[443] Differential Expression of PAX8, WT1, and Calretinin in Peritoneal Cytology: An Immunohistochemical (IHC) Study of Metastatic Ovarian Serous Carcinoma (OSC)

RM McKnight, C Cohen, MT Siddiqui. Emory University Hospital, Atlanta, GA

Background: Metastases from primary ovarian neoplasms are common in peritoneal fluid. Microscopic evaluation has prognostic value as ovarian tumor staging incorporates the presence of malignant cells in peritoneal cytology. Reactive mesothelial cells can mimic OSC, and it is often difficult to distinguish the two on morphology alone. Calretinin by IHC has been recognized as a reliable marker for mesothelial cells, while, WT1 has proven useful in the diagnosis of OSC. This can be a diagnostic pitfall, as mesothelial cells can show immunoreactivity for WT1. Recently, PAX8 has been used as a marker in distinguishing ovarian from mammary carcinoma. To our knowledge no studies using PAX8 have been performed on peritoneal cytology specimens, and its expression in metastatic OSC has not been studied.
Design: IHC for PAX8, WT1, and Calretinin were performed on paraffin-embedded cell block sections from forty-one metastatic OSC peritoneal cytology cases and ten negative controls. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) for the cytology results were calculated using histological diagnosis as the gold standard. Differential expression of PAX8, WT1, and Calretinin was also evaluated in background mesothelial cells.
Results:

Marker expression in OSC and Negative controls.

	PAX8	WT1	Calretinin
OSC cases	37/41 (90%)	38/41 (93%)	0/41 (0%)
Negative controls	0/10 (0%)	10/10 (100%)	10/10 (100%)

Sensitivity and specificity data of markers used

	Sensitivity	Specificity	PPV	NPV
PAX8	90%	100%	100%	71%
WT1	92%	0%	79%	0%

In benign and positive peritoneal cytology fluids both WT1 and Calretinin staining were observed in background mesothelial cells, consistently with variable intensity. PAX8 expression was not observed in mesothelial cells, and Calretinin expression was not observed in ovarian carcinomas.
Conclusions: PAX8 is a sensitive and specific marker for the detection of metastatic OSC and Calretinin is useful for identifying mesothelial cells. While PAX8 and WT1 demonstrate comparable sensitivity (90% and 93% respectively) in diagnosing metastatic OSC, PAX8 has superior specificity as staining is not observed in mesothelial cells. This suggests that the profile of PAX8-positive, Calretinin-negative is highly specific and sensitive for detecting metastatic OSC and can be useful in distinguishing it from mesothelial cells.
Category: Cytopathology

Tuesday, March 23, 2010 9:15 AM

Platform Session: Section F, Tuesday Morning