ENDOMETRIOSIS AND RISK OF OVARIAN CANCER- Can endometriosis predispose to ovarian cancer?

Endometriosis is a common gynecological disorder.  It affects up to 10% of women in the reproductive age.  The common presentations of endometriosis are menstrual or pelvic pain, ovarian cyst and difficulty to conceive.  There has been report of malignant (cancerous) transformation of endometriosis.

Ovarian cancer is the one of the deadliest gynecology cancer as most women present at later stage of the disease.  The current treatment includes pelvic clearance, removing as much tumor as possible while the residue tissues are treated with chemotherapy.  However, the 5 years survival rate for late stage ovarian cancer remains low.  This creates anxiety and concerns amongst health professionals as well as women suffering from endometriosis.


Endometriosis, a precursor to ovarian cancer

There are strong evidences to support that endometriosis predisposed women to the risk of developing ovarian cancer (Nezhat 2014).  The endometriosis tissue exhibits ability to ‘travel’ (spread) to distant organ.  It does not confine itself to the female reproductive organ but can metastasize to other organs in the body.  It attaches and invades the target organ and subsequent lead to end organ damage. The endometriosis itself manifests recurrence, uncontrolled cell proliferations and growth driven by estrogen hormone.

Recently, based on molecular genetic studies, ovarian cancer can be classified into two categories, namely types I and II.  The type I tumors develop gradually, less aggressive and usually confined to the ovary at diagnosis.  They comprise approximately 25% of all ovarian cancer and have better prognosis.  These type II ovarian cancers are endometrioid, clear cells and low- grade serous subtypes.

The type II tumors are more aggressive, rapid growing and demonstrate high- grade malignancy.  There is no known precursor lesions associated with type II ovarian cancers.

They seems to arise from the fimbrial epithelium and highly unstable chromosomally (Kurman 2011).


How high is the risk?

The exact likelihood for a malignant transformation in women with endometriosis is unknown.  It has been found that there is a 2-3 folds risk of women with endometriosis to develop a clear cell or endometrioid ovarian cancers, compared to women without endometriosis.  These tumors are from the Type I category.  The estimated relative risk (RR) are 3.59 and 1.98 respectively (Guo 2015).

With the lifetime risk of ovarian cancer estimated at 1.4% and approximately 25% of all ovarian cancers are Type I origin (clear cell and endometrioid subtypes), the lifetime risk would be 0.34% in the general population. For women with endometriosis, the odds ratio of developing ovarian cancer is about 1.5 compared to the general female population.  An odds ratio of 1.5 means that a woman with endometriosis has a life-time risk of developing an ovarian cancer of about 1.5% instead of 1% (Pearce 2012).

The risk factors associate with malignant transformation include, women with long- standing history of endometriosis, endometriosis diagnosed at an early age, endometriosis associated with infertility, ovarian endometrioma and atypical endometriosis.


Final message

Endometriosis, a seemingly benign gynecological condition has shown to be associated with ovarian cancer.  However, most women with endometriosis do not develop ovarian cancer as the risk of malignant transformation remains low.  The manifest tumors are also less aggressive.



Odd s ratio is the odds that an outcome (ovarian cancer) will occur given a particular exposure (endometriosis), compared to the odds of the outcome occurring in the absence of that exposure (without endometriosis).

Relative risk describes the probability of developing a disease (ovarian cancer) in the exposed (endometriosis) to the non- exposed group (non-endometriosis)



  1. Pearce CL et al. Lancet Oncology 2011.
  2. Guo SW et al., Fert Steril 2015
  3. Kurman RJ et al., Hum Pathol 2011
  4. Nezhat FR et al., Int J Gynecol Cancer 2014
  5. Nezhat F et al., Fert Steril 2014

Author: Dr Ng Soon Pheng

Dr. Ng Soon Pheng MD(USM), M.Med (O&G), Fellow Reprod Medicine (Singapore), AM (Mal) Dr. Ng Soon Pheng qualified from University Kebangsaan Malaysia (UKM), Malaysia. After obtaining his post graduate degree, Dr. Ng Soon Pheng continued his clinical fellowship training in the field of IVF in Singapore. He was also the recipient of Yayasan Sultan Iskandar (Johor) Scholarship for the fellowship stint. Dr. Ng Soon Pheng has more than 10 years of working experience in the field of Obstetrics and Gynaecology in the public and university hospital, with special interests in infertility. He was an Associate Professor with the Department of O&G in Universiti Kebangsaan Malaysia (UKM) specialising in infertility. Dr. Ng Soon Pheng is currently Consultant Obstetrician and Gynaecologist with special interests in infertility at Columbia Asia Hospital - Puchong.