Spontaneous ovarian epithelial neoplasms in guinea pigs (Cavia porcellus)

Diagnosis and classification of ovarian epithelial neoplasms in guinea pigs (Cavia porcellus) are challenging due to inconsistent terminology and few available diagnostic criteria. This study evaluated 23 ovarian epithelial neoplasms in 15 guinea pigs with the objectives of (1) differentiating ovarian surface epithelial (OSE) neoplasms from rete ovarii neoplasms using immunohistochemistry, (2) describing salient histologic features, and (3) classifying neoplasms according to canine, human, and rodent classification schemes. PAX-8 immunohistochemistry separated immunoreactive rete neoplasms from nonreactive OSE neoplasms. OSE neoplasms (n = 12) were well-differentiated and arose directly from the ovarian surface, rather than the primarily cortical location of OSE neoplasms in other species. Focal OSE neoplasms with papillary projections on a fibrous core or tubules within a fibrous stroma were classified as surface papilloma and surface adenoma, respectively. Generalized OSE neoplasms with tubules, papillae, and fibrous stroma were classified as surface borderline tumors. Neoplasms with invasion, 2-4 mitotic figures per 2.37 mm², and/or peritoneal implantation were classified as surface carcinoma. The only carcinoma with follow-up had resolution of clinical signs and no radiologic evidence of recurrence 6 months after ovariohysterectomy. Rete neoplasms (n = 11) included rete cystadenomas and rete adenomas, and consisted of epithelial cells arranged in papillae and tubules within a rete tubule, with or without cysts, respectively. Further investigation is needed to correlate diagnoses with neoplasms' biologic behavior. We propose using "surface" and "rete" in the diagnosis to denote location, rather than "papillary," "cystic," or "serous," which are variably used in other ovarian neoplasia classification schemes, to standardize terminology.