The evolution of endometrial carcinoma classification through application of immunohistochemistry and molecular diagnostics: past, present and future

Uterine cancer was first subclassified based on anatomic site, separating those tumours arising from the endometrium from cervical cancers. There was then further subclassification of endometrial cancers based on cell type, and this correlated with the Type I and Type II categories identified through the epidemiological studies of Bokhman, with endometrioid carcinoma corresponding (approximately) to Type I and serous carcinoma to Type II. These histotypes are not clearly separable in practice, however, with considerable interobserver variability in histotype diagnosis, especially for high-grade tumours. There followed studies of immunomarkers and then mutational studies of single genes, in attempts to improve subclassification. While these have revealed significant differences in protein expression and mutation profiles between endometrioid and serous carcinomas, there is also considerable overlap, so that there remain challenges in subclassification of endometrial carcinoma. Gene panel testing, using next-generation sequencing, was applied to endometrial cancers and highlighted that there are tumours that show genetic alterations intermediate between classic Type I/endometrioid and Type II/serous carcinomas. The Cancer Genome Atlas studies of endometrioid and serous carcinoma offered revolutionary insight into the subclassification of endometrial carcinoma, i.e. that there are four distinct categories of endometrial carcinoma, rather than two, based on genomic architecture. In this review, we provide an overview of immunohistochemical and molecular markers in endometrial carcinoma and comment on the important future directions in endometrial carcinoma subclassification arising from The Cancer Genome Atlas results.