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Ovarian dermoid cyst with malignant transformation: The patient has an ovarian dermoid cyst, or benign cystic teratoma, with malignant transformation. The CT scan shows a complex mass in the region of the left adnexa with fatty tissues, a soft-tissue element, and localized ascites. During surgery, a teratodermoid cyst with malignant transformation was resected. Ovarian dermoid cyst is the most frequently diagnosed ovarian tumor. About 80% of the cases occur in women aged 20-30 years, and 15% are bilateral. The tumors are typically large (10-15 cm in diameter) at presentation. A dermoid cyst is derived from germinal cells and therefore can be composed of tissues arising from all 3 germinal layers. On histologic examination, 50% of cysts have lipid substance, hair, sebaceous secretions, hair follicles, and eggshell calcifications, and 30% have formed elements such as teeth and fragments of bone. Malignant transformation is reported in 1-2% of cases and usually originates from squamous epithelial cells. Malignant transformation should be suspected if the size of the tumor is >10 cm. The cysts are often asymptomatic and discovered only as incidental findings on pelvic sonography performed for other reasons. However, dull pressure and pain may occur as the tumor grows and puts pressure on adjacent structures. In addition, 15% of cases are associated with menstrual abnormalities. Acute abdominal and pelvic pain may be associated with ovarian torsion, hemorrhage from around or inside the tumor, and, in rare cases, rupture of the cyst. Plain radiography of the abdomen is sometimes helpful in detecting a dermoid cyst if calcifications or if the fat-floating sign is present. This sign is a horizontal line between 2 soft tissues of different opacities caused by oily and sebaceous fluid floating over serous and intracystic debris. Initial assessment of a pelvic mass usually involves ultrasonography, which is used to determine the nature and consistency (cystic or solid) of the tumor and to identify ascites. If the tumors are large, their origin may be difficult to identify on sonograms. Sonographic examination of a dermoid tumor reveals a complex appearance, as hyperechoic material (due to hair, teeth, and fat) and hypoechoic areas (due to fluid) are common. Similar to the fat-floating sign on plain radiography, the fat-fluid level can be seen on ultrasonography and CT scanning. Cross-sectional CT or MRI studies may be helpful in the setting of equivocal ultrasound findings. They may also be used for staging tumors with malignant transformation. The criterion standard for diagnosis is laparoscopy with resection of the tumor and histologic examination. Alpha-fetoprotein (AFP) and human chorionic gonadotropin (HCG) levels may be elevated in patients with teratomas and germ cell tumors and are useful in preoperative tissue typing and in monitoring treatment. Imaging of the lungs with plain radiography or CT may be useful if metastases are suspected.