Endometrioid tumors are a class of tumor characterized by a resemblance to endometrium

Endometrium

-Function:The endometrium is the innermost glandular layer and functions as a lining for the uterus, preventing adhesions between the opposed walls of the myometrium, thereby maintaining the patency of the uterine cavity. During the menstrual cycle or estrous cycle, the endometrium grows to a...

/ endometrial carcinoma, and over a third of cases have focal squamous differentiation.

Ovary

They are part of the surface epithelial

Surface epithelial-stromal tumor

Surface epithelial-stromal tumors are a class of ovarian neoplasms that may be benign or malignant. Neoplasms in this group are thought to be derived from the ovarian surface epithelium or from ectopic endometrial or Fallopian tube tissue. This group of tumors accounts for the majority of all...

 tumor

Tumor

A tumor or tumour is commonly used as a synonym for a neoplasm that appears enlarged in size. Tumor is not synonymous with cancer...

 group of ovarian neoplasm

Ovarian cancer

Ovarian cancer is a cancerous growth arising from the ovary. Symptoms are frequently very subtle early on and may include: bloating, pelvic pain, difficulty eating and frequent urination, and are easily confused with other illnesses....

s (10–20% of which are the endometrioid type).
Benign

Benign

A benign tumor is a tumor that lacks the ability to metastasize. Common examples of benign tumors include moles and uterine fibroids.The term "benign" implies a mild and nonprogressive disease. Indeed, many kinds of benign tumors are harmless to human health...

 and borderline

Borderline

Borderline or border line may refer to:*Border-In film:*Borderline , a film starring Paul Robeson*Borderline , a film noir starring Fred MacMurray*Borderline , a film starring Charles Bronson...

 variants are rare, as the majority are malignant

Malignant

Malignancy is the tendency of a medical condition, especially tumors, to become progressively worse and to potentially result in death. Malignancy in cancers is characterized by anaplasia, invasiveness, and metastasis...

.
There is an association with endometriosis

Endometriosis

Endometriosis is a gynecological medical condition in which cells from the lining of the uterus appear and flourish outside the uterine cavity, most commonly on the ovaries. The uterine cavity is lined by endometrial cells, which are under the influence of female hormones...

 and concurrent primary endometrial carcinoma (endometrial cancer).

On gross pathological

Pathology

Pathology is the precise study and diagnosis of disease. The word pathology is from Ancient Greek , pathos, "feeling, suffering"; and , -logia, "the study of". Pathologization, to pathologize, refers to the process of defining a condition or behavior as pathological, e.g. pathological gambling....

 examination, the tumor is cystic and may be solid and some arise in cystic endometriosis. In 40% of cases, endometrioid tumors are found bilaterally.

Endometrium

Endometrioid carcinoma can also arise in the endometrium.

Grades 1 and 2 are considered "type 1" endometrial cancer

Endometrial cancer

Endometrial cancer refers to several types of malignancies that arise from the endometrium, or lining, of the uterus. Endometrial cancers are the most common gynecologic cancers in the United States, with over 35,000 women diagnosed each year. The incidence is on a slow rise secondary to the...

, while grade 3 is considered "type 2".