It is the most common serous benign ovarian neoplasm (50 %), occurs in the reproductive age, is composed of varying amounts of fibrous stroma and cysts and is subclassified by the WHO in serous cystoadenoma, cystoadenofibroma, and adenofibroma. These tumors range in size from 1 to 10 cm and are bilateral in 20 % of the cases. Smooth-walled unilocular or multilocular cysts are lined by nonstratified tubal-type epithelium. Epithelial cells do not show nuclear atypia, mitoses and necrosis. Torsion, rupture, and infection are the most common complications.

Serous borderline tumors occur more frequently during the fourth and fifth decades and usually in the reproductive age. They form bilateral (30–40 %) large cystic or solid and cystic masses with surface papillary epithelial structures. The inner layer of the cyst is composed of large and small confluent papillae with complex hierarchical branching (Fig. 5). The lining cells are tall to columnar with stratification, mild–moderate atypia, and sparse mitoses (Fig. 6). Psammoma bodies can be present. Foci of stromal microinvasion (<10 mm²) do not warrant a diagnosis of carcinoma. Extraovarian peritoneal and lymph node implants may be present at the time of detection. The extraovarian serous implants are classified according to the WHO as invasive or noninvasive, epithelial or desmoplastic, on the basis of the interface between the implant and normal tissue. The micropapillary variant of serous borderline tumors is more frequently associated with bilaterality, ovarian surface involvement and extraovarian implants, some of which may be invasive. The course of disease is indolent. Tumor stage is an important prognostic factor; invasive extraovarian implants confer a comparatively poor prognosis. Serous borderline tumor cells are immunoreactive for CK7, EMA, WT1, CA125 while are CK20 negative. Point mutations in BRAF or K-RAS are frequently associated with serous borderline tumors, but no association with germ line or somatic BRCA1/BRCA2 mutations has been reported.

Serous carcinoma comprises 35 % of all ovarian tumors, the incidence peak being in the sixth to seventh decades. At the time of diagnosis, pelvic ovarian masses, peritoneal spread and ascites are present. Serous carcinoma forms solid and cystic masses (Fig. 7) that frequently show areas of necrosis and hemorrhage. At microscopy, complex papillae, glands with malignant cellular stratification, and psammoma bodies are observed. The immunohistochemical profile is similar to that typically found in benign and borderline serous tumors. Grading, from 1 to 3 according to the WHO, is based on architectural features and nuclear atypia (Fig. 8). Molecular studies show somatic or germ line abnormalities of BRCA1/BRCA2 genes in high-grade tumors. In contrast, low-grade serous carcinoma shows a molecular profile similar to that of serous borderline tumors (BRAF or K-RAS mutation).

Benign mucinous tumors are composed of intestinal-type or endocervical-like mucinous epithelium. They account for the majority of mucinous ovarian tumors (80 %). They are unilocular or multilocular tumors of variable size, ranging from a few to over 30 cm. They are typically unilateral; the capsula is thick and white with a smooth outer surface. Mucinous cysts contain gelatinous material and are composed of glands and cysts lined by columnar, pale-staining epithelium; stratification and atypia are absent (Fig. 10).

Two histological types of mucinous borderline tumors are distinguished: intestinal type and endocervical-like. The first type (Fig. 11) is a large, multicystic and unilateral (>95 %) tumor lined by stratified intestinal-type epithelium with papillary intraglandular growth, mild to moderate nuclear atypia, and rare mitoses. Stromal invasion is absent (Fig. 12). Intestinal-type mucinous borderline tumors at immunohistochemistry are CEA positive, CA125 negative, CK7 and CK20 positive. At presentation, most of them are stage I and have a benign clinical course. Patients with associated peritoneal implants, foci of intraepithelial carcinoma, or stromal microinvasion have an excellent prognosis (100 % survival). Recent studies show that advanced stage intestinal-type mucinous borderline tumors with adverse outcome represent in fact metastases from occult extraovarian primary tumors.

In contrast to intestinal-type mucinous borderline tumors, endocervical-like neoplasms are less common, usually smaller, frequently bilateral and associated with endometriosis. Tumor cells are CK7 positive and CK20 negative. These tumors are often admixed with a component of serous borderline tumor.

Mucinous adenocarcinomas are <15 % of all mucinous ovarian tumors and most of them are stage I at presentation. The tumoral masses are solid and cystic and may contain foci of necrosis and hemorrhage (Fig. 13). Ovarian surface involvement is rare but large tumors may present capsular rupture. Different patterns of stromal invasion can be recognized in intestinal-type adenocarcinoma: (1) confluent or expansile pattern (complex, crowded, confluent glands with cytologic atypia and little stroma) (Fig. 14); (2) infiltrative and destructive pattern (Fig. 15), more frequent in metastatic tumors. Areas of benign, borderline, and invasive mucinous carcinoma coexist in the same tumor (Fig. 16). Immunohistochemical profile is similar to that of borderline tumors (CK7+, CK20+, CEA+). The stage is the most important prognostic factor. Grading of mucinous carcinomas has not been precisely defined yet, but the nuclear features are the best discriminator; tumors with glandular and papillary architecture are low grade.

Mucoid noninvasive nodules adherent to peritoneal surface characterize PMP. Low-grade neoplastic mucinous epithelium with pools of extracellular mucin and fibrosis is their typical morphologic appearance. Many studies show that all cases of PMP derive from low-grade mucinous tumors of the appendix and that ovarian involvement is secondary (“metastatic mucinous tumor”).