Why is magnetic resonance imaging (MRI) performed in newly diagnosed breast cancer patients, particularly those with dense breasts, implants, or high‑risk features?

Why MRI is Performed in Breast Cancer

MRI is primarily used in breast cancer to define disease extent, detect multifocal/multicentric disease, screen the contralateral breast, and evaluate response to neoadjuvant therapy—particularly in women with dense breasts, mammographically occult tumors, or invasive lobular carcinoma. 1, 2

Primary Clinical Indications

Defining Disease Extent and Multifocality

• MRI detects multifocal and multicentric disease in up to 16% of women that is underestimated on mammography and ultrasound. 1
• MRI demonstrates sensitivity approaching 90% for determining extent of disease, especially in young women under age 50. 1
• The modality is particularly accurate for invasive lobular cancer, which is frequently underestimated by conventional imaging. 1
• MRI can reliably assess chest wall involvement, as pectoral or intercostal muscle enhancement correlates well with invasion. 1

Mammographically Occult Tumors

• MRI should be used to define extent of cancer in women with very dense breast tissue where mammography and ultrasound cannot adequately image the breast. 1, 2
• The modality is indicated for women with positive axillary nodes but no identifiable breast primary on mammography or ultrasound. 2
• MRI is recommended for evaluating Paget's disease of the nipple when the breast primary is not identified on conventional imaging. 2

Neoadjuvant Therapy Monitoring

• MRI is helpful before and after neoadjuvant therapy to define extent of disease, assess response to treatment, and determine potential for breast-conserving therapy. 1
• A pretreatment baseline MRI must be obtained for accurate comparison during treatment monitoring. 1
• MRI provides early indicators of treatment response including changes in lesion size, kinetic parameters, and apparent diffusion coefficient. 3

Contralateral Breast Screening

• MRI detects unsuspected contralateral disease in up to 3.1% of women at initial diagnosis. 1
• This screening function is particularly valuable given the bilateral nature of breast cancer risk. 1

Technical Requirements and Limitations

Mandatory Technical Standards

• MRI must be performed using a dedicated breast coil by a breast imaging team capable of performing MRI-guided biopsy. 1, 2
• Consultation with the multidisciplinary treatment team is required before making treatment decisions based on MRI findings. 1, 2
• For premenopausal patients, imaging should ideally be performed in the first half of the menstrual cycle to minimize background parenchymal enhancement, though most centers do not delay imaging in newly diagnosed patients. 1

Critical Limitations to Recognize

• MRI has a high false-positive rate (specificity 50-97%), requiring additional biopsies without proven survival benefit. 1, 2
• No randomized prospective trials demonstrate that MRI improves local recurrence rates or survival, even when it alters surgical management. 1, 2
• MRI overestimates disease extent in DCIS cases by a mean of 1.97-3.2 cm in 65.2% of cases. 2, 4
• Patients should never be denied breast-conservation therapy based on MRI findings alone without tissue sampling. 1, 2

Impact on Surgical Management

Documented Changes in Treatment Planning

• Breast MRI staging alters surgical treatment in 7.8% to 33.3% of women, though outcome differences remain unproven. 1
• In a prospective randomized trial, preoperative MRI provided additional information in 38% of patients and caused treatment plan changes in 18%. 5
• The breast reoperation rate was significantly reduced with preoperative MRI (5% versus 15% without MRI, p<0.001). 5
• MRI may increase mastectomy rates by identifying mammographically occult disease, though final mastectomy numbers do not differ significantly between MRI and non-MRI groups. 2, 5

Specific Clinical Scenarios

Dense Breast Tissue

• Mammography has significantly reduced sensitivity in women with dense breasts. 4, 6
• MRI is particularly indicated in young patients and women with dense breasts where mammography sensitivity is compromised. 6
• However, in DBT-screened populations with non-dense breasts, the positive impact of preoperative MRI is diminished (true-positive rate 8.5% versus 30% in DM-screened populations). 7

Invasive Lobular Carcinoma

• MRI is more accurate than mammography and ultrasound for defining disease extent in invasive lobular cancer. 1
• This histologic subtype is frequently underestimated by conventional imaging modalities. 1

Enhancement Patterns and Predictive Value

• Mass lesions with well-defined margins on pretreatment MRI allow more accurate prediction of residual disease post-treatment. 1
• Non-mass enhancement on pretreatment MRI more commonly reveals scattered cell patterns post-treatment, making residual disease assessment more difficult. 1

Common Pitfalls to Avoid

• Do not use MRI for routine staging in stage I disease without signs or symptoms of metastatic disease. 1
• Never make surgical decisions based solely on MRI findings without histologic confirmation through tissue sampling. 1, 2
• Recognize that MRI is superior at detecting microcalcifications associated with DCIS compared to mammography (sensitivity 92% versus 56%). 4
• Be aware that only 67-84% of DCIS-associated calcifications show enhancement on MRI, so mammography remains superior for detecting microcalcifications. 4