What is the sensitivity and specificity of core needle biopsy (CNB) in diagnosing phyllodes tumors?

Sensitivity and Specificity of Core Needle Biopsy for Phyllodes Tumors

Core needle biopsy has poor sensitivity (ranging from 13-83%) but high specificity (92-100%) for diagnosing phyllodes tumors, making it unreliable for excluding this diagnosis when clinical or imaging suspicion exists. 1, 2, 3

Diagnostic Performance of Core Needle Biopsy

Sensitivity Data

• The largest single-center study (128 patients) found CNB accuracy of only 13.3% for diagnosing phyllodes tumors, with 75.5% misdiagnosed as fibroadenoma or fibroepithelial lesions 3
• Sensitivity varies by tumor grade: benign phyllodes (4.9%), borderline (4.2%), and malignant (25.0%) 3
• A 91-patient study reported 63% sensitivity for CNB, which increased to 76% when combined with imaging 1
• An 83% sensitivity was reported in one smaller study of 64 lesions, though this represents the higher end of published data 2

Specificity Data

• Specificity is consistently high across all grades: 92% for benign, 98.2% for borderline, and 100% for malignant phyllodes tumors 3
• When CNB suggests phyllodes tumor, the positive predictive value is 71% 2
• The false-negative rate is substantial at 39%, meaning CNB frequently misses phyllodes tumors 4

Clinical Implications and Guideline Recommendations

When CNB Suggests Phyllodes Tumor

The NCCN guidelines explicitly recommend excisional biopsy when CNB shows potential phyllodes tumor, regardless of imaging concordance 5

This recommendation applies to:

• Definite phyllodes tumor on CNB 5
• Equivocal fibroepithelial lesions 5
• Fibroadenoma diagnoses that are discordant with clinical presentation 5

Factors Associated with False-Negative Results

• Younger patient age is significantly associated with higher false-negative rates across all diagnostic modalities, likely due to increased prevalence of cellular fibroadenomas in this population 1
• Smaller tumor size correlates with lower diagnostic accuracy 3, 6
• Benign-grade phyllodes tumors are more frequently misdiagnosed than malignant variants 3, 6

Diagnostic Strategy

Clinical Features Suggesting Phyllodes Despite Benign CNB

Proceed to excisional biopsy when CNB shows fibroadenoma but clinical features include:

• Large tumor size (>3 cm median for phyllodes vs 1.6 cm for fibroadenoma) 2, 6
• Rapid tumor growth 3
• Prior surgical history of fibroadenomas 3
• Higher BI-RADS classification (≥4) despite benign CNB 6

Interpretation of Equivocal CNB Results

• When CNB states "equivocal for phyllodes," 32% are confirmed phyllodes on excision 4
• When pathologist favors phyllodes on equivocal CNB, excision correlates 100% with stated preference 4
• When fibroadenoma is favored on equivocal CNB, 90% are confirmed fibroadenoma 4

Critical Pitfall

The major clinical pitfall is accepting a fibroadenoma diagnosis on CNB and deferring excision when clinical suspicion for phyllodes remains. The 39% false-negative rate means that nearly 2 in 5 phyllodes tumors will be misdiagnosed as benign lesions 4. This is particularly problematic because:

• Phyllodes tumors require complete excision with negative margins 5
• Inadequate initial surgery increases recurrence risk
• Malignant phyllodes tumors have metastatic potential affecting mortality

The NCCN panel's recommendation for excisional biopsy when CNB suggests potential phyllodes tumor exists precisely because CNB cannot reliably exclude this diagnosis 5. Combined cytohistological and radiological assessment improves sensitivity to 76%, but this still leaves nearly 1 in 4 cases undiagnosed 1.