Endometrial endometrioid adenocarcinoma (EEC) is the most common malignancy of the female genital tract, partly attributable to chronic estrogen exposure secondary to increased obesity rates. Tumor stage, which in most cases is based on depth of invasion (DOI), is of critical importance in determining if additional treatment is needed. However, the array of invasive morphologies within the spectrum of ECC can make the determination of DOI difficult. Several morphologic patterns of invasion have been described, including diffusely infiltrating irregular glands, "broad front" (or pushing border), adenoma malignum, adenomyosis-like, and microcystic, elongated, and fragmented glands. EEC may often contain a mixture of invasive patterns, which can further complicate evaluation of these common tumors. Recognition of these patterns may lead to more accurate staging, but perhaps more importantly, some patterns may be associated with adverse prognostic features. The purpose of this review is to highlight the various invasive patterns of EEC and note their unique pitfalls and prognostic implications in an effort to improve staging accuracy and treatment and follow-up for the thousands of women affected by this disease each year.
