Grossly, the spleen measured 14×11×6 cm, with the splenic cyst measuring 12×8×4 cm. The cyst was filled with brown coloured fluid. Cut section showed a smooth, glistening inner wall with typical tree-root trabeculations (figure 1). The cyst wall was lined by flattened cuboidal mesothelial cells (figure 2). Immunophenotyping showed positivity for calretinin (figure 3) throughout the lining epithelium, positivity for low molecular weight cytokeratin and a weak positivity for CA 19-9 in the wall. Based on the histological and immunophenotypic findings, a final diagnosis of primary mesothelial cyst of the spleen was made.

The true or primary cysts may be congenital or neoplastic in origin and are lined by mesothelial, squamous or transitional epithelium. Secondary or pseudocysts are usually post-traumatic, due to failure of organisation of subcapsular or parenchymal haematomas ad occasionally due to necrosis following an infarction or rarely due to an abscess.⁵

Clinically, primary cysts occur predominantly in children and young adolescents; often asymptomatic until they assume large sizes, they may then present with local or referred pain, abdominal distension compression of adjacent structures and rarely as thrombocytopenia.^{1 6 7}

Capsular surface mesothelial invagination with subsequent cyst formation,⁶ embryonic inclusion of epithelial cells from adjacent structures, epithelial cell metaplasia from adjacent structures or vascular endothelium from peritoneal inclusions are some of the theories put forth to explain the genesis of these congenital cysts.¹

Grossly, mesothelial are usually unilocular and vary in size; the cut surface is whitish or greyish- white, smooth and glistening and displays characteristically prominent coarse tree-root-like trabeculations due to subepithelial fibrosis. Cyst fluid may vary in density and may demonstrate cholesterol crystals and macrophages on microscopic examination.⁸

The cyst wall is generally lined by a single cell series of mesothelial, transitional or squamous epithelium.^{1 8} Occasional finding of combination of epithelial cells may be explained by the pluripotent nature of the mesothelial cells.² Occasional absence of lining cells may lead to an erroneous diagnosis of secondary cyst unless multiple sections are studied.⁵

Accurate preoperative diagnosis of mesothelial cysts is rare; the occurrence of a unilocular cyst in the absence of previous trauma, infection or exposure to hydatid disease may help in the diagnosis.⁹

Recent studies have indicated rise in tumour markers such as CA 19-9 and carcinoembryonic antigen levels in association with primary mesothelial cysts.³ Serum levels of CA 19-9 have been shown to reduce substantially after cyst removal, offering a screening test to indicate recurrences in case of spleen preserving surgeries.³ CA 19-9 may also be demonstrated in lining of the cyst wall as was demonstrated in our case.⁴ Calretinin is a vitamin D-dependent calcium-binding protein that is expressed in mesothelial cell lines; even benign mesothelial cells may show the characteristic “fried-egg” staining pattern.¹⁰ Cytokeratins specifically permit identification of the epithelial origin based on the expression profile. Immunohistochemistry confirmed the mesothelial origin of the splenic cyst in our case as evidenced by the positive calretinin and low molecular weight cytokeratin and weakly positive CA 19-9 staining in the cyst wall.

All symptomatic cysts and cysts beyond 5 cm in diameter need surgical treatment. Splenic preservation should be the primary factor while considering treatment; in the present era, laparoscopic methods are increasingly being carried out. Various modes of treatment based on the expertise of the surgical team include total splenectomy, partial cyst excision and marsupialisation,⁷ partial splenectomy, partial cystectomy (decapsulation) and laparoscopic partial cystectomy.⁸ Our patient was subjected to a laparoscopic splenectomy because the cyst was occupying the hilar portion precluding preservation.

In conclusion, a rare case of histologically and immunophenotypically confirmed mesothelial cyst of the spleen is presented with a discussion of the various findings and treatment modalities.