What Percent of Ovarian Cysts Are Cancerous?

The vast majority of ovarian cysts are not cancerous. Among cysts that look simple and fluid-filled on imaging, the cancer rate is essentially zero. Even when you broaden the count to include all types of ovarian masses flagged on ultrasound or CT, only about 1% of those abnormal findings turn out to be ovarian cancer. The number climbs higher when a mass has worrying features like solid components or internal walls, but even then, most prove benign after surgery. What really drives your individual risk is the kind of cyst, your age, and what the cyst looks like on imaging.

Simple Cysts Are Almost Never Cancer

The single most reassuring finding in this area is how consistently studies show that simple ovarian cysts carry near-zero cancer risk. A large study that followed over 2,700 women whose cysts were discovered incidentally on CT scans found that none of the 1,031 women with simple-appearing cysts developed ovarian cancer over an average follow-up of more than five years.¹ Zero out of over a thousand. A separate study in asymptomatic postmenopausal women found the same pattern: unilocular cysts under 10 centimeters carried minimal risk for malignancy.²

A simple cyst, in clinical terms, is a smooth-walled, fluid-filled sac with no solid parts, no internal dividers, and no extra blood flow visible on ultrasound. Most of these are functional cysts, meaning they form as a normal part of the menstrual cycle and disappear on their own within a few months.³ The majority of ovarian disorders overall are benign, with functional cysts and benign neoplasms making up the bulk.⁴

In postmenopausal women, simple cysts are common too. One follow-up study of postmenopausal women with simple cysts found that about 46% resolved spontaneously and another 44% persisted without changing, with no cancers arising.⁵ This is why guidelines now generally recommend monitoring simple cysts with periodic ultrasound rather than rushing to surgery, even in older women.

Complex Masses Are Where the Risk Rises

When a cyst has complex features, the picture changes. Complex characteristics include solid components, thick internal walls (septations), irregular surfaces, and abnormal blood flow patterns. In a large population-based study using ultrasonography, women with complex cysts or solid masses had a significantly elevated cancer risk, with the three-year risk ranging from 9 to 430 cases per 1,000 depending on the woman’s age and the specific imaging findings. By comparison, women with simple cysts were at no significantly increased risk compared to women with completely normal ovaries.⁶

A British screening trial that followed over 48,000 women confirmed this gradient. Among 4,367 women with abnormal-looking adnexal findings on ultrasound, the overall absolute risk of epithelial ovarian cancer was about 1%. But in the subgroup of 741 women whose masses contained solid elements, the risk jumped to roughly 4.5%.⁷ So even among masses that look suspicious, the odds are still strongly in favor of a benign diagnosis. But those odds shift meaningfully when solid components are involved.

A study of incidentally detected adnexal masses in women 50 and older found no ovarian cancers among surgically excised lesions. The most common pathological findings were cystadenomas, nonneoplastic cysts, mature teratomas, and other benign conditions.⁸ This reinforces how frequently what looks concerning on a scan turns out to be harmless under a microscope.

How Imaging Helps Sort Cysts

Ultrasound is the first and most important tool for evaluating an ovarian cyst. An experienced examiner’s subjective impression of a mass’s shape and blood flow has been shown to be highly effective at predicting whether it is benign or malignant.⁹ But because subjective assessments vary between operators, several standardized scoring systems have been developed to give doctors a more structured way to estimate cancer risk from an ultrasound image.

These scoring systems assign categories based on what the cyst looks like. The O-RADS system, for example, rates masses on a scale from 1 (normal) to 5 (highly suspicious). The IOTA Simple Rules and the ADNEX model use different criteria to arrive at similar predictions. A comparative study found that the ADNEX model and the IOTA Simple Rules had the highest overall diagnostic accuracy at about 91%, while O-RADS had the highest sensitivity at 98% but lower specificity.¹⁰ In practical terms, these systems are good at catching cancer when it is present and good at reassuring you when it is not, though no system is perfect.

What this means for you is that the ultrasound report often gives a strong indication of whether a cyst needs further workup or can safely be watched. A cyst classified as low-risk on a validated scoring system can usually be monitored with repeat imaging, while a high-risk classification prompts discussion about surgery or further testing.

