Ovarian Cancer Series 01: What Is Ovarian Cancer?

 

When people hear the term “ovarian cancer,” they often think of a single type of cancer that begins in the ovaries. In reality, ovarian cancer includes several different types of cancer, depending on the cells and tissues where the cancer develops.

Our understanding of where ovarian cancer begins has also changed. In particular, some of the most common ovarian cancers are now thought to begin not in the ovary itself, but in the fallopian tube. This is one reason why ovarian, fallopian tube, and primary peritoneal cancers are often discussed together in medical guidelines and patient information.

So what exactly do we mean when we talk about ovarian cancer?

What Are the Ovaries?

The ovaries are reproductive organs located in the pelvis, one on each side of the uterus. They contain and release eggs and produce hormones such as estrogen and progesterone.

Different types of tumors can develop in the ovaries. However, finding an ovarian tumor or cyst does not necessarily mean that cancer is present. Some ovarian tumors are benign, while malignant tumors can invade nearby tissues or spread to other parts of the body.

Ovarian cancer develops when malignant cells arise in tissues of the ovary.

Are All Ovarian Cancers the Same?

No.

Ovarian cancers can be classified according to the type of cell in which the cancer begins. Major categories include epithelial ovarian cancer, germ cell tumors, and sex cord-stromal tumors.

Epithelial ovarian cancer is by far the most common type. Epithelial carcinomas account for about 85% to 90% of malignant ovarian tumors.

Germ cell tumors begin in the cells that give rise to eggs and are relatively uncommon. Sex cord-stromal tumors develop from cells involved in supporting ovarian structure and producing hormones, and they are also relatively rare.

For this reason, when ovarian cancer is diagnosed, it is important to identify not only that cancer is present but also the type of cells and tissue from which it developed.

What Is the Most Common Type of Ovarian Cancer?

Epithelial ovarian cancer itself includes several different histologic types.

Major types include:

• High-grade serous carcinoma

• Low-grade serous carcinoma

• Endometrioid carcinoma

• Clear cell carcinoma

• Mucinous carcinoma

Among these, high-grade serous carcinoma, or HGSC, is the most common type.

These classifications are more than different names for ovarian cancer. Different histologic types can have distinct biological characteristics and patterns of development. Identifying the specific type therefore provides clinically relevant information for diagnosis and treatment.

Can Ovarian Cancer Begin in the Fallopian Tube?

This is one of the important changes in our understanding of ovarian cancer.

In the past, most ovarian cancers were assumed to begin in the ovaries. As research progressed, however, evidence accumulated suggesting that a substantial proportion of high-grade serous cancers may actually begin in the fallopian tube rather than in the ovary itself.

The fallopian tubes extend from the uterus toward the ovaries. The end closest to each ovary contains finger-like projections called fimbriae.

Studies of tissue removed during risk-reducing surgery in women with BRCA1 or BRCA2 gene variants identified early cancers and precancerous changes, including serous tubal intraepithelial carcinoma (STIC), in the distal fallopian tube. Subsequent research has supported a model in which some high-grade serous cancers begin in this region and later involve the ovary or peritoneum.

This does not mean that all ovarian cancers begin in the fallopian tubes. Ovarian cancer includes several different histologic types, and their origins and development can differ.

The fallopian tube origin is therefore particularly important in explaining how high-grade serous carcinoma may develop.

Why Are Fallopian Tube and Primary Peritoneal Cancers Discussed Together With Ovarian Cancer?

When reading about ovarian cancer, you may frequently see three cancers mentioned together: ovarian cancer, fallopian tube cancer, and primary peritoneal cancer.

Fallopian tube cancer begins in the fallopian tubes. Primary peritoneal cancer begins in the peritoneum, the tissue that lines the abdominal cavity and covers many of the organs in the abdomen and pelvis.

Although these cancers begin in different locations, epithelial ovarian, fallopian tube, and primary peritoneal cancers are closely related histologically and biologically.

For this reason, they are generally staged and treated using the same clinical framework.

So if fallopian tube cancer or primary peritoneal cancer appears in ovarian cancer guidelines or patient information, it does not mean that an unrelated cancer has suddenly been introduced. These cancers have important biological and clinical relationships.

Why Does the Type of Ovarian Cancer Matter?

Identifying the type of ovarian cancer is more than simply assigning a more specific name to the disease.

Cancers arising from different cells and tissues can have different biological characteristics. The histologic type therefore provides important information that helps clinicians characterize the disease and plan appropriate care.

However, histologic type alone cannot determine an individual patient’s prognosis or treatment outcome. Multiple pieces of clinical information must be considered together when evaluating an individual case.

Key Takeaways

There are three important points to remember about ovarian cancer.

First, ovarian cancer is not a single disease. It includes several types of cancer based on the cells in which they develop, with epithelial ovarian cancer being the most common.

Second, among epithelial ovarian cancers, high-grade serous carcinoma is the most common histologic type, and a substantial proportion of these cancers are thought to begin in the end of the fallopian tube rather than in the ovary itself.

Third, because of their biological and clinical relationship, epithelial ovarian cancer, fallopian tube cancer, and primary peritoneal cancer are closely related diseases and are generally managed within the same clinical framework.

Learning what type of ovarian cancer is present provides the foundation for later discussions about risk factors, screening, diagnosis, and treatment.

In the next article, we will look at why ovarian cancer develops and which factors may increase the risk of developing it.

References

1. National Cancer Institute (NCI).
   Ovarian Epithelial, Fallopian Tube, and Primary Peritoneal Cancer Treatment (PDQ®) – Health Professional Version.
   NCI — Ovarian Epithelial Cancer Treatment (PDQ®)

2. National Cancer Institute (NCI).
   Ovarian Epithelial, Fallopian Tube, and Primary Peritoneal Cancer Treatment (PDQ®) – Patient Version.
   NCI — Ovarian Cancer Treatment, Patient Version

3. National Cancer Institute (NCI).
   Ovarian, Fallopian Tube, and Primary Peritoneal Cancers Prevention (PDQ®) – Health Professional Version.
   NCI — Ovarian Cancer Prevention (PDQ®)

4. National Cancer Institute (NCI).
   Ovarian, Fallopian Tube, and Primary Peritoneal Cancer.
   NCI — Ovarian Cancer

5. American Cancer Society.
   What Is Ovarian Cancer?
   American Cancer Society — What Is Ovarian Cancer?