Risk of Malignancy with RMI <200 in Postmenopausal Ovarian Cysts
When the Risk of Malignancy Index (RMI) is calculated as less than 200 in a postmenopausal woman with an ovarian cyst, the risk of malignancy is generally low, typically in the range of 1-10%, though the precise risk depends critically on the ultrasound morphology of the cyst. 1
Understanding RMI <200 in Clinical Context
The RMI threshold of 200 is commonly used as a cutoff to distinguish low-risk from high-risk ovarian masses, with values below 200 suggesting lower malignancy risk. 1 However, the actual malignancy risk varies substantially based on the specific ultrasound characteristics of the cyst, which is why modern risk stratification systems like O-RADS have largely superseded RMI alone. 1
Risk Stratification by Cyst Morphology (RMI <200)
When RMI is <200, the malignancy risk breaks down as follows based on ultrasound features:
Simple unilocular cysts ≤3 cm: Essentially 0% malignancy risk, with no cases of malignancy reported in large screening studies 1, 2
Simple unilocular cysts 3-10 cm: <1% malignancy risk (O-RADS 2 category), with only 1 out of 2,349 simple cysts (0.04%) proving malignant at 3-year follow-up in a landmark study of 72,093 women 2, 3
Unilocular cysts with irregular inner wall or multilocular smooth cysts <10 cm: 1-10% malignancy risk (O-RADS 3 category) 1
Complex cysts with wall abnormalities or solid areas: 10-50% malignancy risk (O-RADS 4 category), even when RMI is <200 1, 4
Critical Warning Signs That Elevate Risk Despite Low RMI
Even with RMI <200, certain ultrasound features substantially increase malignancy risk and require escalated management:
- Developing vascularity within previously avascular lesions on color Doppler imaging 1, 2
- Changing morphology on serial imaging (e.g., developing septations or solid components) 5, 2
- Cyst enlargement over time during surveillance 2
- Size >10 cm in maximum diameter 1, 2
- Presence of ascites in conjunction with the cyst 1
- Papillary projections or solid nodules with blood flow 1
Management Algorithm for RMI <200
For simple cysts:
- ≤3 cm: No follow-up required 1, 2
- 3-10 cm: Follow-up ultrasound at 8-12 weeks, then annually if stable 1, 2
10 cm: Gynecologic referral or MRI evaluation 1
For septated/multilocular cysts without solid components:
- Referral to ultrasound specialist, gynecologist, or MRI for further characterization 1, 5
- Annual ultrasound surveillance if diagnosis is confident and <10 cm 5
For any cyst with solid components or vascularity:
- Direct MRI with contrast (if feasible) and gynecologic oncology consultation, regardless of RMI value 1, 2
Important Caveats
CA-125 alone (a component of RMI) performs worse than ultrasound in distinguishing benign from malignant lesions, as it may be low with low-grade and low malignant potential lesions. 1 CA-125 levels improve specificity only in lesions already suspected to be malignant on ultrasound. 1
Hemorrhagic cysts should not occur in postmenopausal women, so if typical hemorrhagic features are seen, further evaluation by ultrasound specialist, gynecologist, or MRI is required even with low RMI. 5, 2
Endometriomas have higher risk of malignant transformation to clear cell and endometrioid carcinomas in postmenopausal women, warranting annual ultrasound surveillance when not surgically excised. 5, 2
The most common pitfall is failing to compare with previous imaging, which can lead to missing subtle changes indicating malignant transformation despite reassuring RMI values. 5, 2