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.