History
An elderly female with history of a primary intestinal neuroendocrine tumor (NET) and suspicion of recurrence was referred for PET/CT staging to evaluate suspected metastases. 18F-SiFAlin-TATE, an 18F-labeled somatostatin receptor agonist with similar NET avidity and physiological distribution as that of 68Ga-DOTATATE, was used as the PET tracer. One hour and 30 minutes following a 5.1 mCi (190 MBq) intravenous (IV) injection of 18F-SiFAlin-TATE, the patient underwent a 5-minute, single-bed, whole-body PET/CT acquisition on Biograph Vision Quadra™.
Findings
Figures 1 to 6 show high uptake within the liver and para-aortic lymph node metastases in a patient with suspicion of recurrent NET. Five lesions were visualized in the liver with the largest measuring 3.7 cm in the highest dimension and the highest SUV among the lesions at 32.7. Three para-aortic lymph node lesions were visualized at the level of bifurcation of the aorta around the right common iliac artery with the largest having a maximum diameter of 1.3 cm and the highest SUV of 26.7. In view of a relatively small number of liver and nodal lesions with high somatostatin receptor density reflected by high SUV, the potential for successful 177Lu-DOTATATE therapy can be concluded from the PET/CT findings.
High image quality with sharp visualization of both large and small liver metastases with high contrast over normal liver parenchymal uptake as well as sharp visualization of small para-aortic lymph node metastases with a low dose of 5.1 mCi (190 MBq) IV injection of 18F-SiFAlin-TATE and a 5-minute total acquisition of a single bed reflects the high performance of Biograph Vision Quadra PET/CT. The system combines a 106-cm long axial PET field of view (FOV) with ultra-fast time-of-flight (TOF) performance along with high resolution, which is reflected by the standard 440 x 440 matrix of a whole-body PET acquisition. The contrast enhancement on CT along with high SUVmax on PET within liver lesions reflects high tumor perfusion along with high somatostatin receptor density, which indicates potential of positive outcome from peptide receptor radionuclide therapy (PRRT) with 177Lu- or 225Ac-DOTATATE.
Figure 1. PET MIP images show high uptake in multiple metastatic lesions in the liver as well as in the para-aortic lymph nodes in abdomen. Physiological uptake within the spleen and kidneys is high while moderate uptake is seen in normal liver parenchyma, intestines, and bladder. Note the focal uptake within the pituitary. Low-level physiological uptake is also visualized within the thyroid, salivary glands, and mediastinum.
Discussion
Although 68Ga-DOTATATE PET/CT has been widely used for evaluation of NET, a fluorinated somatostatin receptor analog has the potential to improve the logistics of PET imaging for NET due to the slightly longer half life and ease of distribution typical of a fluorinated tracer. 18F-SiFAlin-TATE has been evaluated as a suitable alternative to 68Ga-DOTATATE with a favorable half-life and distribution efficacy.
A study1 comparing 68Ga-DOTATATE and 18F-SiFAlin-TATE in 13 patients with NET demonstrated the relative equivalence of both agents. 18F-SiFAlin-TATE shows slightly higher physiological uptake in the liver, spleen, and adrenal glands compared to 68Ga-DOTATATE although the difference was not statistically significant. The renal uptake however was significantly higher for 18F-SiFAlin-TATE.
The SUVmax within metastatic lesions was significantly higher with 18F-SiFAlin-TATE (liver metastases with mean SUVmax of 18.8 compared to 12.8 with 68Ga-DOTATATE). Thus, tumor to normal liver ratio was significantly higher with 18F-SiFAlin-TATE, which reflects the potential for higher diagnostic accuracy for smaller lesions as compared to 68Ga-DOTATATE.
The SUVmax of liver metastases in the present clinical example is comparable to the values obtained in the literature. The highest SUVmax among liver lesions in the present case is 32.7 while the mean SUVmax reported in the study comparing 68Ga-DOTATATE and 18F-SiFAlin-TATE was around 20.0. Normal liver SUVmax was 7.33 and SUVpeak was 6.42 in the present case while the mean liver SUVmax in the other study was 9.5 ± 4.5.
Conclusion
The high lesion uptake and low physiological uptake in the liver, intestines, thyroid, mediastinum (except the spleen), kidneys, and adrenals, along with the logistical advantages of using a fluorinated tracer, demonstrates the diagnostic potential of 18F-SiFAlin-TATE PET for NET imaging DOTATATE.
The high resolution and high lesion contrast achieved with a 5-minute, low-dose, single-bed, whole-body PET/CT acquisition on Biograph Vision Quadra demonstrates the clinical potential of 18F-SiFAlin-TATE as an appropriate alternative to 68Ga-DOTATATE.
Examination protocol
Scanner: Biograph Vision Quadra PET/CT
