Histopathological challenges in assessing borderline ovarian tumours

The diagnosis of borderline ovarian tumours is problematic. Traditionally, the absence of stromal invasion has distinguished borderline ovarian tumours from their malignant counterparts. The recent recognition of microinvasion associated with borderline neoplasms, the analysis of peritoneal implants (PIs), and issues associated with tumour nomenclature contribute to this diagnositc challenge. Furthermore, a proposed reclassification of serous ovarian tumours abandoning the borderline category in favour of atypical proliferative serous tumour (APST) and micropapillary serous carcinoma (MPSC); the latter being subdivided into invasive (invasive MPSC or low-grade serous carcinoma) and non-invasive (non-invasive MPSC or intraepithelial low-grade serous carcinoma) variants, has resulted in the inconsistent use of tumour nomenclature. To facilitate understanding, both the old and new terminologies of serous tumours will be used in this review. Unfortunately, diagnostic dilemmas are not restricted to serous ovarian tumours, in mucinous ovarian tumours, benign, borderline and malignant epithelium can co-exist in the same lesion and metastatic mucinous carcinoma from the gastrointestinal tract can mimic a primary mucinous ovarian tumour. Pseudomyxoma peritonei, which was originally considered as the peritoneal lesion (or implant) associated with a borderline mucinous ovarian tumour, is now believed to be secondary to a low-grade primary mucinous tumour of the appendix. Finally, accurate and complete histological assessment requires the pathologist to be aware of newly described lesions e.g. the seromucinous tumours. In this article, the difficulties associated with the histological diagnosis of the above tumours will be considered with emphasis on the identification of early invasion.