What Blood Tests Can and Cannot Tell You

CA-125 is the most widely used blood test when ovarian cancer is a concern. It is a protein that tends to be elevated in women with epithelial ovarian cancer. Another marker, HE4, is often used alongside it. When the two are combined using a formula called the ROMA index, the diagnostic performance improves. One Italian multicenter study found that ROMA showed well-balanced sensitivity and specificity: roughly 87% sensitivity and 86% specificity in premenopausal women, and 90% sensitivity and 94% specificity in postmenopausal women.¹¹

The catch is that CA-125 levels can be elevated by many non-cancerous conditions, including endometriosis, fibroids, pelvic inflammatory disease, and even menstruation. This makes false positives common, particularly in younger women. One study evaluating these markers in women with suspicious cystic masses found that when at least one of CA-125 or HE4 was positive, the negative predictive value was about 92%, meaning a negative result was highly reassuring. However, the markers were weaker at detecting early-stage cancer.¹² Other research has found that adding VEGF (a protein involved in blood vessel growth) to CA-125 can improve detection of early-stage disease, pushing sensitivity up to about 90%.¹³

Blood tests are best understood as one piece of the puzzle rather than a standalone answer. They are most useful when combined with imaging findings and clinical history to help decide whether a cyst needs surgery or can be safely monitored.

Ovarian Cysts in Children and Teenagers

Ovarian cysts in younger patients are overwhelmingly benign. A meta-analysis of pediatric ovarian masses found that non-neoplastic lesions (mostly simple cysts) were the most common, accounting for roughly 37% to 74% of cases. Among the neoplastic lesions, germ cell tumors were the most frequent type, with malignancy rates ranging from about 3.5% to 11% across studies.¹⁴ The most common neoplasm in this age group is the mature teratoma, also called a dermoid cyst, which is benign. The most common malignancy is a dysgerminoma, a type of germ cell tumor.¹⁵

An important study of girls aged 8 to 18 with ovarian cysts found zero malignancies among 409 patients who were followed to resolution, whether that resolution came through imaging or surgery.¹⁶ This underscores that the vast majority of ovarian cysts in young patients are functional or benign and can often be managed conservatively. The concern in pediatric care is less about cancer and more about ovarian torsion, where a large cyst causes the ovary to twist on itself, cutting off its blood supply, which is a surgical emergency.

Cysts Found During Pregnancy

Ovarian cysts turn up in about 5% of pregnancies, usually spotted incidentally during routine ultrasound.¹⁷ The vast majority of these are benign and resolve on their own. Corpus luteum cysts, which form after ovulation and help sustain the early pregnancy, are particularly common and almost always harmless.¹⁸

The challenge during pregnancy is that the diagnostic toolkit is limited. MRI can be used without radiation, but biopsy is generally avoided unless there is a strong suspicion of malignancy. CA-125 levels are naturally elevated during the first trimester, which makes blood tests less reliable. Most physicians take a watchful-waiting approach: if the cyst looks simple and stays small, it is observed. If it grows rapidly, develops complex features, or causes symptoms, surgery during the second trimester is considered the safest window. Ovarian cancer during pregnancy is rare, estimated at roughly 1 in 10,000 to 1 in 25,000 pregnancies in the broader literature.

When Endometriosis Is Involved

Endometriomas, sometimes called chocolate cysts, are ovarian cysts filled with old blood that form as part of endometriosis. These are benign, but endometriosis does carry a small risk of malignant transformation. A systematic review of 1,000 endometriosis cases found malignancy in about 5.5%, with ovarian endometriosis specifically associated with cancer in 5% of cases.¹⁹ The most common cancers linked to endometriosis were clear cell carcinoma and endometrioid carcinoma.

That 5% figure comes from a surgical and pathological case series, not from the general population of people living with endometriosis. Most women with endometriomas will never develop cancer. The risk factors that should prompt closer attention have been studied: women who developed cancer from endometriosis were significantly older (average age around 54 vs. 39 for those with benign endometriomas), had larger cysts (about 14 cm vs. 7.5 cm), and those cysts more often had solid components and multiple internal compartments.²⁰ A new solid nodule growing within a known endometrioma, or a rapid increase in size in a postmenopausal woman, would be red flags.

The Gray Area of Borderline Tumors

Between clearly benign and clearly malignant, there is a middle category: borderline ovarian tumors. These have some cellular features of cancer, like abnormal cell growth, but they do not invade surrounding tissue the way true cancers do. They account for a meaningful minority of ovarian tumors removed surgically, and they create diagnostic headaches.

