Endometrial carcinoma is a malignant epithelial tumour, arising in the endometrium with glandular differentiation, but it may have variable morphology.

Endometrioid adenocarcinoma is the most common type accounting for almost three-fourths of the cases [11]. It resembles a proliferative phase endometrium with small, back-to-back glands without the stroma intervening. The grade of the tumour is based on nuclear features and architectural pattern. The nuclear grade is determined by the variation in nuclear size and shape, nucleoli and distribution of chromatin. The architecture grade and pattern are seen as how well the gland formation is seen as compared to solid clusters of tumour cells. Glandular complexity may be seen as luminal budding, papillae and cribriform patterns. Mitotic activity usually increases with the increase in nuclear grade and is an independent variable. The grading of the tumour is done according to the degree of gland formation by the tumour (Table 4.3). In grade 1 lesions, nuclei of the lining epithelial cells are uniform with minimal atypia and small discrete nucleoli (Fig. 4.2). The degree of tumour necrosis is usually mild to moderate. Marked amount of necrosis is unusual, even in high-grade endometrioid adenocarcinoma (Fig. 4.3).

Different morphologic patterns of endometrioid adenocarcinoma are seen including villoglandular, secretory, ciliated cell and adenocarcinoma with squamous differentiation. These patients share similar epidemiologic characteristics of typical endometrioid carcinoma, and these patterns may be seen in association with the usual form of endometrioid cell type.

Mucinous carcinoma is an adenocarcinoma with abundant intracellular mucin. To categorise it as mucinous carcinoma, more than 50 % of the cell population must contain mucin which should be PAS positive and diastase resistant. Its appearance is similar to the mucinous endocervical adenocarcinoma, and it has to be differentiated from the clear cell carcinoma where the pattern is usually papillary or solid as compared to glandular in this variant. Endometrioid and clear cell adenocarcinoma may have large amounts of intraluminal mucin, but only mucinous adenocarcinoma contains the mucin within the cytoplasm. They tend to be of low grade with a good prognosis.

Serous carcinoma usually involves older women and is uncommon as compared to endometrioid carcinoma. It often displays papillary architecture like the variants of endometrioid carcinoma, but the papillae here have broad cores, and the pattern may be even solid. The cytologic atypia is marked (Fig. 4.4). Psammoma bodies may be present. These tumours are aggressive and have a poor prognosis. They are considered as high-grade neoplasms and are not graded. These tumours have a predilection for peritoneal spread, akin to ovarian serous adenocarcinoma [15].