Mucinous borderline tumour of the ovary

What is a mucinous borderline tumour?

A mucinous borderline tumour is a non-cancerous type of ovarian tumour. While the tumour is considered non-cancerous, it is associated with a small risk of turning into cancer over time. The behaviour of this tumour is thought to fall somewhere in between a mucinous cystadenoma which is a non-cancerous tumour and mucinous carcinoma which is a type of cancer.

The ovary

The ovaries are part of the female reproductive tract. The ovaries are small organs that are located at the ends of the fallopian tubes and are attached to the uterus by the ovarian ligaments. The outer surface of the ovaries is lined by specialized epithelial cells that form a barrier called an epithelium. The epithelial cells on the surface of the ovary are also called the capsular surface. The tissue below the epithelium is called the stroma.

How do pathologists make this diagnosis?

For most women, the diagnosis of mucinous borderline tumour is only made when the entire tumour has been surgically removed and sent to a pathologist for examination. The fallopian tube and uterus may be removed at the same time.

Your surgeon may request an intraoperative or frozen section consultation from your pathologist. The diagnosis made by your pathologist during the intraoperative consultation can change the type of surgery performed or the treatment offered after the surgery is completed.​

The tumour is usually made up of many small spaces. Pathologists call these spaces cysts. The walls of the cysts can be thin or thick and more solid areas may be found inside some of the cysts.

When the tumour is examined under the microscope, the tissue on the inside of the cysts and the solid areas are made up of an abnormal type of epithelium that forms glands and produces a thick, gelatinous fluid called mucin. The mucin fills the inside of the tumour.

Cancer in a mucinous borderline tumour

Your pathologist will carefully examine the tumour under the microscope to look for two microscopic features that will help determine your prognosis. Intraepithelial carcinoma and microinvasion are early signs of cancer and both are associated with worse prognosis.

1. Intraepithelial carcinoma – These are groups of very abnormal tumour cells that look different from the rest of the tumour. These cells have changed into cancer cells.
2. Microinvasion – These are single tumour cells that have spread out of the epithelium and into the stroma below. The movement of tumour cells into the stroma is called invasion. Invasion is something that happens when the tumour changes in cancer.

Histologic type

There are two types of mucinous borderline tumour and the type depends on the kinds of cells seen in the mucin-producing epithelium when the tumour is examined under the microscope. There is no difference in prognosis between these two types.

1. Endocervical – In this type of tumour, the cells in the epithelium look similar to the cells that line the endocervical canal (the canal that leads from the cervix into the uterus).
2. Intestinal – In this type of tumour the cells in the epithelium look similar to the cells that line the digestive (intestinal) tract.  The intestinal type is more common than the endocervical type.

Intact or ruptured tumour

All ovarian tumours are examined to see if there are any holes or tears in the outer (capsular) surface of the ovary. The capsular surface is described as intact if no holes or tears are identified. The capsular surface is described as ruptured if it contains any large holes or tears. If the ovary or tumour is received in multiple pieces, it may not be possible for your pathologist to tell if the capsular surface has ruptured or not.

This information is important because a capsular surface that ruptures inside the body may spill tumour cells into the abdominal cavity. A ruptured capsule is associated with a worse prognosis and is used to determine the tumour (T) stage (see Pathologic stage below).

Ovarian surface involvement

Your pathologist will carefully examine the tissue under the microscope to see if there are any tumour cells on the surface of the ovary. Tumour cells on the surface of the ovary increase the risk that the tumour will spread to other organs in the pelvis or abdomen. It is also used to determine the tumour stage (see Pathologic stage below).

Other organs or tissues involved

Small samples of tissue are commonly removed in a procedure called a biopsy to see if tumour cells have spread to the pelvis or abdomen. These biopsies which are often called omentum or peritoneum are sent for pathological examination along with the tumour.

Other organs (such as the bladder, small intestine, or large intestine) are not typically removed and sent for pathological examination unless they are directly attached to the tumour. In these cases, your pathologist will examine each organ under the microscope to see if there are any tumour cells attached to those organs.

Tumour cells in other organs are used to determine the tumour stage (see Pathologic stage below).​

The appendix

If you have been diagnosed with mucinous borderline tumour or if your doctor suspects you may have a mucin-producing tumour, your appendix might also be removed and sent for pathological examination. In these cases, your pathologist will examine the appendix for any tumour cells. Tumours of the appendix can look very similar to the mucinous borderline of the ovary. Tumours that start in the appendix can spread from the appendix to the ovary.

Pathologic stage

​The pathologic stage for mucinous borderline tumour is based on the TNM staging system, an internationally recognized system originally created by the American Joint Committee on Cancer. This system uses information about the primary tumour (T), lymph nodes (N), and distant metastatic disease (M)  to determine the complete pathologic stage (pTNM). Your pathologist will examine the tissue submitted and give each part a number. In general, a higher number means more advanced disease and a worse prognosis.

Tumour stage (pT)

​Nodal stage (pN)

Metastatic stage (pM)

by Emily Goebel MD FRCPC (updated August 24, 2021)