CCCs usually present as a large cystic mass, and they rarely occur in bilateral. CCC tends to present in early FIGO stages, with 35–60 % in stage I disease and 9–22 % in stage II. However, stage I CCC is more likely to be stage IC, probably because of higher risk of tumor rupture [6]. Their tendency of early clinical stage at presentation is explained by that CCC usually arises from preexisting endometriomas. However, clear cell carcinoma in early stages had a poorer outcome or a tendency toward poor outcome compared with other ovarian carcinomas [7, 8].

The gross appearances of CCC are characterized by multiple mural nodules within a thick-walled unilocular cyst, which represent endometriomas (Fig. 2) [4]. On MR imaging, the content of the cysts typically shows high intensity on T1-weighted images, as usually seen in endometriomas. However, the signal intensity on T2-weighted images is usually high, rather than low intensity commonly seen in endometriomas [9]. The neoplastic nodule in CCC can show variable size, ranging from tiny nodule to large mass that is about to fulfill the entire cystic cavity. On MR imaging, they usually show low intensity on T1-weigthed images, but variably low to high intensity on T2-weighted images (Fig. 3). Post-contrast image reveals enhancement in the nodules. The confirmation of enhancement on post-contrast image is important for differentiating neoplastic mural nodules from coagulated clots, which are commonly seen in endometriomas but never enhanced. Imaging features in CCCs described above are also seen as endometrioid adenocarcinomas, which can also arise from endometrioma. However, the presence of simultaneous thromboembolitic events, including thrombophlebitis, pulmonary embolisms, and infarctions of many organs, may suggest the diagnosis of CCC (Figs. 1 and 3).