Squamous Cell Carcinoma Originating From Mature Cystic Teratoma of the Ovary Diagnosed 10 Years After Initial Tumor Detection: A Case Report

Malignant transformation is a rare complication of ovarian mature cystic teratoma that occurs in 1-3% of cases. We herein report a case of squamous cell carcinoma originating from mature cystic teratoma of the ovary diagnosed 10 years after initial tumor detection. A 69-year-old woman presented to the Department of Internal Medicine with a seven-month history of abdominal fullness. Magnetic resonance imaging performed at another Department of Internal Medicine 10 years ago revealed a left ovarian teratoma measuring 8 cm; however, she refused referral to the Department of Gynecology. The current plane magnetic resonance imaging showed a large pelvic-abdominal cystic tumor measuring 17 cm, consisting of fat, fluid-fluid level, and a hair ball. Although the anterior wall of the cyst was slightly thick, the tumor had no solid projection into the cyst. She was referred to our Department of Gynecology. Contrast-enhanced abdominal computed tomography showed the lack of an enhanced solid component in the tumor. Laparotomy presented a large cystic tumor originating from the left ovary. A left salpingo-oophorectomy was performed without the intraperitoneal spillage of cyst fluid. Macroscopically, the left ovary was a unilocular cyst containing hair and sebaceous yellow fluid. A histological examination revealed that part of the thickened cyst wall was lined by squamous cell carcinoma <i>in situ</i>. Although the area of carcinoma <i>in situ</i> was dominant, nests of squamous cell carcinoma partially invaded the stroma. We diagnosed this tumor as squamous cell carcinoma originating from mature cystic teratoma of the left ovary. We speculate that the reason for the lack of enhancing solid components in the present case may be the abundance of squamous cell carcinoma <i>in situ</i>. The time required for mature cystic teratoma to become malignant remains unclear; therefore, the further accumulation of cases is needed to elucidate the mechanisms underlying the malignant transformation of mature cystic teratoma of the ovary.