Accurate Distinction of Ovarian Clear Cell From Endometrioid Carcinoma Requires Integration of Phenotype, Immunohistochemical Predictions, and Genotype

Abstract

Ovarian clear cell carcinoma (OCCC) and ovarian endometrioid carcinoma (OEC) are both associated with endometriosis but differ in histologic phenotype, biomarker profile, and survival. Our objectives were to refine immunohistochemical (IHC) panels that help distinguish the histotypes and reassess the prevalence of mismatch repair deficiency (MMRd) in immunohistochemically confirmed OCCC. We selected 8 candidate IHC markers to develop first-line and second-line panels in a training set of 344 OCCC/OEC cases. Interobserver reproducibility of histotype diagnosis was assessed in an independent testing cohort of 100 OCC/OEC initially without and subsequently with IHC. The prevalence of MMRd was evaluated using the testing cohort and an expansion set of 844 ovarian carcinomas. The 2 prototypical combinations (OCCC: Napsin A+/HNF1B diffusely+/PR−; OEC: Napsin A−/HNF1B nondiffuse/PR+) occurred in 75% of cases and were 100% specific. A second-line panel (ELAPOR1, AMACR, CDX2) predicted the remaining cases with 83% accuracy. Integration of IHC improved interobserver reproducibility (κ=0.778 vs. 0.882, P<0.0001). The prevalence of MMRd was highest in OEC (11.5%, 44/383), lower in OCCC (1.7%, 5/297), and high-grade serous carcinomas (0.7%, 5/699), and absent in mucinous (0/126) and low-grade serous carcinomas (0/50). All 5 MMRd OCCC were probable Lynch syndrome cases with prototypical IHC profile but ambiguous morphologic features: 3/5 with microcystic architecture and 2/5 with intratumoral stromal inflammation. Integration of first-line and second-line IHC panels increases diagnostic precision and enhances prognostication and triaging for predisposing/predictive molecular biomarker testing. Our data support universal Lynch syndrome screening in all patients with OEC when the diagnosis of other histotypes has been vigorously excluded.

The Evolution of Ovarian Carcinoma Subclassification

“…The three-marker first-line panel of napsin A, HNF1B, and PR can aid in the distinction of CCC from EC, although this can be misleading in a few ECs with non-specific cytoplasmic clearing when the IHC panel suggests CCC. Accurate distinction requires the integration of morphology (underlying architecture: tubulocystic for CCC versus glandular for EC), IHC and genotype (MMRd for EC) [ 45 ]. The best markers to distinguish EC from MC are PR and vimentin [ 51 ].…”

“…In contrast to the endometrium, however, the group of NSMP is substantially larger (73% versus 56%), requiring further stratification. The most promising biomarkers, which have only been assessed outside the context of molecular subtype thus far, are PR and CTNNB1 , with the latter being the most commonly mutated gene in ovarian EC [ 45 , 46 , 63 , 81 , 82 , 83 , 84 , 85 ].…”

“…MMRd with a high tumor mutation burden and neoantigen expression are predictors of response to immune checkpoint blockade but do not explain all responsive cases. While there are obscure cases with diffuse intratumoral stromal inflammation that are MMRd and might be classified as CCC [ 95 ], MMRd does not occur in prototypical CCCs; hence, MMRd is better considered in the context of the endometrioid histotypes (see above) [ 45 ]. Recent studies evaluating the immune microenvironment of CCC suggest that tumor-associated macrophages may be a marker for immunosuppressive microenvironments [ 96 , 97 ].…”

Validated biomarker assays confirm that ARID1A loss is confounded with MMR deficiency, CD8+ TIL infiltration, and provides no independent prognostic value in endometriosis‐associated ovarian carcinomas

“…As high neoantigen load generated by the hypermutation MMRd phenotype is well recognized to drive immune infiltration, this may be the more dominant driver for this observation [56,63]. Furthermore, MMRd is relatively uncommon in CCOC, with 5% in our cohort and 1.7% in a recent series [73].…”

“…Loss affecting MSH2, MSH6, or PMS2 (in the presence of intact MLH1) affected 49 out of 104 MMRd EAOC (39 ENOCs, 10 CCOCs) cases of probable Lynch syndrome. However, germline testing was not carried out in our cohort, and a recent analysis of germline MMR defects confirmed Lynch syndrome in 8 out of 12 probable cases (67%) [72,73], none amongst those with concurrent PMS2/MLH1 loss by IHC.…”