What types of breast lesions are not typically detected by mammography, especially in women under 40 or those with dense breast tissue?

Breast Lesions Not Detected by Mammography

Small intraductal lesions without calcifications, lesions obscured by dense breast tissue, and very small invasive cancers (<5mm) are the primary types of breast lesions that mammography fails to detect.

Lesions Frequently Missed by Mammography

Small Intraductal Lesions Without Calcifications

• Mammography often fails to demonstrate intraductal papillomas and early ductal carcinoma in situ (DCIS) when they are very small, contain no calcifications, or are completely intraductal 1
• The sensitivity of mammography for detecting pathologic nipple discharge (often caused by intraductal lesions) ranges from only 15% to 68%, with many lesions being too small or lacking calcifications to be visible 1
• Microcalcifications are generally not demonstrated well on ultrasonography either, but when present on mammography, they help identify otherwise occult intraductal disease 2

Lesions Obscured by Dense Breast Tissue

• The sensitivity of mammography is significantly decreased in women with dense breast tissue, as fibroglandular tissue masks noncalcified lesions 1
• Women with dense breasts have a 4- to 6-fold greater risk of developing breast cancer compared to those with fatty tissue, and these cancers are at greater risk of not being detected on mammography 1
• Dense breast tissue creates a "masking effect" where overlapping normal tissue obscures underlying malignancies, particularly asymmetric masses and architectural distortions 3
• There is an increased rate of interval cancers (cancers appearing between screening rounds) in women with dense tissue, and these cancers often have worse prognosis than screen-detected cancers 1

Very Small Invasive Cancers

• No tumor that was 5mm or less in maximal diameter was clinically palpable, and many of these small cancers are also mammographically occult 3
• The threshold for mammographic detection is influenced by tumor size, with smaller cancers more likely to be missed 3
• Cancers missed by mammography are typically smaller than those detected, more often have negative lymph nodes, and present with lower stage disease 3

Asymmetric Masses and Architectural Distortions

• Asymmetric masses account for only 3% of all mammographic signs of malignancy but represent 14% of false-negative mammograms 3
• These subtle findings are easily overlooked, particularly in dense breast tissue where they blend with normal parenchyma 3
• Architectural distortion without an associated mass or calcifications is particularly challenging to detect on standard mammography 1

Specific Clinical Scenarios with Poor Mammographic Detection

Women Under 40 Years

• The American College of Radiology recommends ultrasound as the first-line imaging for palpable masses in women under 30 years, as most benign lesions in young women are not well visualized on mammography 4
• Dense breast tissue is more common in younger women, further limiting mammographic sensitivity 1

Patients with Breast Implants

• Breast augmentation implants are associated with a significantly higher rate of missed breast cancers, with 5 out of 8 (62.5%) clinically detected breast carcinomas being undetected by mammography in patients with implants 3

Proliferative Lesions Without Calcifications

• Atypical ductal hyperplasia, atypical lobular hyperplasia, and lobular carcinoma in situ (LCIS) are often mammographically occult when they do not produce calcifications 5
• These "borderline" proliferative lesions are difficult to diagnose even in surgical specimens and are frequently invisible on mammography 5

Important Caveats

False-Negative Rate

• The overall misinterpretation rate of mammography is approximately 1.8%, with a total false-negative rate of 9% when including truly occult lesions 3
• Cancers associated with false-negative mammograms occur more often in younger women and in dense breast parenchyma 3

Negative Predictive Value Limitations

• Despite the high negative predictive value of mammography with ultrasound (97.4% to 100%), negative imaging should never overrule a strongly suspicious finding on physical examination 1, 4
• Any highly suspicious breast mass detected by imaging or palpation should undergo biopsy unless there are exceptional clinical circumstances 1

Supplemental Screening Necessity

• Women with dense breasts require supplemental screening modalities such as ultrasound, MRI, contrast-enhanced mammography, or molecular breast imaging to overcome the limitations of mammography 1
• Digital breast tomosynthesis (DBT) improves lesion characterization in noncalcified lesions compared to conventional mammography but still has limitations in very dense tissue 1