Why would a breast mass appear larger on mammography (mammogram) compared to breast Magnetic Resonance Imaging (MRI)?

Why Mammography May Show Larger Breast Masses Compared to MRI

The question premise is actually reversed from what typically occurs in clinical practice—mammography generally underestimates tumor size compared to MRI, not the other way around. However, there are specific scenarios where mammography can appear to show larger lesions, which I'll address algorithmically below.

The Typical Pattern: Mammography Underestimates Size

Mammography consistently underestimates tumor size compared to both MRI and final pathology, while MRI tends to overestimate or more accurately reflect true tumor extent. 1

Evidence for Mammographic Underestimation:

• Digital mammography underestimates tumor size by approximately 7 mm compared to histopathologic findings 1
• In a study of 61 breast cancers, mean tumor size was 2.1 cm by mammography versus 2.76 cm by pathology, with MRI showing 2.65 cm 2
• Mammography underestimated tumor size by 3.5 mm on average (p < 0.002) 3
• The Pearson correlation coefficient for accuracy was significantly better for MRI (r = 0.80) compared to mammography (r = 0.26) 2

MRI Characteristics:

• MRI showed larger median tumor size than mammograms in a multicenter ECOG-ACRIN trial of 339 women with DCIS 1
• MRI overestimated tumor size by approximately 3 mm on average, though this was not statistically significant (p = 0.0570) 3
• In some studies, MRI overestimated disease extent by a mean of 1.97-3.2 cm in 65.2% of DCIS cases 1

Scenarios Where Mammography May Appear Larger

1. Compression Artifact and Tissue Spreading

• Mammographic compression spreads breast tissue in two dimensions, which can make lesions appear larger in the craniocaudal or mediolateral oblique projections
• This is a technical artifact rather than true size measurement
• MRI shows true three-dimensional anatomy without compression

2. Calcification-Associated Lesions

• Mammography is superior at detecting microcalcifications, which are often associated with DCIS 4
• When extensive calcifications span a large area, mammography may measure the full extent of calcifications
• MRI may not show enhancement corresponding to all calcified areas—only 67-84% of DCIS-associated calcifications show enhancement on MRI 1
• This can create the appearance that mammography shows a "larger" lesion when it's actually detecting calcifications that don't enhance on MRI

3. Low-Grade or Non-Enhancing DCIS

• Four cases in one study showed suspicious calcifications on mammography but no enhancement on MRI, despite biopsy-proven DCIS 1
• Low-grade DCIS is less likely to enhance on MRI 1
• The mammographic extent of calcifications may span a larger area than the enhancing component on MRI

4. Contrast-Enhanced Mammography Considerations

• Contrast-enhanced mammography overestimated tumor size by 11 mm compared to histopathology 1
• If comparing contrast-enhanced mammography to standard MRI, this could explain apparent size discrepancies

Clinical Algorithm for Interpretation

When encountering size discrepancies between mammography and MRI:

1. Determine if calcifications are present 1

   • If yes: Mammographic extent may reflect calcification span rather than viable tumor
   • MRI enhancement may underrepresent true disease extent in calcified DCIS

2. Assess tumor grade and type 1

   • Low-grade DCIS: Trust mammography more for extent
   • High-grade DCIS or invasive cancer: Trust MRI more for extent
   • 48% of high-grade DCIS lesions were detected on MRI but not apparent on mammography 1

3. Consider technical factors 1

   • Compression artifact on mammography
   • Whether contrast-enhanced mammography was used
   • MRI limitations in far posterior breast and axilla

4. Use complementary information 1

   • MRI depicted 88% of DCIS lesions and predicted multicentricity in 90% of cases 1
   • Mammography sensitivity for DCIS is only 56% compared to 92% for MRI 1

Common Pitfalls to Avoid

• Do not assume mammographic size is more accurate simply because it appears larger—the evidence strongly supports MRI superiority for size estimation in most scenarios 2, 3
• Do not dismiss mammographic findings of extensive calcifications even if MRI shows limited enhancement—absence of enhancement is insufficient to exclude DCIS 1
• Be aware that MRI can overestimate disease extent by 1-3 cm in some cases, leading to potential overtreatment 1
• Remember that mammography's major limitation is its inability to show non-calcified disease in dense breast tissue, where MRI excels 4, 5