Somatostatin Receptor Expression on [ ⁶⁸ Ga]Ga-DOTATATE PET Among Patients with Poorly Differentiated Extrapulmonary Neuroendocrine Carcinomas: A Prospective Study

Limited data are available regarding somatostatin receptor (SSTR) expression in poorly differentiated extrapulmonary neuroendocrine carcinoma (NEC). Some retrospective series have reported strong SSTR expression in up to 40% of patients with NEC. However, these figures may be influenced by selection bias, such as imaging of patients with lower Ki-67 index or uncertainty about pathologic tumor differentiation. Methods: In this prospective study, [⁶⁸Ga]Ga-DOTATATE PET was compared with [¹⁸F]FDG PET in patients with metastatic extrapulmonary NEC and at least 1 measurable site of metastasis. The primary endpoint was the proportion of patients with tumors showing uniform [⁶⁸Ga]Ga-DOTATATE avidity (uptake greater than that in the normal liver). The secondary endpoint was the proportion of patients with no SSTR expression or a heterogeneous pattern of expression. Results: This study enrolled 30 patients who underwent [⁶⁸Ga]Ga-DOTATATE PET, 28 of whom also underwent [¹⁸F]FDG PET within a prespecified 2-mo interval from [⁶⁸Ga]Ga-DOTATATE PET. Primary tumor sites included the pancreas (n = 7), colon/rectum (n = 6), and uterus (n = 2); the sites of 9 primary tumors were unknown. Histologic subtypes included small cell (n = 15), large cell (n = 6), and mixed or unspecified types (n = 9). All patients had radiographically measurable metastases on cross-sectional imaging, although metabolic uptake on [¹⁸F]FDG PET may have been attenuated by prior chemotherapy administration in some cases. Only 4 patients (13%) had evidence of uniformly avid uptake (Krenning grade 3 or 4) on [⁶⁸Ga]Ga-DOTATATE PET, indicating strong SSTR expression. Conclusion: Avid, uniform SSTR expression was relatively uncommon in unselected patients with extrapulmonary NEC.