A rare case of a primary retroperitoneal mucinous cystic tumour with borderline malignancy and literature review.

Primary retroperitoneal mucinous cystic neoplasms (PRMCN) with borderline malignancy are exceptionally rare tumours with lack of pathognomonic clinical and imaging-specific features. Here, we report a case of PRMCN with borderline malignancy in a 62-year-old woman who presented with abdominal pain. Imaging studies revealed a well-defined cystic mass on the right flank in close relation with the cecum and caecal appendix, without other findings suggestive of malignancy. A possible diagnosis of an ovarian epithelial tumour was ruled out intraoperatively. After surgical excision, microscopic examination allowed the final diagnosis. As there is no evidence of disease during follow-up, complete tumour resection without cystic rupture appears to be the best therapeutic option. Thus, although rare, this tumour should be considered when imaging findings suggest an ovarian mucinous neoplasm in women with normal ovaries. An international registry for rare tumours and longer follow-ups may contribute for more consistent approach for managing these patients.