What is the appropriate workup for a complex or oil‑filled cyst identified on screening mammography?

Management of Complex/Oil Cysts in Breast on Mammogram

Complex cysts require immediate core needle biopsy due to their 14-23% malignancy risk, while oil cysts identified on mammography require no further evaluation if they have pathognomonic features. 1, 2

Critical First Step: Distinguish Between Complex Cysts and Oil Cysts

Oil Cysts (Fat Necrosis)

• Oil cysts with pathognomonic mammographic features (radiolucent center with thin calcified rim) require no further workup. 2
• If an oil cyst is suspected but not clearly identified on mammogram, perform directed mammography to confirm the diagnosis and avoid unnecessary biopsy. 2
• Oil cysts discovered incidentally on ultrasound can appear highly variable—only 8% mimic simple cysts, and 12% can mimic intracystic masses with suspicious features. 2
• When ultrasound features are concerning but mammography confirms classic oil cyst appearance, no biopsy is needed. 2

Complex Cysts (Cystic and Solid Components)

Complex cysts are defined by the presence of discrete solid components including thick walls, thick septa (≥0.5 mm), and/or intracystic masses. 1, 3

Management Algorithm for Complex Cysts

Step 1: Confirm with Targeted Ultrasound

• Perform high-resolution ultrasound to definitively characterize the lesion and identify solid components. 1
• Assign BI-RADS category 4 or 5 to all complex cystic masses. 1, 4

Step 2: Mandatory Tissue Diagnosis

• Perform core needle biopsy (not aspiration) for all complex cysts regardless of size or symptoms. 1, 4
• The malignancy rate of 14-23% makes observation unacceptable. 1, 3
• Core needle biopsy is preferred over fine needle aspiration as it provides architectural information. 1

Step 3: Post-Biopsy Management

If benign and image-concordant:

• Physical examination with or without ultrasound/mammogram every 6-12 months for 1-2 years to ensure stability. 1
• Return to routine screening if stable. 1
• If lesion increases in size or suspicion, proceed to surgical excision. 1

If malignant:

• Proceed to surgical excision and treatment per breast cancer guidelines. 1

If indeterminate or image-discordant:

• Surgical excision is required. 1

Critical Distinction: Do NOT Confuse with Complicated Cysts

Complicated cysts (internal debris/low-level echoes WITHOUT solid components) have <2% malignancy risk and can be managed with either aspiration or short-term surveillance. 1, 5

Key differentiating features:

• Complicated cysts: Low-level echoes, debris, NO thick walls, NO thick septa, NO solid components. 1, 5
• Complex cysts: ANY discrete solid component, thick walls, thick septa (≥0.5 mm), or intracystic masses. 1, 3

Evidence Strength

The NCCN guidelines (most recent 2018 version) provide the strongest evidence base for this approach. 1 Multiple studies consistently demonstrate the 14-23% malignancy rate in complex cysts, with malignant etiologies including papillary carcinoma, invasive ductal carcinoma, DCIS, and metastases. 1, 3, 6

Common Pitfalls to Avoid

• Do not attempt aspiration of complex cysts—the presence of solid components mandates tissue diagnosis, not fluid drainage. 1, 4
• Do not confuse oil cysts with complex cysts—oil cysts on mammography with classic features need no biopsy, while complex cysts always require biopsy. 2, 1
• Do not downgrade complex cysts to complicated cysts—any solid component, thick wall, or thick septation (≥0.5 mm) requires biopsy. 1, 3
• Do not rely on ultrasound alone for oil cysts—their sonographic appearance is highly variable and can mimic malignancy; use directed mammography for confirmation. 2