Mature cystic teratomas are found often in gonadal sites, but rarely in extragonadal sites, such as the retroperitoneum, mediastinum, central nerve system, lung, and liver. To our knowledge, only seven cases of pathologically benign mature cystic teratoma originating in the diaphragm have been reported. We report the case of a 45-year-old woman found to have a mature cystic teratoma below the left side of her diaphragm. She had undergone oophorectomy for a huge mature cystic teratoma in the left ovary 34 years earlier, followed 1 year later by excision of a tumor in the right side of the diaphragm. All sections of the pathological specimen showed evidence of a mature cystic teratoma and pathologically resembled the specimens obtained in the previous two operations. Although this tumor might have been a recurrence of the mature cystic teratoma in the left ovary, it is more likely that multicentric teratomatous foci metachronously developed in the gonadal and extragonadal sites.
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