Endometriosis is a common, chronic condition where tissue similar to the lining inside the uterus, called the endometrium, grows in other areas of the body, most frequently the pelvic cavity. This misplaced tissue still responds to hormonal cycles, causing it to bleed and inflame surrounding organs, which often leads to severe pain, scarring, and fertility problems. Endometriosis is not a cancer; it is classified as a benign, non-cancerous disorder. While the two conditions are fundamentally different, there is a complex and low-level association between long-standing endometriosis and the risk of developing certain rare cancers.
Understanding the Difference Between Endometriosis and Cancer
The distinction between a benign condition like endometriosis and a malignant disease like cancer rests entirely on the behavior of the cells involved. Endometriosis is characterized by localized growth and is non-metastatic, meaning the misplaced tissue does not invade distant, unrelated organs. Although endometriotic lesions can be locally invasive within the pelvic cavity, causing significant tissue damage and forming deep adhesions, the cells themselves do not possess the uncontrolled, destructive growth pattern of malignancy.
In contrast, cancer is defined by its ability to grow uncontrollably, invade surrounding tissues, and spread to distant sites through a process called metastasis. Cancer cells acquire mutations that allow them to ignore normal growth-regulating signals and resist programmed cell death, or apoptosis. While endometriosis tissue shares some features with cancer, such as the ability to grow new blood vessels and invade local tissues, it generally retains a normal cellular structure and remains hormone-responsive. The tissue in endometriosis is considered benign because it lacks the capacity for systemic, life-threatening metastasis that characterizes true malignancy.
The Specific Link to Ovarian and Endometrial Cancers
While endometriosis itself is not cancer, it is recognized as a risk factor for a rare event known as malignant transformation, primarily affecting the ovaries. The absolute lifetime risk of an endometriotic lesion turning cancerous is estimated to be low, affecting less than 1% to 2.5% of people with the condition. This transformation is most strongly linked to specific, less common subtypes of ovarian cancer, namely clear cell carcinoma and endometrioid ovarian cancer (EOAC).
These cancers are often referred to as endometriosis-associated ovarian cancers because they are believed to arise directly from the endometriotic tissue, particularly ovarian endometriomas. Studies suggest that people with endometriosis have a two to four times greater risk of developing these specific ovarian malignancies compared to the general population. The risk may be even higher—up to nine times greater—for those with more severe forms, such as ovarian endometriomas or deep infiltrating endometriosis.
The proposed mechanism involves chronic inflammation, oxidative stress, and genetic changes within the endometriotic cells over a long period, eventually leading to atypical endometriosis that can become cancerous. Overall, the vast majority of people with endometriosis will never develop cancer, and the link is primarily to the rare clear cell and endometrioid types of ovarian cancer.
Identifying Symptoms That Require Immediate Medical Review
People with endometriosis should be aware of changes in their symptoms that may signal a rare complication requiring immediate medical review. A sudden, rapid worsening of pelvic pain that is unresponsive to typical pain management or hormonal treatments warrants prompt investigation. This is particularly concerning if the pain is new, constant, and localized to one area, especially one where a large endometrioma is known to exist.
Any unexplained, noticeable abdominal swelling or the new onset of fluid accumulation in the abdomen, known as ascites, should be evaluated urgently. Other signs that are not typical of a regular endometriosis flare include:
- Unexplained, significant weight loss.
- New or severe fatigue that does not improve with rest.
- Persistent changes in bowel or bladder habits.
- An ovarian cyst, such as an endometrioma, that is rapidly increasing in size.
These changes do not automatically mean cancer, but they are important indicators that a rare transformation or another serious medical condition needs to be ruled out by a specialist.