Contrast-enhanced fluorodeoxyglucose positron emission tomography/computed tomography in solid pseudopapillary neoplasm of the pancreas

INTRODUCTION

Solid pseudopapillary neoplasm (SPN) is a rare exocrine tumor of the pancreas, with 2-3% incidence rate among primary pancreatic tumors and 10-15% incidence rate among cystic tumors of the pancreas.[12] The neoplasm was first described by Frantz in 1959.[3] Because of its rarity, the literature contains few reports of this tumor; however, reports regarding fluorodeoxyglucose positron emission tomography/computed tomography (FDG PET/CT) in SPN have increased significantly in the past 10 years.[4] We present radiological, FDG PET/CT, and cytological findings of a case of SPN.

CASE REPORT

An 18-year-old female presented with a complaint of abdominal distention on the left side that was gradually increasing in size over a period of 18 months. There was an associated sensation of abdominal fullness and pain over the last 1 month. Her menstrual cycles were regular. She had undergone appendicectomy for acute appendicitis, 5 years back. All laboratory investigations, including tumor markers, were within the normal ranges. CT of the abdomen showed a heterogeneously enhancing mass lesion replacing the body and tail of the pancreas with solid and cystic hemorrhagic areas. She was referred for FDG PET/CT scan to characterize the pancreatic lesion. The FDG PET/CT [Figures 1 and 2] showed intense FDG uptake (maximum standardized uptake value [SUVmax] 20.0) in the solid enhancing portions of the mass lesion in the pancreas. There were no other regional or distant FDG avid lesions. Based on the CT and PET findings, a solid cystic neoplasm of the pancreas was considered. The large FDG avid solid portion of the lesion was just near the anterior abdominal wall, suitable for percutaneous fine needle aspiration (FNA) that was done subsequently. FNA cytology showed features of a solid and cystic neoplasm of the pancreas Figure 3. The patient underwent distal pancreatectomy and splenectomy. Histological findings of the surgical specimen showed a partially encapsulated tumor consisting of uniform polygonal cells, with moderate to abundant amphophilic cytoplasm and arranged in solid nests, with areas of degeneration characterized by separation of the cells into pseudo papillary aggregates, and with an intervening accumulation of mucopolysaccharide rich ground substance consistent with features of SPN of the pancreas. The postoperative course was uneventful. The patient is currently disease free after 7 months of surgery.

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Figure 1 Maximum intensity projection image (a) of positron emission tomography/ computed tomography shows a large abnormal fluorodeoxyglucose avid focus in the umbilical region. Contrast-enhanced computed tomography (a) shows a large well-defined heterogeneously enhancing mass lesion (arrow) with solid, cystic, and hemorrhagic areas replacing the body and tail of the pancreas, measuring 13.2 cm × 12.8 cm × 9.8 cm (TR × CC × AP). There was no evidence of calcification. Positron emission tomography shows intense fluorodeoxyglucose uptake (maximum standardized uptake value 20.0, arrow) in the solid enhancing portions of the mass lesion in the pancreas

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Figure 2 Coronal computed tomography and fused positron emission tomography/computed tomography (a and b) and sagittal computed tomography and fused positron emission tomography/computed tomography (c and d) images show the pancreatic lesion (shown by arrow) seen to indent and displace the stomach anterosuperiorly, and compress the splenic vein posteriorly with multiple perigastric and spleno renal venous collaterals

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Figure 3 Hematoxylin and Eosin stain of percutaneous fine needle cytology specimen from the pancreatic lesion shows features of solid and cystic papillary neoplasm of the pancreas

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DISCUSSION

SPN usually affects young women at an average age of 28 years with a female:male ratio of 10:1.[5] About 20-25% of the cases are seen in pediatric patients.[6] The clinical presentation of SPN is nonspecific. Most of the patients present with nonspecific symptoms including abdominal discomfort, mild abdominal pain, or palpable abdominal mass.[7] Due to slow growth, SPN often remains asymptomatic, until the tumor has enlarged considerably as seen in our patient. The most common site of SPN is the tail of the pancreas, followed by the head and the body.[6] The diagnosis can be confirmed by percutaneous core needle biopsy with ultrasound or CT-guidance, as shown in our case.[8]

SPN has a good prognosis after adequate resection, and preoperative diagnosis can be helpful in surgical planning. In a case series of 8 tumors, CT and magnetic resonance imaging (MRI) findings included encapsulation, solid and cystic components, focal calcification, and weak enhancement during the arterial phase on enhanced CT or MRI and increasing enhancement during the portal venous phase (most specific finding). All the tumors showed increased FDG uptake. The mean SUVmax of all tumors was 8.9, ranging from 2.5 to 29.1. On pathological correlation, the tumors with high cellularity, high proliferative index, pancreatic parenchymal, vascular, or perineural invasions had stronger FDG uptake.[9] A similar observation has been made by Nakagohri et al.[10] Thus, FDG PET/CT can potentially reflect the histopathological composition of the tumors. On delayed FDG PET/CT images, the SUVmax of SPN increased in 4 patients and slightly decreased in 2 patients.[9] Sato et al. have also showed a decrease in SUVmax in delayed images and suggested that SUVmax of early and delayed scan may be of little value in differentiating benign from malignant SPN.[11]

SPN is usually benign, based on its pathological features. However, it always shows hypermetabolism on the fluorine-18 (¹⁸F)-FDG PET scan, which is a characteristic feature of malignant tumors. The high cellular density, rich mitochondria, and the hypervascular nature as shown in radiological findings have been thought to contribute to the FDG accumulation.[9] In addition, because of the high FDG avidity of SPNs, Guan et al. have concluded that SPN cannot be differentiated from pancreatic adenocarcinoma and neuroendocrine tumors based on the SUVmax.[4] In a study comparing pancreatic ductal adenocarcinomas, SPNs had significantly higher tumor size-adjusted metabolic tumor volume and tumor lesion glycolysis.[12]

The curative treatment is a complete surgical resection.[13] In rare cases, SPN metastasizes to liver or seeds the peritoneum. PET/CT can reveal unsuspected metastases in the liver and mesentery.[12] In our patient, ¹⁸F-FDG PET staged the tumor to be confined to the pancreas without metastases, and she had safely undergone surgical resection. In an analysis of the 114 patients with pathologically confirmed SPN, 26 (22.8%) had solid pseudopapillary carcinoma (SPC). There were no differences in any clinical factors between the benign SPN and SPC groups, and only 4 recurrences identified were in the SPC group.[14] Because of the indolent entity, patients usually have a very good prognosis with less likelihood of recurrence and metastasis after the resection of SPN.[614] In addition, our patient is disease free after 7 months of follow-up. To conclude, FDG PET/CT helps in identifying metabolically active areas in solid cystic neoplasms of the pancreas. Biopsy from these active areas can give an adequate yield of viable tumor cells that helps in achieving a faster diagnosis.