Discussion
The accessory spleen is not uncommon. Halpert et al. reported 291 (10.8%) of 2700 autopsied cases with an accessory spleen, 215 (62.1%) in the vicinity of the splenic hilum, followed by 78 (22.5%) in the pancreatic tail6. However, a cyst occurring in the pancreatic accessory spleen is extremely rare (Table 1).7-52 The lesion site was the pancreatic tail in all cases. The primary complaints were abdominal pain and vomiting. On the other hand, 33 of 57 cases were asymptomatic. Smaller tumor diameters are often asymptomatic. The case reported in our study has a tumor diameter of 1.5 cm, which is the smallest among the reported cases, and it is estimated that there were no symptoms. CEA or CA 19-9 was elevated in some cases, but remained within the conventional limits in others. However, the lesion was asymptomatic in many cases, and detected by image examination. Cyst morphology was multilocular in 33 cases, and unilocular in 15. The tumor diameter ranged from 15 mm to 134 mm. A majority of the findings were reduced in T1, and elevated in T2-weighted MRI images. Preoperative diagnoses, such as IPMN, pNET, epidermoid cyst, and malignant tumor suggest case complexity. Consequently, presurgical diagnosis of pNET was performed. Histopathological findings revealed LEC in the present case; no such instance has been reported previously.
Pancreatic LECs are extremely rare. LECs are reported to account for 0.5% of pancreatic cysts; they were initially identified and reported by Luchtrath et al. in 19854, and so named by Truong et al. in 1987.5 Mege et al. reported a study with pancreatic LECs in middle-aged to elderly men (mean age, 55 years; range, 20-82 years; 91 men vs. 26 women); it was observed that the lesion was occasionally accompanied by abdominal pain (43%) and an elevated serum CA 19-9 level (55%).53 Pancreatic cysts can be classified into true cysts, pseudocysts, and cystic neoplasms. LEC is considered as a type of true cyst characterized by a lining of squamous epithelium with dense subepithelial lymphoid tissue. The cystic contents are typically white in color and may include keratinized material or cholesterol crystals. Adsay et al. classified cystic lesions covered by the squamous epithelium of the pancreas as LECs; those occurring in the sub-pancreatic epithelium are epidermoid and dermoid cysts with cutaneous appendages.54 The pathological diagnostic criteria for LECs are ambiguous. Presently, the predominant diagnostic criteria of a LEC comprises the lumen surface of the cyst with stratified squamous epithelium with abundant lymphoid tissue underneath. The reports so far follow the classification of cysts as defined by Adsay et al.54 Adsay et al. defined that “LECs are characterized microscopically by stratified squamous epithelium surrounded by a band of mature lymphoid tissue with intervening well-formed germinal centers”. In addition to the above, Adsay et al. defined “The second type of squamous-lined cyst in the pancreas is the epidermoid cyst arising in intrapancreatic accessory spllen”. Adsay et al. do not mention lymphoid-rich cysts in the accessory spleen. Among the cases reported as epidermoid cysts around 1987 proposed by Truong, there were some cases that were thought to be LECs.5 Due to the unclear classification, it cannot be ruled out that LECs may also be included in the cases reported as epidermoid cysts.
In the present case, the lumen epithelium of the multilocular cyst consisted of mature squamous material with developed sub-epithelial lymphoid tissue. In addition, the white and the red pulp were detected in the pancreatic accessory spleen and diagnosed as splenic LEC. No such LECs have been reported; consequently, the present case is extremely rare. There are multiple theories regarding the pathogenic mechanism of pancreatic LECs. The first theory suggests an origin from the misplaced branchial cleft tissue because of the histologic resemblance. The second suggests squamous metaplasia on an obstructed pancreatic duct, which subsequently protrudes into a peripancreatic lymph node. The third indicates cyst development from an ectopic pancreas in a peripancreatic lymph node. At present, a decisive view has not been reached. Tateyama et al. reported, “In the present immunohistochemical study, the cytokeratin phenotypes of the epithelial lining of LEC were similar to those of the epithelial retention cysts but different from those of branchial cleft cysts.14 In addition to the cytokeratin pattern, the presence of some islets and ducts in the fibrous wall of the LEC might support the second hypothesis”. In this case, a LEC was found in the accessory spleen in the pancreas, and we reckon that that the second hypothesis that squamous metaplasia on the pancreatic duct protrudes into the pancreatic lymph tissue of the accessory spleen in the pancreas is a highly plausible explanation for our case. However, it still needs consideration and discussion.