The incidence of ovarian cancer of DMBA-treated rats and representative images of the tumors formed. Tumors were detected by palpating the abdominal wall of rats once a week for a total of 32 weeks to monitor tumor progression and validated by a histological examination. Open circles, rats of the S group; closed circles, rats of the CS group (A). Tumors were usually a single invasive nodule (B, C), some were intraperitoneal spread or seeding of tumor (D), with or without ascites. Arrows refers to the tumor.