Ovarian
serous neoplasms can have morphologic overlap with
malignant mesothelioma. The distinction is clinically important, yet most studies have failed to identify immunostains that reliably distinguish these 2
tumor types. Recently,
transcription factor PAX8 was shown to be a sensitive and relatively specific marker for Müllerian
tumors. In addition, some studies suggest that h-
caldesmon is sensitive and specific for
mesothelioma when compared with serous ovarian
tumors. The goal of this study was to evaluate whether PAX8 and h-
caldesmon expression can successfully distinguish
mesothelioma from serous ovarian
tumors. Immunohistochemistry was carried out using PAX8 and h-
caldesmon antibodies on archival tissue from 254 ovarian serous
tumors and 50 mesothelial
tumors. Nuclear and cytoplasmic immunoreactivity were considered positive for PAX8 and h-
caldesmon, respectively. PAX8 staining was present in 99% of high-grade serous ovarian
carcinomas and all (100%) low-grade ovarian
carcinomas and serous borderline
tumors; however, only 74% of these cases (188/254) were diffusely positive in more than 50% of
tumors cells, and intensity ranged from strong to weak. None of the pleural
malignant mesotheliomas were reactive with PAX8. However, 2/23 (9%) peritoneal
malignant mesotheliomas showed focal and/or weak staining for PAX8; the remaining cases were negative. Two well-differentiated papillary
mesotheliomas and 1 multicystic
mesothelioma each showed some staining for PAX8. h-
caldesmon was negative in all
serous neoplasms and all
mesothelial neoplasms, except 1 pleural
malignant mesothelioma which showed patchy immunoreactivity. Strong PAX8 staining is highly specific (P<0.00001) for ovarian serous
tumors when compared with
malignant mesotheliomas of the peritoneum and pleura. The presence of weak staining for PAX8 in the 3 "noninvasive"
mesotheliomas questions the use for PAX8 in this differential diagnosis. On the basis of this study, h-
caldesmon is not a useful marker for
mesothelioma.