BACKGROUND: Primary neuroendocrine tumors (NETs) in the retroperitoneal space are extremely rare. We report the case of a patient diagnosed with primary presacral NET in the retroperitoneum that was initially suspected to be hepatic metastasis, who was followed up for more than 8 years.

CASE REPORT: A 78-year-old man was referred to our hospital for the treatment of a hepatic mass. Following resection, the patient was diagnosed with a grade 2 well-differentiated NET. A thorough evaluation to identify the primary tumor detected small well-demarcated presacral nodules on In-111 octreotide single-photon emission tomography/computed tomography (SPECT/CT). Metastases to other locations were not observed. Presacral nodules were difficult to remove using the surgical approach; therefore, we decided to follow up closely. After 4 years, the patient was diagnosed with recurrent hepatic metastasis and peritoneal seeding. Although combination therapy of everolimus and octreotide long-acting repeatable was administered, it was discontinued owing to disease progression. Baseline Ga-68 DOTATOC positron emission tomography-computed tomography revealed adequate avidity for the lesions observed on SPECT/CT; therefore, 5 cycles of peptide receptor radionuclide therapy (PRRT) were administered, after which stable disease was maintained.

CONCLUSIONS: We identified an extremely rare primary retroperitoneal NET on In-111 octreotide SPECT/CT. During long-term follow-up, although the patient presented with recurrent hepatic metastases and peritoneal seeding, PRRT was successful in stabilizing the disease.

Keywords: Diagnostic Techniques, Radioisotope, neuroendocrine tumors, Positron-Emission Tomography, Radionuclide Imaging, Retroperitoneal Neoplasms, Tomography, Emission-Computed, Single-Photon, Liver Neoplasms, octreotide, Radiopharmaceuticals, Rare Diseases, Single Photon Emission Computed Tomography Computed Tomography, Watchful Waiting