Comparison of [¹⁸F]DOPA and [⁶⁸Ga]DOTA-TOC as a PET imaging tracer before peptide receptor radionuclide therapy

Background: In treatment of neuroendocrine neoplasms (NENs), confirmation of somatostatin receptor expression with ⁶⁸Ga-DOTA somatostatin analogues is mandatory to determine eligibility for peptide receptor radionuclide therapy (PRRT). [¹⁸F]DOPA can detect additional lesions compared to [⁶⁸Ga]DOTA-TOC. The aim of this study was to explore differences in tumour detection of both tracers and their relevance for selecting patients for PRRT. We retrospectively studied eight patients with NENs who underwent both [⁶⁸Ga]DOTA-TOC and carbidopa-enhanced [¹⁸F]DOPA PET/CT, before first-time PRRT with [¹⁷⁷Lu]DOTA-TATE. Tracer order was influenced due to stock availability or to detect suspected metastases with a second tracer. On CT, disease control was defined as a lesion showing complete response, partial response, or stable disease, according to RECIST 1.1. criteria. Results: Seven patients with in total 89 lesions completed four infusions of 7.4 GBq [¹⁷⁷Lu]DOTA-TATE, one patient received only two cycles. Before treatment, [¹⁸F]DOPA PET/CT detected significantly more lesions than [⁶⁸Ga]DOTA-TOC PET/CT (79 vs. 62, p <.001). After treatment, no difference in number of lesions with disease control was found for [¹⁸F]DOPA-only (5/27) and [⁶⁸Ga]DOTA-TOC-only lesions (4/10, p =.25). [¹⁸F]DOPA detected more liver metastases (24/27) compared to [⁶⁸Ga]DOTA-TOC (7/10, p =.006). Six patients showed inpatient heterogeneity in treatment response between [¹⁸F]DOPA-only and [⁶⁸Ga]DOTA-TOC-only lesions. Conclusions: Response to PRRT with [¹⁷⁷Lu]DOTA-TATE was comparable for both [⁶⁸Ga]DOTA-TOC- and [¹⁸F]DOPA-only NEN lesions. [¹⁸F]DOPA may be capable of predicting response to PRRT while finding more lesions compared to [⁶⁸Ga]DOTA-TOC, although these additional lesions are often small of size and undetected by diagnostic CT.