“Early” Clear Cell Proliferations (Clear Cell Carcinoma in Situ) in Ovarian Endometriotic Cysts

Clear cell carcinoma (CCC) is an uncommon malignancy accounting for ∼12% of ovarian carcinomas. Most cases arise from endometriosis, frequently an endometriotic cyst. We report a series of 6 cases where clear cell proliferations, morphologically, and immunophenotypically consistent with CCC, involve the epithelial lining of an endometriotic cyst without invasion into the surrounding stroma. The patients were aged 29 to 63 years (mean 45). In all cases, epithelial proliferations composed of cells with atypical nuclei, sometimes with a hobnail morphology, and clear or eosinophilic cytoplasm involved the epithelial lining of an ovarian endometriotic cyst. In areas, the proliferations comprised a monolayer, but in all cases, there was also significant epithelial stratification and multilayering, sometimes with a pseudopapillary architecture. There was no invasion of the atypical cells into the surrounding ovarian stroma. The proliferations were positive for Napsin A (6 of 6; 4 diffuse, 2 focal), racemase (5 of 5; 3 diffuse, 2 focal), hepatocyte nuclear factor 1-beta (5 of 5; all diffuse), oestrogen receptor (5 of 6; 2 diffuse, 3 focal), and PAX8 (3 of 3; all diffuse). p53 was wild-type in all 6 cases and WT1 and progesterone receptor were negative in the 4 and 6 cases tested, respectively. Mismatch repair immunohistochemistry was retained in the 3 cases tested. Next-generation sequencing was performed in 2 cases. In 1 case, a sole pathogenic MSH6 variant (p.Ser65fs) was identified. Follow-up (2 to 24 months) was available in 5 cases and there was no tumour recurrence. In reporting these "early" clear cell proliferations in endometriotic cysts, we provide recommendations for the reporting pathologist regarding the most appropriate terminology, which is important in patient management. We suggest that these proliferations be termed "CCC in situ" and that identification of such a lesion should prompt extensive sampling in order to exclude an invasive CCC component within the stroma outside the endometriotic cyst lining. We also stress the importance of close dialogue between the pathologist and the clinician and between the clinician and the patient in order to avoid overtreatment in such cases.