I read with interest the recent case report by Drs Dutt and Berney “Clear cell carcinoma of the ovary arising in a mucinous cystadenoma”.¹ As these authors state, ovarian clear cell carcinomas often arise in endometriosis with a quoted incidence of associated pelvic endometriosis in 50–70% of cases and an incidence of endometriosis in the same ovary of 25%.² This is undoubtedly a gross underestimate of the association between ovarian endometriosis and clear cell carcinoma because in many cases the tumour will overgrow and completely obliterate the endometriosis. However, my personal experience is that when ovarian clear cell carcinomas are extensively sampled (with special reference to cystic areas) the frequency of endometriosis in these neoplasms is substantially higher and indeed the origin of the tumour in an endometriotic cyst can be identified in most cases.

Drs Dutt and Berney might consider the possibility that the pre-existing cystic areas in their case, in fact, represent an ovarian endometriotic cyst with mucinous metaplasia. Figure 1 looks like the picture often seen in ovarian endometriotic cysts with mucinous metaplasia and fig 2 (right hand side) looks like the atypical changes sometimes seen in such cysts. The single cell lining of clear cells (fig 2, left hand side) is often seen in clear cell carcinomas arising in endometriotic cysts. Mucinous metaplasia can be extensive in ovarian endometriotic cysts³ and, in such instances, a diagnosis of mucinous cystadenoma may be considered. In addition, borderline mucinous tumours may arise in endometriotic cysts.^{3, 4} Within the ovary a definitive diagnosis of endometriosis (especially an endometriotic cyst) is often difficult because there may be secondary changes in both the glandular and stromal elements. In particular, in endometriotic cysts, typical endometrial type stroma is often sparse or absent altogether and instead the stroma usually has a fibrous appearance.

There is evidence in the literature that many ovarian endometriotic cysts are, in fact, benign neoplasms and are not related to usual pelvic/abdominal endometriosis. Ovarian endometriotic cysts are often solitary and not associated with generalised pelvic/abdominal endometriosis. Studies, using X linked polymorphic markers, have demonstrated that ovarian endometriotic cysts may be monoclonal,^{5, 6} supporting the hypothesis of a benign neoplasm, and DNA aneuploidy has been found in atypical areas.⁷ Because benign and borderline serous and mucinous cystadenomas are common but the corresponding endometrioid neoplasms are rare, it may be that endometriotic cysts with and without atypia correspond to benign and borderline endometrioid cystadenomas, respectively. This would provide an explanation for the coexistence of ovarian endometrioid and clear cell carcinomas with endometriosis.

The authors reply

We are grateful to Dr McCluggage for his interest in our paper. We would like to reiterate our report that in this case there were no identifiable endometriotic components: the tumour appeared to be a classic benign mucinous cystadenoma.

He suggests that the mucinous areas might represent an endometriotic cyst with complete mucinous metaplasia. After extensive sampling (as reported), we found no areas of endometriosis. In fact, as we stated, there was a mucin filled multicystic area lined by typical picket fence mucinous cells. This area comprised one third of the total tumour, which was 24 cm in maximum diameter. This seems incompatible with an endometriotic origin. The benign mucinous areas in the tumour are illustrated (fig 1).

We entirely agree that clear cells forming a single cell lining are common in clear cell carcinomas arising in endometriotic cysts, but this represents simply a change to clear cell morphology, and cannot be taken to impute the derivation of the benign lesion.

Dr McCluggage suggests that endometriotic cysts can show a lack of endometrioid stroma or that they can be “fibrous”. If a multicystic endometriotic cyst has global mucinous metaplasia and a complete lack of endometrioid stroma, then we can only say that in our eyes this would be taken to be a mucinous cystadenoma.

To suggest that a clear cell tumour always arises from an endometriotic cyst seems to be too didactic a viewpoint. Our case demonstrated that other pathogeneses may give rise to clear cell carcinoma.

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Figure 1

Multicystic area of tumour with benign mucinous epithelium, mucin within cysts, and an absence of endometrial stroma.