Introduction: Primary appendiceal tumors are rare, as they account for only 0.5 to 1% of all appendiceal biopsy specimens. The mucinous type of adenocarcinoma has an incidence of only 10%. This type can have different manifestations, of which an exceptionally small fraction manifests as an ovarian tumor. Thus, we report a rare case of primary appendiceal mucinous adenocarcinoma in a 49-year-old woman, presenting clinically with an ovarian mass.

Case Presentation: The patient manifested with left sided abdominal pain and progressive abdominal distention, initially suspected to be due to an ovarian pathology. She underwent exploratory laparotomy with ovarian mass resection, total abdominal hysterectomy with salpingo-oophorectomy, and appendectomy. The mass was then identified to be a primary appendiceal mucinous adenocarcinoma by histopathologic examination and immunohistochemistry studies. The patient was then referred to medical oncology for further management. She underwent eight cycles of palliative chemotherapy using capecitabine and oxaliplatin. Clinically, the patient had significant improvement with improved appetite, weight gain, and no recurrence of abdominal distention. Mid-cycle evaluation scan showed stable disease as the calcified nodule on the left lung did not increase in size. The pelvic cavity was also negative for any lymphadenopathies. There was a decrease in the perihepatic and perisplenic ascites. However, the post-treatment scan showed a notable radiographic progression of the carcinomatosis in the peri-gastric and rectovesical space. The patient is currently undergoing regular surveillance and upon progression, she was apprised for second line palliative chemotherapy.

Conclusion: This case highlights a rare case of a primary mucinous adenocarcinoma of the appendix, highlighting the importance of considering this as a differential when presented with an adnexal mass. Determining this, with the use of combined immunohistochemistry markers would ascertain the primary origin of the tumor aiding in the appropriate treatment approach moving forward.

Keywords: Appendiceal tumors, Case Report, Immunohistochemistry markers, Mucinous adenocarcinoma, Ovarian mass

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