Clinical Characteristics of Patients in Japan with Ovarian Cancer Presumably Arising from Ovarian Endometrioma

<b><i>Objective:</i></b> To investigate the clinical features of patients in Japan with malignant transformation of ovarian endometrioma. <b><i>Patients:</i></b> Thirty-three patients diagnosed with ovarian cancer presumably arising from endometrioma were recruited retrospectively. These patients had been followed for at least 2 years after the ovarian endometrioma diagnosis, then continued to be followed after they had been found to have malignant transformation. <b><i>Results:</i></b> The average age of the patients was 47.7 ± 9.3 years; 75.7% were premenopausal at the time of diagnosis of ovarian cancer. Among the 33 patients, ovarian cancer developed in the ipsilateral ovary of 6 patients with endometrioma after cystectomy. Twenty-eight patients were diagnosed with stage I ovarian cancer, and major histotypes were clear cell in 23 cases and endometrioid in 8. Before surgery for cancer, mural nodules within the endometriomas were detected in 32 patients, and 1 patient had a small 3-mm nodule. In 30 patients, the diameter of the tumor doubled in size 6 months prior to the diagnosis of malignant transformation. The diameter of the endometrioma and the preoperative CA125 value did not significantly correlate. <b><i>Conclusions:</i></b> To detect malignant transformation of ovarian endometrioma early and precisely, the clinician should determine the existence of a mural nodule and assess the rapid growth of the endometrioma.