Borderline tumors are notoriously difficult to diagnose before surgery. Studies have found that they are correctly classified preoperatively only about 29% to 69% of the time.²¹ Even during surgery, when a sample is sent for rapid frozen-section analysis, borderline tumors are the hardest category to get right. A meta-analysis of frozen-section studies found that the accuracy for borderline tumors was only about 69%, with under-diagnosis happening in roughly 20% of cases, meaning the frozen section called it benign but the final pathology showed it was borderline.²²

The good news is that borderline tumors generally have an excellent prognosis. Most can be treated with surgery alone, and recurrence is uncommon for early-stage disease. Younger women who want to preserve fertility can sometimes have just the affected ovary removed rather than undergoing a complete hysterectomy. The difficulty is not in treating them but in identifying them accurately before and during surgery.

BRCA Mutations and Occult Cancers

Women with inherited BRCA1 or BRCA2 mutations face a substantially elevated lifetime risk of ovarian cancer, which is why many choose risk-reducing surgery (removal of the ovaries and fallopian tubes) once they have finished having children. When pathologists closely examine the tissue removed during these preventive surgeries, they occasionally find microscopic cancers that had not yet been detected by any imaging or blood test.

A study of women undergoing risk-reducing surgery found that about 8% of those with BRCA1 mutations had microscopic cancers discovered at the time of the operation. None of the BRCA2 carriers in that cohort had occult cancers, though BRCA2 does still elevate ovarian cancer risk, just somewhat less than BRCA1.²³ These hidden cancers were frequently found in the fallopian tubes rather than the ovaries themselves, which has reshaped how scientists think about the origin of high-grade serous ovarian cancer. Much of what we call “ovarian” cancer may actually start in the fallopian tubes.

For the general population, these numbers do not apply. A cyst found on a routine ultrasound in a woman without a BRCA mutation or significant family history carries far lower risk. But for BRCA carriers, the existence of occult cancers in seemingly normal tissue is one of the strongest arguments for preventive surgery.

What Happens When a Cyst Is Removed Surgically

When a cyst is concerning enough to warrant surgery, the tissue is examined by a pathologist. During the operation, a frozen-section analysis can provide a rapid preliminary diagnosis in about 20 minutes, helping the surgeon decide how extensive the procedure needs to be. If the frozen section suggests cancer, the surgeon may proceed with a more complete staging operation in the same session.

Frozen sections are quite reliable for benign and malignant diagnoses. One study found 95% sensitivity and 100% specificity for benign tumors, and 90% sensitivity and 97% specificity for malignant tumors.²⁴ Another center reported 96% sensitivity for benign tumors and 89% for malignant ones, with an overall accuracy of 90%.²⁵ As noted earlier, the weak spot is borderline tumors, where frozen sections sometimes under-call the diagnosis. This means a small number of women learn after surgery that their tumor was more complex than initially thought, and they may need follow-up imaging or occasionally a second procedure.

Among ovarian tumors that reach the operating room, one surgical series found 75% were benign, about 18% were malignant, and about 7% were borderline.²⁴ Keep in mind that the cysts reaching surgery are a pre-selected group: they already looked suspicious enough on imaging or blood tests to justify an operation. The malignancy rate among all ovarian cysts, including the many that are simply monitored and resolve on their own, is far lower.

Why the Numbers You Find Online Vary So Widely

If you search this topic, you will encounter malignancy rates ranging from less than 1% to over 15%, depending on the source. The variation is not because anyone is wrong; it reflects different study populations and different definitions of what counts as a “cyst.” A study that only enrolls women whose cysts were surgically removed will report a much higher cancer rate than a study that counts every cyst found incidentally on imaging, because surgery selects for the masses that already looked worrying. A study of postmenopausal women will generally report higher rates than one of premenopausal women. And a study that lumps borderline tumors together with frank malignancies will report higher numbers than one that separates them.

The most useful way to think about this is in layers. At the broadest level, if you include every ovarian cyst that shows up on imaging in the general population, the cancer rate is well under 1%. Among cysts that look abnormal enough to be flagged for follow-up, the rate is roughly 1%. Among those that look complex or have solid parts, it rises to the low single digits. And among the subset that ends up in the operating room, roughly one in five turns out to be malignant or borderline. Your personal risk depends on which layer your cyst falls into, which is something your imaging findings, age, and clinical history help determine.