**3. Pathogenesis of endometriosis-associated ovarian cancer; the role of iron overload-induced oxidative stress**

Endometrial cysts, or so-called chocolate cysts, are well-known lesions in endometriosis that contain fluid with an excess of free iron because of recurring hemorrhage in the cyst. It is interesting to note that Sampson mentioned in his first report of endometriosis-associated cancer that old hemorrhages should be considered additional evidence that meets his criteria (Sampson, 1925). Hemosiderin, heme, or iron deposition in endometriotic lesions have been assumed to trigger oxidative damage and chronic inflammation (Van Langendonckt, 2002a; Van Langendonckt, 2002b; Van Langendonckt, 2004; Toyokuni, 2009). In particular, iron storage in macrophages is significantly increased in patients with endometriosis; and intracellular iron activates the nuclear factor-κB pathway and exaggerates chronic inflammation (Lousse, 2009; Lousse, 2008). As a result, prominent oxidative stress, or an excess of reactive oxygen species, is consistently produced. This

direct prospective study of 20,686 Swedish patients hospitalized with endometriosis between 1969 and 1983 with a mean follow-up period of 11.4 years demonstrated a standardized incidence ratio (SIR) of 1.9 and a 95% confidence interval [CI] of 1.3 to 2.8 (Brinton, 1997). Similar results were reported in a case-control study analyzing patients from the United States, in which the relative risk for ovarian cancer development in endometriosis patients was 1.7 (Ness, 2000). A nationwide case-control study of Australian patients with ovarian cancer revealed that endometriosis increased the risks of both endometrioid adenocarcinoma and clear cell carcinoma, with odds ratios of 3.0 and 2.2, respectively (Nagle, 2008). A recent retrospective study from Canada also showed a significant increase in the relative risk (rate ratio [RR], 1.6; 95% CI, 1.12 to 2.09) of ovarian cancer in patients with endometriosis (Aris, 2010). In line with these reports, a recent prospective study from Japan showed a significant and much greater elevation in the relative risk (SIR, 8.95; CI, 4.12 to 115.3) of cancer development in Japanese patients with endometrioma, or endometrial cyst of the ovary (Kobayashi, 2007). The reason for this discrepancy is unclear, but one possibility is that the endometriosis patients in the Japanese study included only those with clinically detectable ovarian endometrial cysts. It is also important to note that Danazol (17-α-ethinltestosterone), a synthetic androgen that has been used to treat endometriosis, has been revealed to be an independent risk factor for the development of ovarian cancer. A negative correlation between oral contraceptive use and ovarian cancer, regardless of histologic type other than mucinous tumors, was recently shown by a collaboration of various groups worldwide (Cottreau, 2003). These factors may

In addition to an epidemiologic approach, the retrospective pathological analysis of samples from ovarian cancer patients is also useful to confirm the presence of endometriosis associated with ovarian cancers of various histological types. A comprehensive review of 2,807 ovarian cancer patients from 15 independent publications from western countries from the 1970s to 1990s, including 3 articles from Japan, revealed that endometriosis was incidentally found in 14.1% of ovarian cancer patients (39.2%, 21.2%, 3.3% and 3.0% of clear cell, endometrioid, serous and mucinous carcinoma patients, respectively), with a tendency toward a higher incidence of endometriosis in Japanese patients with clear cell carcinoma

**3. Pathogenesis of endometriosis-associated ovarian cancer; the role of iron** 

Endometrial cysts, or so-called chocolate cysts, are well-known lesions in endometriosis that contain fluid with an excess of free iron because of recurring hemorrhage in the cyst. It is interesting to note that Sampson mentioned in his first report of endometriosis-associated cancer that old hemorrhages should be considered additional evidence that meets his criteria (Sampson, 1925). Hemosiderin, heme, or iron deposition in endometriotic lesions have been assumed to trigger oxidative damage and chronic inflammation (Van Langendonckt, 2002a; Van Langendonckt, 2002b; Van Langendonckt, 2004; Toyokuni, 2009). In particular, iron storage in macrophages is significantly increased in patients with endometriosis; and intracellular iron activates the nuclear factor-κB pathway and exaggerates chronic inflammation (Lousse, 2009; Lousse, 2008). As a result, prominent oxidative stress, or an excess of reactive oxygen species, is consistently produced. This

also influence the relative risk of ovarian cancer development.

(Yoshikawa, 2000).

**overload-induced oxidative stress** 

process is thought to have a causative role in endometriosis development and progression, leading to carcinogenesis (Murphy, 1998; Ness & Cottreau, 1999; Ngo, 2009). Alternatively, the high concentration of free iron in endometrial cysts may directly provide oxidative stress that induces genomic mutation in epithelial cells (Yamaguchi, 2008), and whether the direct pathway or the indirect pathway involving macrophages has a major role in carcinogenesis remains to be resolved. Iron overload in experimental animals enhances epithelial cell proliferation (Defrere, 2006) and causes malignant tumors with genomic abnormalities (Hu, 2010), which suggests a similar mechanism leading to carcinogenesis in human endometriosis (Fig. 1). However, further studies are awaited to elucidate the precise role of iron-deposition induced oxidative stress in carcinogenesis of endometriosis-associated cancer.

Fig. 1. A proposed mechanism of carcinogenesis in endometriotic (chocolate) cysts
