Presentation
Incidental finding on renal ultrasound. Otherwise well, premenopausal.
Patient Data

There is a multiloculated, 17cm mass in the left adnexa with thin internal septations which demonstrate mild vascularity. There are no solid components or papillary projections.
A normal right ovary is noted. The uterus is suboptimally seen, but appears to be normal.
There is no ascites.
Case Discussion
This is a sonographically inconclusive lesion, based on IOTA simple rules criteria 2. The presence of mild (IOTA color score 2) internal vascularity and the size of the multilocular mass put this outside the "likely benign" range, however none of the IOTA malignant features are present.
Likely differential is between a cystadenoma or borderline epithelial cystic neoplasm, with mucinous favored over serous based on the presence of internal septations.
Path-proven borderline mucinous cystadenoma.