having a length of 15 cm and a superposition of 1.1 cm.
The PET/CT systems were calibrated before the study and subjected to the quality assurance program. To verify the accuracy of CTDI and DLP values recorded in the dose report, CTDI was measured following international guidelines.
Personalized doses were prepared using the µ DDS-A automatic dispenser (Tema Sinergie S.p.A.) according to the weight of the patient following an internal protocol adapted from EANM guidelines and measured with a CAPINTEC CRC 15 PET activity meter. The accuracy of the instrument was assessed using a NIST traceable standard source of 68Ge/68Ga , while a source of 11C was used to measure the linearity in the (1 GBq – 10 MBq) activity range.
DRL for CT were assessed as the 75th percentile of the distribution. Activity values were analyzed and fit with a normal distribution and DRL were set as the mean activity in the patients’ sample.
These values were then used to estimate the effective and organ doses for a standard patient through ImPACT CT patient dosimetry calculator and the dose-activity coefficients reported in the literature.
A small variability of CTDI values was observed between scanners due to the adoption of a standard protocol; larger variations were observed for DLP values, taking into account both patient diameter and height. Considering the consistency of our data and the general agreement between all the scanners, DRL were set as the 75th percentile of the whole set of values, that is 3.82 mGy for CTDI and 381 MBq∙cm for DLP.
DRL for 68Ga-DOTANOC activity resulted 102 MBq and 1.4 MBq/kg, which is close to the inferior limit stated by international guidelines, revealing the capability of modern PET scanner to obtain diagnostic images with a relatively low amount of radioactive tracer.
Effective doses of 5.4 mSv and 2.6 mSv were calculated for the CT and PET part respectively. The organs receiving the highest dose were thyroid, bone surface and thymus (8.5, 7.8 and 6.7 mGy) for the CT acquisition, while kidneys, urinary bladder wall and spleen (9.1, 8.5 and 7.4 mGy) for 68Ga-DOTANOC administration.
Local DRL have been proposed for 68Ga-DOTANOC PET/CT. These levels will be used as a clinical aid for internal optimization and are of potential interest as benchmark for other centers approaching this technique.