Claudin‐18 overexpression in intestinal‐type mucinous borderline tumour of the ovary

Aims Mucinous borderline tumours of the ovary are subclassified as intestinal‐type ( IMBT ) and endocervical‐like ( EMBT ), which differ in their clinicopathological features. In this study, we attempted to elucidate characteristics of the mucinous epithelium in each subtype. Methods and results The expression of claudin‐18, a marker of gastric differentiation, MUC s, CDX 2, CK 7, CK 20, oestrogen receptor ( ER ), progesterone receptor (PgR), CA ‐125 and vimentin in IMBT s ( n  =   54), EMBT s ( n  =   25) and serous borderline tumours ( SBT s) ( n  =   22) were compared by immunohistochemistry. Claudin‐18 positivity was identified in 98% of the IMBT s, whereas only 4% of the EMBT s were claudin‐18‐positive. Expression of intestinal markers such as CDX 2 and MUC 2 was relatively infrequent in IMBT s (48% and 33%, respectively). Müllerian‐lineage markers such as ER , P g R and vimentin were expressed rarely in IMBT s, while most EMBT s and SBT s were positive for these markers. Hierarchial clustering revealed a close association between EMBT s and SBT s, while IMBT s were clearly separate. Conclusions Claudin‐18 positivity is a specific phenotype that is characteristic of IMBT s. Frequent and diffuse expression of gastric markers, along with less frequent and usually focal expression of intestinal markers, suggests that IMBT s are essentially composed of gastrointestinal‐type mucinous epithelium (gastric‐type epithelium with a variable degree of intestinal differentiation).