What is a Breast Mouse?
A "breast mouse" is colloquial terminology referring to a fibroadenoma—a benign, mobile breast mass that characteristically moves freely under the skin during palpation, resembling a mouse moving under a sheet.
Clinical Characteristics
- Benign masses typically are mobile and have discrete, well-defined margins and a soft or rubbery texture, which distinguishes them from malignant lesions 1
- The term "mouse" derives from the highly mobile nature of these lesions during physical examination, as they slip easily under the examining fingers 1
- True breast masses are generally asymmetrical in relation to the other breast, distinct from the surrounding tissues, and three-dimensional 1
Distinguishing Features from Malignancy
- A typical cancer may be firm, have indistinct borders, and have attachments to the skin or deep fascia with dimpling or nipple retraction, which contrasts sharply with the mobile "breast mouse" 1
- Benign masses like fibroadenomas demonstrate smooth, well-circumscribed borders rather than the irregular, spiculated margins seen in malignancy 2
- Cysts cannot reliably be distinguished from solid breast masses by palpation alone—only 58% of palpable cysts were correctly identified by physical examination in one study 1
Critical Clinical Pitfall
- Significant disagreement among experienced examiners may occur—four surgeons performing independent physical examinations agreed on the need for biopsy in only 73% of masses subsequently proven malignant 1
- Because many breast masses may not exhibit distinctive physical findings, imaging evaluation is necessary in almost all cases to characterize the palpable lesion 1
- Negative imaging evaluation should never overrule a strongly suspicious finding on physical examination; any highly suspicious breast mass detected by imaging or palpation should undergo biopsy 2
Recommended Evaluation Approach
- Any woman presenting with a palpable lesion should have a thorough clinical breast examination to assess for asymmetry, mobility, texture, and three-dimensional characteristics 1
- Diagnostic mammography and targeted breast ultrasound are the recommended initial imaging modalities, with the specific approach dependent on patient age 1
- For women 30-39 years of age with a palpable mass, both ultrasound and diagnostic mammography are rated as "usually appropriate" (rating 8/9) for initial evaluation 1
- The negative predictive value of mammography with ultrasound in the context of a palpable mass ranges from 97.4% to 100% 1