To evaluate the features of ovarian mature cystic teratomas which are useful in diagnosing benign tumors. Mature cystic teratomas of the ovary are a common ovarian neoplasm, particularly in patients under the age of 45¹. The majority of these neoplasms are benign; only a small minority demonstrates malignant potential. This distinction is difficult to make on imaging alone.^2,3 Transmural growth and invasion into adjacent organs distinguishes malignant tumor from benign.^2,4,5 In the absence of such overt features, the presence of an enhancing solid component has been postulated as an indicator of malignancy. ^5-8 Given the high incidence of teratomas, determining benignity has important clinical implications with regards to follow up and surgical excision. For example, many gynecologic surgeons will not perform a minimally invasive operation on patients with suspected malignant teratoma; thus patients who desire laparoscopic surgery may instead undergo a large midline laparotomy, which has both cosmetic and morbidity implications. This study was HIPPA compliant and IRB approved. A systematic review was performed of our institution’s imaging and pathology archives from January 2008 through September 2015 for examinations in which key words “dermoid” and teratoma” appeared in the body or impression of the report. Inclusion criteria included the presence of suspected teratoma and either 2 year imaging follow up or surgical excision. Two readers blinded to the pathology and radiologic follow-up independently reviewed each case for the presence of a solid component and the following characteristics: presence of enhancement, type of enhancement (peripheral or solid), angle between the solid component and cyst wall, border of the solid component (smooth or irregular), border of the cyst (smooth or irregular), presence of intravoxel or bulk fat, and presence of restricted diffusion. The presence of ascites and metastasis was also evaluated. Statistical analysis was performed utilizing a Student's t-test.66 cases were included in the study. 59 were benign, and 7 of the tumors were malignant Final pathology of malignant lesions were: malignant degeneration into squamous cell carcinoma (2), immature teratoma (3), and collision tumors (2). There was very good to excellent inter-reader agreement (kappa values 0.8-0.9 for all variables). A solid component was seen in all malignant nodules (p<0.001), however solid component was also seen in 43 of 59 benign nodules. There was no significant difference in the presence of enhancement of the solid component between benign and malignant tumors (p=0.03). However, peripheral enhancement was seen only in benign nodules (p<0.0001) and in no malignant nodules. Confinement of the nodule to the cyst wall (p=0.002), presence of intravoxel or bulk fat in the solid nodule (p<0.001), and a smooth cystic border (p=0.03) were all significantly correlated with benignity. An obtuse angle of the nodule and irregular margin were both statistically significant indicators of malignancy (p=0.003 and 0<0.001, respectively). Tumor size was larger in malignancy (range 1.2-25.6 cm for benign dermoids; range 4.8-29.6 cm for malignant). Diffusion restriction in the nodule did not correlate with malignancy (p=0.1), nor did a unilocular versus multilocular configuration (p= 0.4). Physiologic ascites was seen in most cases, therefore presence of ascites also did not correlate with malignancy. No distant metastases were seen in any cases. Mature cystic teratomas are a common benign tumor. Increased imaging of these benign lesions with sensitive imaging modalities such as MRI has led to increased sensitivity in detecting subtle enhancement within the Rokitansky nodule. This finding has previously been described as worrisome for malignancy, therefore the detection of enhancement within young women with an otherwise benign-appearing mature cystic teratoma has led to uncertainty about the possibility of these lesions harboring malignancy. Reports questioning the presence of malignancy cause the referring gynecologic surgeon to change their operative technique from a minimally invasive technique to an open laparotomy. This study is the first in the literature to describe the incidence of, and type of, solid nodular enhancement within benign mature cystic teratomas. Fat containing ovarian lesions with an internal nodule demonstrating peripheral enhancement and internal fat, acute angles and no extension beyond the cystic wall, can be confidently diagnosed as having a benign nodule and thus compatible with benign mature cystic teratoma. We theorize that this peripheral enhancement within the nodule reflects reactive hyperemia of the margin of the Rokitansky nodule from adjacent mild inflammatory changes in the mature cystic teratoma. Enhancement of solid Rokinasky nodules can be seen commonly in benign mature cystic teratomas. In particular, peripheral enhancement of a smoothly marginated nodule which is confined to the cyst wall in a fat containing lesion should not raise concern for malignancy.