Introduction

Solid pseudopapillary tumor (SPT) of the pancreas is an uncommon, low-grade malignant neoplasm that predominantly affects young women. It has a low incidence of recurrence or distant metastasis [1,2,3,4]. SPT often remains asymptomatic, though some cases may manifest symptoms like abdominal pain or distension [1, 3, 4]. Diagnostic approaches encompass diverse methods including barium meal examinations, ultrasonography, computed tomography (CT), magnetic resonance imaging (MRI), angiography, endoscopic retrograde cholangiopancreatography (ERCP), and fine-needle aspiration cytology. However, there is no imaging modality of choice, so it should be determined according to the clinical situation [1, 4].

In general, complete surgical excision is regarded as the treatment of choice [4, 5]. Surgical procedures depend on the location of the tumor in the pancreas. Distal pancreatectomy with splenectomy is a feasible option for tumors located in the body or tail of the pancreas. It is important to be aware of the potential occurrence of splenic tissue implantation following the surgery [6]. Various imaging exams could be performed in this clinical context, although none is specific [7].

We report the usefulness of [18F]FDOPA PET/CT in a case of SPT with peritoneal seeding that mimicked splenosis.

Case Report

A 55-year-old woman underwent laparoscopic distal pancreatectomy with splenectomy for incidentally detected SPT. The patient was followed up by an annual CT scan, and 34 months later, six enhancing nodular lesions were identified near the splenectomy site. Initially, the lesions were reported as splenosis. On follow-up CT scans over 2 years, the nodules progressively enlarged and the long diameter of the largest lesion had expanded from 2.2 to 7.7 cm (Fig. 1).

Fig. 1
figure 1

Follow-up abdominal CT scans with about a 1-year interval. These images are portal-phase abdominal CT scans, taken in June 2018 (a), July 2019 (b), and August 2020 (c, d). These show multiple rounded nodules considered as splenosis (arrows; the largest nodule, arrowhead)

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The nodules were suspected to be peritoneal seeding and a spleen scan with single photon emission computed tomography (SPECT)/CT was performed using 99mTc-labeled damaged RBC. This scan showed no uptake in the peritoneal nodules (Fig. 2). On [18F]FDOPA PET/CT, the peritoneal nodules showed high avidity (Fig. 3). The maximum standardized uptake value (SUV) of the lesion was 6.90, whereas the mean liver SUV was only 1.49. The multiple peritoneal nodules with high [18F]FDOPA-avidity were suggested as recurred tumors. The peritoneal nodules were surgically removed and pathologically confirmed as SPT (Fig. 4).

Fig. 2
figure 2

Spleen scan and SPECT/CT using 99mTc-labeled damaged RBC. These images are maximum intensity projection (MIP) (a), fusion SPECT/CT (b), CT (c), and SPECT (d) images. Multiple peritoneal nodules show no significant radiotracer uptake (arrows)

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Fig. 3
figure 3

[18F]FDOPA PET/CT for differential diagnosis. These images are MIP (a), fusion PET/CT (b), PET (c), and CT (d) images. There is highly increased [18F]FDOPA uptake in the multiple peritoneal nodules (arrows)

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Fig. 4
figure 4

Pathologic images of recurred SPT. Gross image of one of the surgical specimens (a). The hematoxylin and eosin–stained slide shows the solid and pseudopapillary appearance of tumor cells (b; ×200). The tumor cells show a positive result for beta-catenin expression (c; ×200)

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Discussion

[18F]FDOPA PET/CT has been used in various diseases, especially for the diagnosis of neuroendocrine tumors [8,9,10]. [18F]FDOPA, a radiolabeled analog of amino acid, is taken up into cells via the L-type amino acid transporter (LAT), decarboxylated by amino acid decarboxylase (AADC), and transported into secretory vesicles [11, 12]. Therefore, cells with neuroendocrine features or only high expression of LAT show a high uptake of [18F]FDOPA. Its uptake is also shown in other organs or tumors without a neuroendocrine feature. Considering its normal distribution, the basal ganglia, pancreas, adrenal glands, hepatobiliary, and urinary system show mild-to-moderate radioactivity, but the spleen shows faint or no radioactivity [13, 14].

Some cases of SPT have shown uptake of [18F]FDOPA in PET/CT scans, but the mechanisms behind this remain uncertain [15,16,17,18,19]. SPT originates from the exocrine tissue of the pancreas and, unlike neuroendocrine tumors (NET), it does not consistently display neuroendocrine features and has a different amino acid profile [18, 20]. This suggests that the mechanism of [18F]FDOPA uptake in SPT may differ from that in NET. One hypothesis for [18F]FDOPA uptake in SPT is elevated LAT-1 expression. Elevated LAT-1 expression is a common feature in multiple solid tumors and serves as a potential prognostic indicator [21]. It is associated with tumor growth and proliferation [22]. In the context of SPT, increased LAT-1 expression could explain the observed [18F]FDOPA uptake. However, research focusing on LAT-1 expression in SPT is limited. Further studies are needed to clarify the role of LAT-1 in SPT.

Splenosis is a benign condition in which splenic tissues appear at heterotopic sites, mainly in the abdominal cavity or organs. It is incidentally detected after trauma or splenic surgery [23]. Sometimes it is necessary to differentiate with tumors [24]. For differentiating, we use many imaging modalities, such as CT, MRI, or USG. But none of these imaging modalities is the gold standard [7, 25, 26]. Pathologic examination is the gold standard, but surgery or biopsy is invasive, so it is not preferred. Spleen scan using 99mTc-labeled damaged RBC was a noninvasive, highly sensitive, and specific exam. If the results are negative, it largely rules out the possibility of splenic tissue implantation, but additional diagnostic work-up is still required for differential diagnosis.

In situations where differentiation between splenic tissue and tumor is required during SPT evaluation, [18F]FDG PET/CT is not helpful because both conditions can be mildly [18F]FDG-avid. Also in our case, the baseline [18F]FDG PET/CT taken before surgery showed low FDG-avidity of the SPT. If a spleen scan was done, it would find splenic tissue but not other tumor lesions. Therefore, if progression is clinically suspected, further testing is needed. However, [18F]FDOPA PET/CT shows a low spleen uptake and a high SPT uptake. Therefore, it may be more efficient to perform a [18F]FDOPA PET/CT in this kind of case. Given these points, [18F]FDOPA PET/CT in SPT evaluation has potential, although further studies are needed. However, because low-grade NET also shows [18F]FDOPA-avidity, in some situations, it may not be efficient to differentiate SPT from NET. In conclusion, the use of [18F]FDOPA PET/CT can be a valuable diagnostic tool in cases of uncertain diagnosis or suspected metastatic disease, as seen in our case of SPT with peritoneal seeding.

To the best of our knowledge, this is the first case report where [18F]FDOPA PET/CT was employed to diagnose recurred SPT, and the utility of [18F]FDOPA PET/CT for distinguishing between splenosis and tumor recurrence was shown. Further studies are warranted to validate the utility of [18F]FDOPA PET/CT in SPT.