Clear cell ovarian cancer is a distinct type of ovarian cancer with its own biology, its own risk factors, and its own relationship to the CA125 test. It is closely linked to endometriosis, and its CA125 levels can be modest enough to be misleading — so it is another subtype where a “normal-ish” result deserves caution.

What it is and how common it is

Clear cell carcinoma gets its name from the way its cells look under the microscope: their interiors appear clear because they are rich in glycogen. It makes up a modest share of ovarian cancers overall, but that share varies notably by population — clear cell carcinoma is considerably more common in East Asian populations, for example.

One of its defining features is a strong association with endometriosis, the condition in which tissue similar to the uterine lining grows outside the uterus. Clear cell (and endometrioid) cancers are thought to sometimes arise from areas of long-standing endometriosis. On the molecular level, clear cell cancers frequently carry mutations in a gene called ARID1A.

Clear cell cancer is often diagnosed at an earlier stage than high-grade serous disease, which can be favorable — but when it is advanced, it tends to be more resistant to standard chemotherapy, which makes it a serious diagnosis to treat.

How it behaves on a CA125 test

CA125 in clear cell ovarian cancer is variable and often only modestly elevated. Some clear cell cancers raise it and some do not, and the levels are frequently lower than those seen with high-grade serous disease. Because of that, CA125 is not a reliable way to detect clear cell cancer or, on its own, to track it.

This unreliability matters especially for women with known endometriosis, who may already have a mildly raised CA125 from the endometriosis itself — making it even harder to read the number meaningfully. Doctors interpret CA125 here only in combination with imaging and clinical findings.

Symptoms and how it’s diagnosed

Clear cell cancer shares the general symptoms of ovarian cancer, and women with endometriosis should be alert to changes in familiar symptoms:

Persistent bloating, or pelvic or abdominal pain
A pelvic mass
Feeling full quickly or changes in appetite
New or worsening pain in someone with a history of endometriosis

Because CA125 is unreliable, imaging does much of the work — transvaginal ultrasound to characterize a mass, and CT or MRI to assess spread. Clear cell cancers are also associated with two specific complications that can prompt investigation: an increased risk of blood clots (venous thromboembolism), and, less commonly, raised blood calcium. As with other ovarian cancers, diagnosis is confirmed by a pathologist examining tissue removed at surgery.

Treatment and outlook

Surgery to remove the tumor is central, and because clear cell cancer is often found early, complete removal is frequently achievable. Chemotherapy — typically platinum-based — is commonly given, though clear cell cancer’s relative resistance to chemotherapy is a well-recognized challenge, and researchers are actively studying better options, including targeted and immune-based therapies.

Prognosis is generally good for early-stage, fully removed clear cell cancer. Advanced clear cell cancer is harder to treat because of its chemo-resistance, so stage at diagnosis strongly shapes the outlook. Your care team will also watch for the clotting risk associated with this subtype.

Questions to ask your care team

Is my cancer confirmed as clear cell type specifically?
Given my history of endometriosis, how should my CA125 be interpreted?
What is my risk of blood clots, and should anything be done to lower it?
What chemotherapy or targeted-therapy options fit clear cell cancer best?
Are there clinical trials for clear cell ovarian cancer I could consider?

Why this matters to us

Clear cell cancer sits at the intersection of two conditions — ovarian cancer and endometriosis — that are both too often dismissed. The Cure Ovarian Cancer Foundation funds research into these harder-to-treat subtypes and stands beside the families living with them. If this page helped you understand your diagnosis, please consider giving to help the next person find clear answers, too.

This page is for education and awareness and is not medical advice. Please talk with your own care team about your symptoms, tests, and treatment.