Multilocular Cyst
An ovarian cyst with septations is termed a "multilocular cyst." This standardized terminology is defined by the O-RADS (Ovarian-Adnexal Reporting and Data System) US lexicon as a cystic lesion with more than one compartment, requiring at least one complete septum 1.
Key Terminology and Classification
The O-RADS system provides precise definitions to distinguish septated cysts:
- Multilocular cyst without solid elements: Contains multiple compartments separated by septa, but no solid components exceeding 3mm in height 1
- Multilocular cyst with solid component(s): Same as above but includes solid components greater than 3mm in height 1
This distinction is clinically critical because it directly impacts malignancy risk stratification and management decisions.
Clinical Significance and Risk Stratification
The presence and characteristics of septations fundamentally alter the risk assessment:
Low-Risk Features (O-RADS 3: 1-10% malignancy risk)
- Multilocular cyst <10 cm with smooth inner walls and color score 1-3 1
- These require management by a gynecologist with follow-up 1
Intermediate-Risk Features (O-RADS 4: 10-50% malignancy risk)
- Multilocular cyst of any size with smooth inner wall but color score 4
- Any size with irregular inner wall and/or irregular septations 1
- These warrant US specialist evaluation or MRI, with gynecologic oncology consultation 1
High-Risk Features (O-RADS 5: ≥50% malignancy risk)
Important Clinical Context
Research demonstrates that septated cystic ovarian tumors without solid areas or papillary projections have remarkably low malignancy risk. A large surveillance study of 2,870 septated cystic tumors found only one borderline malignancy and one cancer that developed in the contralateral ovary years later, yielding an extremely low malignancy rate over 7,642 follow-up years 2. This supports conservative sonographic surveillance for appropriately selected cases.
Common Pitfall to Avoid
Do not confuse septations with solid components. Septa are thin dividing walls between compartments, while solid components are tissue masses >3mm in height that protrude into the cyst cavity. This distinction is critical—the presence of solid components dramatically increases malignancy risk and changes management from potential observation to surgical consultation 1.