**4.1 Pathology of ovarian endometrioid carcinomas**

Endometrioid carcinomas of the ovary present mainly as unilateral mass in the ovary and less frequently as bilateral (20%) [82]. They display a smooth external surface and a solid/cystic cut surface, sometimes with a residual endometriotic cyst at the periphery of the tumour [80]. Microscopically, these tumours show a variety of morphologic patterns such as glandular, cribriform, villoglandular or solid with characteristic back-to-back glands, areas of squamous differentiation and expansile rather than an infiltrative pattern of invasion [5]. Sometimes a destructive invasion pattern can be seen characterised by neoplastic cells infiltrating the stroma accompanied by a desmoplastic reaction. The neoplastic cells are tall columnar and focally mucinous with a mitotic activity of 5–10 mitoses per high power field [5, 82]. Histologic characteristics confirmatory of OECs are metaplastic features such as squamous, morular, hobnail or mucinous metaplasia, presence of endometriosis lesions, ovarian endometrioid adenofibroma or endometrioid borderline tumour, and presence of a synchronous uterine endometrioid carcinoma found in 15–20% of the cases [5]. Ovarian endometrioid carcinomas are divided according to the presence of solid growth pattern in grade 1 (less than 5% solid growth), grade 2 (5–50% solid growth) and grade 3 (more than 50% solid growth), excluding areas of squamous differentiation [5].
