| Summary: | Abstract Background Tumor-to-tumor metastasis (TTM) is a rare but well-documented phenomenon that is defined as metastasis in a histologically distinct tumor. Ovarian mature teratomas (OMTs) can coexist with various cancers by malignant transformation, which may make it difficult to distinguish these from TTM. Herein, we report a case of TTM from appendiceal adenocarcinoma to the OMT, mimicking the malignant transformation of OMT. Case presentation A 67-year-old Japanese woman underwent abdominal total hysterectomy and bilateral salpingo-oophorectomy for an ovarian tumor in another hospital. She was initially diagnosed with mucinous carcinoma/carcinoid arising in the OMT. One year after surgery, she was referred to our hospital after the presentation of increased appendiceal mass. Cecal biopsy targeting an appendiceal tumor revealed scattered mucinous cells with signet ring features, which were morphologically similar to the malignant components in the previously diagnosed right OMT. Both the appendiceal adenocarcinoma and malignant components of the OMT stained positive for CK7, CK20, CDX-2, and SATB2 but negative for estrogen receptor, progesterone receptor, and pax-8. Finally, we confirmed the diagnosis of appendiceal goblet cell carcinoid metastasizing to the right OMT. The patient had tumor-bearing survival due to systemic chemotherapy administered for 35 months after the initial surgery. Conclusions Awareness of the TTM phenomenon is important to avoid an incorrect diagnosis and to select the appropriate therapy when unusual malignancy is encountered in the OMTs.
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