Background Parenchymal Enhancement (BPE) - Breast MRI

Key Concept

BPE refers to the normal enhancement of fibroglandular tissue on contrast-enhanced MRI.

It is distinct from the amount of fibroglandular tissue (density).
Significance: High BPE can mask lesions (reduce sensitivity) and increase false positives.
Risk Factor: Emerging evidence suggests high BPE is an independent risk factor for future breast cancer.

1. Definition & Assessment

What is it? Enhancement of normal breast stroma/glandular tissue after Gadolinium administration.
Phase: Best assessed on the first post-contrast subtraction image.
Pattern: Typically diffuse and bilateral.
Terminology: We describe both the Level (intensity/volume) and Symmetry.

The ACR BI-RADS lexicon categorises BPE into four levels. This must be reported in every breast MRI.

Minimal: Almost no enhancement (<25% of glandular tissue enhances).
- Implication: Excellent sensitivity for detecting small lesions.
Mild: Little enhancement (25–50%).
- Implication: Minimal interference with interpretation.
Moderate: Considerable enhancement (50–75%).
- Implication: Can obscure small masses; consider mentioning "possible masking" in the report.
Marked: Intense enhancement (>75%).
- Implication: Significant reduction in sensitivity. "Ghosting" on subtraction images can be problematic.

BPE is highly hormonally sensitive.

Menstrual Cycle: BPE peaks in week 1 and 4.
- Golden Window: Scan between Day 7–14 (follicular phase) when BPE is lowest.
Lactation: Marked BPE is typical (makes screening lactating women very difficult).
Age: Decreases with age and menopause.

HRT (Hormone Replacement Therapy): Increases BPE.
- Tip: Cessation of HRT for 4–6 weeks prior to MRI can reduce BPE (though rarely practical).
Tamoxifen / Aromatase Inhibitors: Decrease BPE.
- Note: A lack of BPE reduction after starting Tamoxifen may predict poor treatment response.
Radiation Therapy: Decreases BPE in the treated breast (can lead to asymmetry).

Red Flag

BPE should be symmetric. Asymmetric BPE is suspicious.

Causes of Asymmetry:
- Pathological: Large region of non-mass enhancement (DCIS, Lobular CA).
- Previous Radiation: Reduced BPE on the treated side (the "normal" looking side might actually be the suppressed side).
- Mastectomy: Obviously no BPE.
Action: If no history of radiation/surgery, asymmetric BPE requires careful scrutiny to rule out diffuse malignancy.

Don't confuse Density with BPE: A patient can have "Extremely Dense" breasts (on mammogram) but "Minimal" BPE (on MRI). They are not correlated.
The "Masking" Disclaimer: If BPE is Marked, it is prudent to add a caveat: "Sensitivity for small lesions may be reduced due to marked background parenchymal enhancement."