FIGO Grade 3 Endometrioid Adenocarcinomas With Diffusely Aberrant β-Catenin Expression: An Aggressive Subset Resembling Cutaneous Pilomatrix Carcinomas.

Uterine endometrioid adenocarcinomas are known for their morphologic plasticity. In addition to a multiplicity of metaplasias, uterine endometrioid adenocarcinomas may also undergo high-grade divergent differentiation in the form of high-grade neuroendocrine carcinoma, neuroectodermal differentiation or carcinosarcoma; others may dedifferentiate completely. Here we describe 5 cases of uterine endometrioid adenocarcinomas with high-grade divergent differentiation showing a striking morphologic and immunophenotypic resemblance to cutaneous pilomatrix carcinoma. Specifically, the high-grade component in all cases exhibited solid, basaloid morphology with conspicuous tumor cell necrosis and the presence of shadow cells, accompanied by diffusely aberrant (nuclear and cytoplasmic) β-catenin expression as well as variably diffuse CDX2 expression. In addition, the high-grade component in all cases showed loss of ER and PAX8 expression, retained MMR expression, wild-type p53 expression, patchy p16 expression, and diffusely positive cytokeratin expression (AE1/AE3 and CK7); at least focal neuroendocrine marker expression was present in all cases. CK20 was negative in all cases, with the exception of very focal staining in a single case (2% of tumor cells). All 5 of our tumors had at least a focal conventional FIGO grade 1 component. In all 4 cases tested, the low-grade component retained both PAX8 and ER expression and had, at best, focally aberrant β-catenin expression. Two of our cases had molecular analysis performed and both harbored mutations in exon 3 of CTNNB1 as expected; molecular analysis also revealed that both cases lacked POLE or TP53 mutations and showed no microsatellite instability. The tumors in this series were uniformly aggressive. Four of the 5 patients in our cohort had available follow-up information; of these, 3/4 died of their disease within 14 mo of diagnosis and the fourth patient had distant metastatic disease at presentation and is alive with disease 1 mo following diagnosis. The 1 patient without follow-up information also had distant metastatic disease at presentation and was lost to follow-up 17 mo later. The cases described in this series (1) represent a highly aggressive CTNNB1-mutated subset of the "no specific molecular profile" category of endometrioid adenocarcinomas; (2) illustrate a form of high-grade divergent differentiation resembling cutaneous pilomatrix carcinoma already described in carcinomas at other anatomic sites; and (3) underscore the difficulty in recognizing this phenotype at distant metastatic sites, which are frequent even at the time of presentation, given the consistent loss of ER and PAX8 expression and concurrent CDX2 expression.