Low grade endometrial endometrioid adenocarcinoma: A review and update with emphasis on morphologic variants, mimics, immunohistochemical and molecular features.

Endometrioid adenocarcinoma (ECa) may feature a number of morphologic variations that can pose diagnostic challenge. The purpose of this review and update is to examine the spectrum of morphologic variants and mimics of low-grade (FIGO grades 1 and 2) ECa, with a focus on histologic, immunohistochemical, and molecular features that may inform diagnosis and treatment. In addition to ECa of usual type, variants with unique cytologic and/or architectural features presented include the following: 1) ECa with mucinous differentiation of conventional (Müllerian) type; 2) ECa with squamous differentiation; 3) ECa with morular metaplasia; 4) ECa with patterns resembling cervical transformation zone tissue and/or microglandular hyperplasia; 5) ECa with cytoplasmic clearing; 6) ECa with papillation, including villoglandular variant of ECa, ECa with small nonvillous papillae, and ECa with a "low-grade serous"-like component or surface changes mimicking ovarian serous borderline tumor; 7) corded and hyalinized variant of ECa; 8) ECa with spindled epithelial cells; 9) ECa with sex cord-like pattern; and 10) ECa with other unusual cytologic and associated features. For each variant, relevant differential diagnoses and diagnostic strategies are discussed. The most clinically significant distinctions come into play in the differential diagnosis between low-grade ECa and one of its high-grade mimics. In this setting, the most fundamental tool in the pathologist's diagnostic arsenal is recognition of the low-grade cytologic features typical of low-grade ECa. Circumspect evaluation of cytologic features, complemented by an awareness of the morphologic spectrum, an appropriate battery of immunohistochemical stains when needed, and mindfulness of the clinical scenario, should guide the pathologist to the correct histotype in even challenging cases.