A previously well 9-year-old boy presented with intermittent left upper quadrant pain, anorexia, and weight loss over six months. His past medical history was significant for being born preterm at 34 weeks’ gestation and requiring ventilatory support as a neonate. Computed tomography (CT) scan (Figure 1 and Figure 2) showed a 10 cm encapsulated spherical mass appearing to arise from the pancreatic tail. The working diagnosis was a cystic pancreatic tumour, and he was referred for further management.

At subsequent laparotomy a retroperitoneal mass was found adherent to the left colon and duodenum but free from the pancreas, with associated reactive lymphadenopathy. The mass was biopsied, and surgery was abandoned to allow for formal histological evaluation. Frozen section histology reported a spindle cell lesion comprising of bland spindle cells with elongated cigar-shaped nuclei. Necrosis was present at one end of the core biopsy. Mitotic activity was not seen. Immunohistostaining showed spindle cells expressing smooth muscle actin, desmin, and vimentin. There was no significant expression of S100, CD117, CD34, or AE1/AE3. There was no nuclear or cytoplasmic beta catenin expression. Up to three IgG4 expressing plasma cells were seen per high power field.

Further work-up included a computed tomography positive emission tomography (CT PET) scan which showed a non-avid mass appearing to arise from the tail of the pancreas. Computed tomography-guided biopsy showed degenerate and viable red cells. No other cell type was identified to help determine the nature of the lesion. A decision was made to perform local resection as no malignancy was identified. At surgery the lesion was excised locally without compromising adjacent pancreas, colon, or small bowel. He recovered well post-operatively and was discharged home on day 4. Final histology showed a 130 mm encapsulated thick-walled organized hematoma with no evidence of malignancy.

Retroperitoneal cysts are rare with few reports in children. A literature review of PubMed was performed using the search terms retroperitoneal cyst in children and pediatric population. Articles not available in English, pancreatic cysts and patients over age 18 were excluded. The results are summarized in Table 1 [1],[2],[3],[4],[5],[6],[7],[8],[9],[10].

The presenting symptoms are non-specific and varied with abdominal pain and distension being the most common. Other presentations included nausea and vomiting, anorexia, acute abdomen, and incidental detection on examination or on imaging.

The differential diagnosis includes cysts of lymphatic origin and lymphangiomas, benign or malignant mesothelial cysts, enteric cysts, cysts arising from a urogenital origin, dermoid cysts and cysts with traumatic, hemorrhagic or infectious etiology [1].

The reported ages of patients varied from 10 months old to 15 years old with a median age of 9 years old for those included.

Investigations varied from case to case. Ultrasound was the most commonly used modality across all of the patients.

Diagnosis can be difficult. The literature review confirmed that the majority of cases were diagnosed as retroperitoneal cysts at operation. On imaging, the cyst appeared to be associated with the tail of the pancreas but at operation, it was clearly separate and in our case, there was concern that it could malignant. Frozen section revealed spindle cells within the wall and it required formal immunohistostaining and repeat biopsy to exclude malignancy.

Retroperitoneal cysts are treated surgically. Acutely there is a risk of cyst rupture, infection, and hemorrhage. They may cause abdominal pain by compression of surrounding organs [1]. In this case, complete excision was achieved without complication.

1. Retroperitoneal cysts are a rare entity with a wide diagnosis. Here we report a non-traumatic hemorrhagic cyst which has not been reported previously and should be included in the differential diagnosis.
2. The presenting symptoms of retroperitoneal cysts vary widely from patient to patient in this literature review.
3. This patient underwent a long diagnostic process which we hope to avoid with documentation of this case.