Background: Mature cystic teratoma is the most common benign germ cell ovarian tumor. Malignant transformation is
uncommon. The pathology is mostly composed of squamous cell carcinoma. Mucinous cystadenocarcinoma should be
differentiated between malignant transformation and the coincidental occurrence.
Case Report: A case of an early stage mucinous ovarian cancer co-existing with mature cystic teratoma at the same ovarian side was reported. A 57-year-old woman presented with incidental palpable pelvic mass for two weeks. Right ovary consisted of multiloculated cyst and mature cystic teratoma. The patient underwent an exploratory laparotomy for a large ovarian cyst evaluation. Mucinous ovarian cancer was diagnosed as FIGO stage IC3 after operation. Histopathology report showed mucinous cystadenocarcinoma and mature cystic teratoma of the right ovary. There was no additional abdominal abnormality.
Immunohistochemistry staining supported the diagnosis of metastatic adenocarcinoma of colon or intestinal type of mucinous ovarian cancer. Further investigation for locating other primary cancer site was then performed. The result was negative. The intestinal-type mucinous ovarian cancer co-existing with mature cystic teratoma of the right ovary was the final histopathological reading. The patient was then started on carboplatin/paclitaxel combination chemotherapy for 6 cycles after surgery. The patient showed complete remission at the end of the chemotherapy treatment.
Conclusion: This was a rare case of mucinous ovarian cancer co-existing with mature cystic teratoma. Clinical acumen, immunochemistry staining and metastatic survey investigation played important roles for the final diagnosis.

Keywords: Ovarian cancer, Mucinous, Dermoid