Primary retroperitoneal mucinous cystic tumor with borderline malignancy, microinvasion, and KRAS mutation: A case report

Introduction: Primary retroperitoneal mucinous cystic tumors with borderline malignancy are rare neoplasms of uncertain histopathogenesis with few cases reported in the literature. Histologically, these tumors are an analogous counterpart to the more common ovarian mucinous borderline tumor. A microinvasive component of these neoplasms may contribute to higher probabilities of recurrence and poor prognosis, while the presence of various mutations, particularly in KRAS and GNAS exons, could have therapeutic relevance in the future. Case Report: Imaging studies of a 42-year-old female with abdominal pain and distension revealed a large abdominopelvic cystic tumor. This tumor was initially misdiagnosed as adnexal. Gynecologic surgery showed the tumor to depend on the retroperitoneum. The patient had intact gynecologic organs. Further, a macroscopic study revealed a cystic cavity filled with serous liquid, with dominant papillary excrescence, and multiple microscopic papillae distributed along the inner lining. Moreover, a histologic analysis showed a proliferation of more than 10% of cylindrical mucosecretory epithelium, with dysplasia and focal infiltration to the stroma. Additionally, the immunohistochemistry revealed diffuse epithelial positivity for CK7 and focal CK20 positivity. Superior and inferior gastrointestinal endoscopies were done, where the possibility of a neoplastic origin in here was ruled out. Next-generation sequencing showed a missense mutation in exon 2 of KRAS. Conclusion: While the histopathogenesis of primary retroperitoneal mucinous tumors remains uncertain, the overall behavior and prognosis is remarkably similar to their better-known mucinous counterparts in the ovary. The possibility of targeted treatments in cases with malignancy, recurrence, and metastases also remains an important topic in molecular oncology, highlighting the need for further molecular profiling.