Solid Pseudopapillary Pancreatic Tumor Mimicking a Neuroendocrine Neoplasm on 18F-FDOPA PET/CT

A 36-year-old man with a history of ulcerative colitis was referred to the visceral surgery department of our institution for further evaluation. Ultrasonography showed a well-circumscribed heterogeneous solid-appearing lesion of the pancreatic isthmus. Contrast-enhanced computed tomography (CT) confirmed the presence of an 18-mm rounded low-attenuation area in the inferior edge of the pancreatic isthmus (Figure 1A). Contrast-enhanced magnetic resonance imaging was thus performed. During the arterial phase, the mass was hypointense and showed a slight progressive enhancement, almost imperceptible on the portal phase (Figure 1B). No tumoral cells were obtained from endoscopic ultrasound-guided fine-needle aspiration. Pancreatic hormone secretion was normal. To investigate the possibility of a nonfunctioning atypical neuroendocrine tumor (NET), the patient underwent both somatostatin receptor scintigraphy (SRS) single photon emission CT (SPECT)/CT and 6-L-18F-fluorodihydroxyphenylalanine (FDOPA) positron emission tomography (PET)/CT. Although SRS was strictly normal (Figure 1C), PET/CT showed an increased focal uptake exclusively localized to the pancreatic nodule (Figure 1, D and E). The lesion was particularly evident thanks to carbidopa premedication reducing the physiological pancreatic FDOPA uptake (1). Hence, the diagnosis of pancreatic NET was done, and the patient was referred to surgery (Figure 1F). Unexpectedly, pathological examination revealed a solid pseudopapillary tumor (SPT) with low mitotic activity, strongly and diffusely positive for β-catenin, CD10, and vimentin, and negative for synaptophysin and chromogranin (Figure 1, G–I). Contrast-enhanced magnetic resonance imaging (portal phase) (A), contrast-enhanced CT (delayed phase) (B), SRS single photon emission CT/CT (C), and FDOPA PET/CT (D and E) fusion images. FDOPA PET/CT was performed after carbidopa pretreatment (200 mg orally 90 min before iv radiotracer injection). The red arrows show the pancreatic tumor. Pathological examination of a surgical specimen (F) demonstrates a SPT with low mitotic activity (G). In the tumor cells, the immunohistochemical staining was negative for chromogranin (H) and positive for CD10 (I). Although FDOPA PET is mainly used for the management of patients with NET, FDOPA is not a NET-specific radiotracer (2, 3). To the best of our knowledge, this is the first report of pathological FDOPA uptake in a pancreatic tumor not of neuroendocrine (NE) origin. To date, no definitive explanation for FDOPA uptake by the SPT of our patient can be provided. Although SPT is thought to be an exocrine epithelial lesion capable of a minor degree of NE differentiation (4), no NE features were found at pathological examination. The overexpression of transmembrane transporters (3) eventually associated to high L-DOPA decarboxylase activity (5) could be responsible for FDOPA uptake, justifying further more complex investigations. Disclosure Summary: A.I., P.A., G.A., I.J.N., and P.B. have nothing to declare. computed tomography 6-L-18F-fluorodihydroxyphenylalanine neuroendocrine neuroendocrine tumor positron emission tomography solid pseudopapillary tumor somatostatin receptor scintigraphy.