Synchronous Mucinous Borderline Tumor of the Ovary and Low-Grade Appendiceal Mucinous Neoplasm

We present a rare case of synchronous mucinous borderline tumor of the ovary and low-grade appendiceal mucinous neoplasm (LAMN). We performed a left adnexectomy to diagnose the left ovarian borderline malignancy and an ileostomy because of the swollen appendix during the operation. It was diagnosed as left ovarian mucinous borderline malignancy and LAMN. LAMN causes peritoneal dissemination, ovarian metastasis, and peritoneal pseudomyxoma. The appendix and ovary are close to each other anatomically and can metastasize if there is a tumor in either. For ovarian mucinous tumors, it is necessary to search the gastrointestinal tract, especially the appendix, as the primary lesion. For appendix tumors, it is necessary to search for the ovary. Since LAMN may be associated with borderline ovarian malignancies, as in this case, there is a possibility of the duplication of tumors when searching for ovarian mucinous tumors as the primary tumor and if ovarian tumors are found. Since LAMN and mucinous ovarian tumors have similar histological features, immunohistochemical staining is useful for their differentiation because they show different immunostaining patterns.