BACKGROUND: Neuroendocrine tumors (NETs) are a heterogeneous group of neoplasms derived from endocrine stem cells.These tumors are characterized by overexpression of somatostatinreceptors (SSTR), which is utilized for imaging using SSTR analogs. Peptide receptor radionuclide therapy (PRRT) somatostatinanalogs labeled with 90Y and 177Lu in neuroendocrine tumors (NETs) results in symptomatic improvement, prolonged survival,and enhanced quality of life. The post-therapeutic imaging leadsto possibility of biodistribution of therapy. The aim of our study was to describe different possibilities of post-therapeutic imaging in patients underwent tandem therapy90Y/177Lu-DOTATATE with preliminary results of 90Y PET imaging.

MATERIAL AND METHODS: Thirty patients (11 men, 19 women; the mean age 55 ± 10.9 y) with histological confirmationof metastatic neuroendocrine tumors (G1 and G2) were treated with tandem therapy 90Y/177Lu-DOTATATE. WHBA scan and SPECT acquisition of the abdomen were performed 24 hours post therapy injection, on the dual-headVaricam camera (ELSCINT) using 177Lu photopeak and 90Y bremsstrahlung. PET imaging of 90Y component was done on Siemens Biograph Truepoint PET/CT (window 511 keV ± 15%) 4 hours after 90Y/177Lu-DOTATATE. Additionally phantom studies were performer to analyze the spatial resolution of different protocols.

RESULTS: Out of all the patients, median OS was 49.8 months and median EFS time 24.3 months. Spatial resolution achieved for 90Y, 177 Lu and PET imaging of 90Y componentmeasured using the phantom of the torso filled up with water was 20 mm, 8 mm and 4–5 mm FWHM, respectively. Spatial resolution in human body in our study was about 30 mm for 90Y, 15 mm for 177Lu and 25–30 mm for PET imaging of 90Y component.

CONCLUSIONS: The theoretically best spatial resolution offers PET scanner, however it is important to keep in mind that90Y-imaging PET is not used for diagnosis purposes (small activities)but rather to present new possibility of post-therapeutic imaging (substantially higher activities). For post-therapeutic imaging after intravenous radiopharmaceutical administration the best spatial resolution offers standard scintigraphic camera for 90Y/177Lu DOTATATE imaging, withusing 177Lu photopeaks. The worst spatial resolution offers standard scintigraphic camera for 90Y/177Lu DOTATATE imaging, with using 90Y bremsstrahlunggammas.