What is the likelihood of ovarian cancer in a postmenopausal woman with a new septated ovarian cyst and a Carbohydrate Antigen 125 (CA125) level of 20, given a history of a stable 2 cm clear septated ovarian cyst with a previous CA125 level of 25?

Risk Assessment of Ovarian Cancer in Postmenopausal Woman with Septated Ovarian Cysts

The risk of ovarian cancer is low in this postmenopausal woman with a stable 2 cm clear septated ovarian cyst and a new 1.7 cm x 1.3 cm septated cyst on the same ovary, especially with normal CA125 levels that have decreased from 25 to 20 U/mL. 1, 2

Clinical Features Suggesting Low Malignancy Risk

• The patient has small cysts (both <5 cm), which have a significantly lower risk of malignancy compared to larger cysts in postmenopausal women 1
• The CA125 level is normal (<35 U/mL) and has actually decreased from 25 to 20 U/mL over time, which strongly suggests benign disease 2
• The original cyst has remained stable in size over a one-year period, indicating a non-aggressive nature 3

Risk Stratification Based on Imaging and Biomarkers

• Transvaginal ultrasonography features that suggest malignancy include complex ovarian masses with both solid and cystic components, internal echoes, septations, ascites, or evidence of peritoneal metastases 1
• While this patient has septated cysts, they lack other concerning features such as solid components, large size (>5 cm), or papillary projections 1
• The specificity of CA125 for ovarian cancer is approximately 98.5% for women over the age of 50 years when levels are elevated 1
• A CA125 level below 35 U/mL in postmenopausal women with ovarian cysts strongly predicts benign disease, regardless of cyst size or ultrasound features 2

Natural History of Simple Ovarian Cysts in Postmenopausal Women

• Studies show that 46.3% of simple ovarian cysts in postmenopausal women resolve spontaneously, while 43.9% persist unchanged over follow-up periods 3
• Only about 8% of simple cysts turn into complex cysts, and the risk of malignant transformation is extremely low (less than 1%) 3, 4
• In a study of 314 postmenopausal women with simple ovarian cysts followed over time, only one patient developed ovarian cancer, occurring three years after her last ultrasound 3

Management Considerations

• For postmenopausal women with clear cysts less than 5 cm in diameter, there is no clear consensus on management according to guidelines 1
• Current evidence supports conservative management with follow-up imaging for small, simple or septated cysts with normal CA125 levels 1, 5
• The American College of Radiology notes that simple cysts found on ultrasound may be safely followed without intervention, even in postmenopausal women 5

Important Caveats and Pitfalls

• While the risk is low, continued surveillance is still warranted as approximately 8% of simple cysts can develop complex features over time 3
• The presence of multiple cysts or new cysts developing over time warrants closer follow-up, though this alone does not significantly increase cancer risk when other features are reassuring 1, 3
• It's important to note that CA125 has limitations as a screening tool, with false-positive results possible in several benign conditions including endometriosis, pelvic inflammatory disease, and other benign cysts 1, 6

In conclusion, based on the small size of both cysts, normal and decreasing CA125 levels, and stability of the original cyst over time, this patient has a very low risk of ovarian cancer, likely less than 1% 3, 2. However, continued surveillance with periodic ultrasound and CA125 measurement is still recommended due to the development of a new cyst, even though the overall risk profile remains favorable.