What percentage of fibroadenomas (Benign Breast Tumors) are actually carcinomas (Cancer) based on low suspicion Breast Imaging-Reporting and Data System (BIRADS) and subsequent biopsy?

Percentage of Fibroadenomas That Are Actually Carcinomas Based on Low Suspicion BIRADS

For fibroadenomas classified as low suspicion (BIRADS 3 or 4A) on imaging that undergo subsequent biopsy, less than 2% are actually carcinomas.

Understanding BIRADS Classification and Fibroadenomas

According to the American College of Radiology's Breast Imaging-Reporting and Data System (BI-RADS), lesions are categorized based on their likelihood of malignancy:

• BIRADS 3 (Probably Benign): Risk of malignancy is less than 2% 1
• BIRADS 4A (Low Suspicion): Risk of malignancy is 3-10% 2

Fibroadenomas are among the most common benign breast lesions and typically present with the following characteristics:

• Oval or round shape
• Well-defined margins
• Homogeneous echogenicity on ultrasound
• Orientation parallel to the chest wall
• No posterior acoustic shadowing 1

Malignancy Rates in Fibroadenomas by BIRADS Category

The risk of malignancy in lesions initially classified as fibroadenomas varies by BIRADS category:

• BIRADS 3: Less than 2% risk of malignancy 1, 3
• BIRADS 4A: Approximately 10% risk of malignancy 2
• BIRADS 4B: About 21% risk of malignancy 2
• BIRADS 4C: Approximately 70% risk of malignancy 2

Evidence from Biopsy Studies

A study of 349 biopsy-proven fibroadenomas with at least 24 months of follow-up found:

• False-negative rate of needle biopsy: 1.1%
• Negative predictive value: 98.9% 4

This means that when a lesion is diagnosed as a fibroadenoma on biopsy, there is approximately a 1.1% chance that it is actually a carcinoma that was missed by the biopsy.

Management Considerations

For lesions classified as BIRADS 3 or 4A with a histological diagnosis of fibroadenoma after biopsy:

• Short-term follow-up may not be necessary if the biopsy was adequate and concordant with imaging findings 4
• However, certain factors may warrant closer follow-up or excision:
  • New or enlarging masses
  • Patients with high-risk factors
  • Patient anxiety
  • Patients awaiting organ transplant
  • Patients with known synchronous cancers
  • Patients planning pregnancy 1

Special Considerations

1. Complex fibroadenomas or those with hyperplasia (with or without atypia) carry a higher risk of subsequent breast cancer:

   • Fibroadenoma without hyperplasia: RR 1.48-1.7
   • Fibroadenoma with hyperplasia: RR 3.47-3.7
   • Fibroadenoma with hyperplasia and atypia: RR 6.9-7.29 5

2. Myxomatous fibroadenomas may mimic carcinoma on imaging with:

   • Higher depth-to-width ratio
   • Relatively round shape
   • Increased posterior echo enhancement
   • Internal hyperechogenicity on ultrasound 6

Clinical Implications

1. Understanding the low risk of malignancy in BIRADS 3 lesions helps avoid unnecessary biopsies.

2. For BIRADS 4A lesions, biopsy is typically recommended despite the relatively low risk of malignancy.

3. Subcategorizing BIRADS 4 lesions (4A, 4B, 4C) provides valuable information about the level of concern for carcinoma and helps guide management decisions 2.

4. The overall positive predictive value (PPV) for malignancy increases with BIRADS category:

   • BIRADS 3: PPV of 0.03 (3%)
   • BIRADS 4: PPV of 0.23 (23%)
   • BIRADS 5: PPV of 0.92 (92%) 7

In summary, when a lesion is classified as low suspicion (BIRADS 3 or 4A) and diagnosed as a fibroadenoma on biopsy, the risk of it actually being a carcinoma is very low (less than 2%), assuming proper imaging-pathology correlation and adequate sampling.