Endometriosis, which affects 1 in 10 women, is one of the most common conditions that gynecologists treat. It is known to cause pain, pelvic adhesive disease, endometriotic cyst formation, and infertility. However, even more sinister, it also increases a woman’s risk for the development of epithelial ovarian cancer (known as endometriosis-associated ovarian cancer or EAOC). A woman with endometriosis has a two- to threefold increased risk of developing epithelial ovarian cancer, compared with nonaffected women.1 This risk appears to be concentrated in the premenopausal age group, particularly the fifth decade of life. After menopause their risk of developing cancer returns to a baseline level.
Dr. Emma C. Rossi
EAOC classically presents as clear cell or endometrioid adenocarcinomas, rather than high-grade serous carcinomas. However, low-grade serous carcinomas are also frequently observed in this cohort.2,3 Unlike high-grade serous carcinoma, EAOC is more likely to be diagnosed at an early stage, with the majority at stage I or II, and prognosis is better. After matching for age and stage with cases of high-grade serous carcinoma, there is improved disease-free and overall survival observed among cases of EAOC of clear cell and endometrioid histologic cell types.4 The phenomenon of dual primaries (synchronous endometrial and ovarian cancer) occurs more frequently in EAOC than it does in patients with nonendometriosis-related high-grade serous cancer (25% vs. 4%).
The genomics of these endometriosis-associated cancers are quite distinct. Similar to benign endometriosis implants, EAOC is associated with genomic mutations in ARID1A, PIK3CA, and PTEN, as well as progesterone resistance.1,2 Multiple studies have shown that the adjacent eutopic endometrium carries similar gene mutations as those found in both benign endometriotic implants and EAOC.2 This may explain the higher incidence (twofold) of endometrial cancer in patients with endometriosis as well as the increased incidence of dual ovarian and endometrial cancer primaries.
Just as there are multiple theories regarding the mechanism of benign endometriosis, we have theories rather than conclusions regarding the origins of EAOC. One such theory is that it develops from malignant transformation in an existing endometriotic cyst.5 Endometriotic cysts provide an iron-rich environment which promotes reactive oxygen species that promote carcinogenesis by inducing gene mutations and epigenetic alterations. However, if prolonged exposure to oxidative stress within endometriotic cysts were to be the cause for EAOC, we would expect to see a progressively increasing incidence of ovarian cancer over time in patients with expectantly managed cysts. However, in cases of expectant management, an initial, early, increased risk for cancer within the first 5 years is followed by a subsequent decreasing incidence over time.6 This early incidence spike suggests that some endometriotic cysts may have been misclassified as benign, then rapidly declare themselves as malignant during the observation period rather than a transformation into malignancy from a benign endometrioma over time.
An alternative, and favored, theory for the origins of EAOC are that endometrial cells with carcinogenic genomic alterations reflux through the fallopian tubes during menstruation and settle onto the ovarian epithelium which itself is damaged from recent ovulation thus providing an environment that is highly suitable for oncogenesis.2 Genomic analyses of both the eutopic endometrium and malignant cells in patients with EAOC have shown that both tissues contain the same genomic alterations.1 Given that menstruation, including retrograde menstruation, ends after menopause, this mechanism supports the observation that EAOC is predominantly a malignancy of premenopausal women. Additionally, salpingectomy and hysterectomy confers a protective effect on the development of EAOC, theoretically by preventing the retrograde transfer of these mutant progenitor endometrial cells. Furthermore, the factors that increase the number of menstrual cycles (such as an early age of menarche and delayed or nonchildbearing states) increases the risk for EAOC and factors that inhibit menstruation, such as oral contraceptive pill use, appear to decrease its risk.
EAOC most commonly arises in the ovary, and not in the deep endometriosis implants of adjacent pelvic structures (such as the anterior and posterior cul de sac and pelvic peritoneum). It is suggested that the ovary itself provides a uniquely favorable environment for carcinogenesis. As stated above, it is hypothesized that refluxed endometrial cells, carrying important progenitor mutations, may become trapped in the tissues of traumatized ovarian epithelium, ripe with inflammatory changes, post ovulation.2 This microenvironment may promote the development of malignancy.
Given these theories and their supporting evidence, how can we attempt to reduce the incidence of this cancer for our patients with endometriosis? Despite their increased risk for ovarian and endometrial cancers, current recommendations do not support routine cancer screening in women with endometriosis.7 However, risk-mitigation strategies can still be pursued. Hormonal contraceptives to decrease ovulation and menstrual cycling are protective against ovarian cancer and are also helpful in mitigating the symptoms of endometriosis. While removal of endometriotic cysts may not, in and of itself, be a strategy to prevent EAOC, it is still generally recommended because these cysts are commonly a source of pain and infertility. While they do not appear to undergo malignant transformation, it can be difficult to definitively rule out an early ovarian cancer in these complex ovarian cysts, particularly as they are often associated with tumor marker abnormalities such as elevations in CA 125. Therefore, if surgical excision of an endometriotic cyst is not performed, it should be closely followed for at least 5 years to ensure it is a benign structure. If surgery is pursued and ovarian preservation is desired, removal of the fallopian tubes and uterus can help mitigate the risk for EAOC.8
Endometriosis is a morbid condition for many young women. In addition to causing pain and infertility it increases a woman’s risk for ovarian and endometrial cancer, particularly ovarian clear cell, endometrioid, and low-grade serous cancers and synchronous endometrial and ovarian cancers. Endometriotic cysts should be removed or closely monitored, and clinicians should discuss treatment options that minimize frequency of ovulation and menstruation events as a preventative strategy.
Dr. Rossi is assistant professor in the division of gynecologic oncology at the University of North Carolina at Chapel Hill.
References
1. Endocrinology. 2019;160(3):626-38.
3. Lancet Oncol. 2012;13:385-94.
4. Gynecol Oncol. 2014;132(3):760-6.
6. Int. J Clin Oncol. 2020;25:51-8.