How do they tell the difference between a malignant and benign ovarian lesion on an ultrasound?

Not always clear cut: Purely simple, smooth ovarian cysts are considered benign. Cysts with debris, septations or nodules along their walls can be concerning. Sometimes we do follow up ultrasound to see if they grow. If there is enough concern, MRI helps. Correlation with patient age, family history, blood tests and possibly biopsy also help us be sure.