The risk of malignancy in unilocular cysts: a study on 1148 adnexal masses classified as unilocular cysts at transvaginal scan with review of the literature

Objectives: To estimate the rate of malignancy in adnexal lesions described as unilocular cysts at transvaginal ultrasound examination, to investigate if there are differences in clinical and ultrasound characteristics between benign and malignant unilocular cysts. Methods: 3511 patients with an adnexal mass underwent transvaginal ultrasound examination between 1999 and 2007. The sonologists used the International Ovarian Tumor Analysis terms and definitions to describe their ultrasound findings. Gold standard was the histopathological diagnosis of the mass. Results: Of the 3511 masses, 1148 (33%) were classified as unilocular cysts at scan. Of these, 11 i.e. 0.96 % (95%CI 0.48-1.71) were malignant. The malignancy rate was lower in pre- than post-menopausal women: 0.54% (5/931) (0.17-1.25) versus 2.76% (6/217) (1.02-5.92), P = 0.009. More patients with malignant unilocular cysts had a personal history of breast cancer (18% versus 2%, P = 0.02) or ovarian cancer (18% versus 0.6%, P = 0.003). Hemorrhagic cyst contents at scan was more common in malignant than benign unilocular cysts (18% versus 2%, P = 0.03). In seven of the 11 malignancies judged to be unilocular cysts at scan, papillary projections or other solid components were seen at macroscopic inspection of the surgical specimen. Conclusions: The malignancy rate in surgically removed adnexal lesions judged to be unilocular cysts at transvaginal scan is around 1%. Postmenopausal status, personal history of breast or ovarian cancer and hemorrhagic cyst contents at scan increase the risk of malignancy. To avoid misclassifying adnexal lesions as unilocular cysts at scan, it is important to scrutinize unilocular cysts for the presence of solid components.