Evaluating the Role of Theranostics in Grade 3 Neuroendocrine Neoplasms

The diagnosis and subsequent therapy of neuroendocrine neoplasms (NENs) have long relied on somatostatin receptor (SSTR) expression. The field of theranostics now uses newer SSTR-based PET imaging with ⁶⁸Ga-DOTATATE or ⁶⁸Ga-DOTATOC as a prerequisite for the administration of peptide receptor radionuclide therapy (PRRT). In the United States, Food and Drug Administration approval of ¹⁷⁷Lu-DOTATATE, a form of PRRT, in 2018 for use in gastroenteropancreatic NENs was obtained on the basis of prolonged progression-free survival versus high-dose octreotide long-acting release in a phase III clinical trial of well-differentiated midgut NENs. Well-differentiated grade 1 and grade 2 NENs have a low proliferation index (Ki-67 < 20%) and longer overall survival (>10 y), whereas higher-grade (grade 3 [G3]) NENs have a high Ki-67 (>20%) and shorter overall survival (<1 y). Here, we present a review on the role of SSTR-based imaging and PRRT in G3 NENs, including a discussion of well-differentiated G3 NENs, the newest histologic classification. Some studies suggest that G3 NENs are less likely to be positive on SSTR-based imaging (but more likely on ¹⁸F-FDG PET) than are well-differentiated NENs, but these data are limited. We found only 13 studies mentioning the use of PRRT in G3 NENs and a total of only 151 patients across these studies in whom radiologic response was measured. Of these 151 patients, 99 (66%) demonstrated at least stable disease or a partial response, indicating that some G3 NENs can be responsive to PRRT. We suggest that patients with G3 NENs should receive both ¹⁸F-FDG PET and SSTR-based imaging to aid in both diagnosis and treatment selection, as positivity on SSTR-based imaging helps with patient identification for PRRT and discordance may suggest important clues to tumor biology and prognosis. However, prospective studies are needed to fully understand the role of PRRT in G3 NENs, especially in well- versus poorly differentiated G3 disease.