MRI Evaluation of the Patient with Breast Cancer

Index Tumor Size

Local tumor extent is classified into three types: unifocal, multifocal, and multicentric. Unifocal type is defined as only one malignant focus identified in the breast. Multifocal type is more than one malignant focus identified within the same quadrant of the breast. Multicentric type is greater than one malignant focus identified in multiple quadrants of the breast or separated by more than 4 cm. Multicentric disease is a contraindication to breast conservation therapy ( Figures 9.1 and 9.2 ).

The goal of breast MRI is to characterize the size of the known index malignancy and identify additional sites of occult malignancy to guide surgical planning. MRI demonstrates improved accuracy in determining pathologic tumor size over breast exam and conventional imaging. However, both overestimation and underestimation of disease do occur, and it has not yet been defined in which cases this is most likely to happen. There is limited evidence suggesting that improved correlation between tumor size and pathologic measurement occurs with both high-grade invasive tumors and high-grade ductal carcinoma in situ (DCIS) and underestimation of disease may occur more often with low-grade invasive tumors and low-grade DCIS. Also, there is a greater tendency for tumor size overestimation for tumors larger than 2 cm in size than for tumors smaller than 2 cm.

Multifocal and Multicentric Disease

Multifocal and/or multicentric malignancy may be present at the time of diagnosis but undetected by conventional mammographic and sonographic imaging. This may consist of one or more discrete enhancing masses separate from the known index tumor and/or nonmasslike enhancement, typical for DCIS ( Figures 9.3 and 9.4 ). The preoperative identification of additional sites of malignancy in the breast provides information for surgical planning. It has been consistently shown that breast MRI detects additional unsuspected malignancy within the ipsilateral breast. A recent meta-analysis of 50 studies found that MRI identified occult multicentric or multifocal malignancy in 20% of preoperative MRIs with a positive predictive value (PPV) of 75% and accuracy of 93% when using at least 1.5 Tesla (T) MRI. The same meta-analysis examined 26 studies (n = 4839) that gave information about surgical impact. They found an appropriate change in surgical management occurred in 12.8% of patients with confirmed additional malignancy, 8.3% of whom were converted to receive mastectomy and 4.5% of whom were converted to receive a wider local excision. On the other hand, overestimation of disease (false positives) resulted in inappropriate alteration in surgical management in 6.3% of patients, 4.6% of whom had a wider local excision and 1.7% of whom had mastectomies. The importance of not subjecting patients to unnecessary extensive surgery, combined with the known risk of overestimation of disease with MRI, emphasizes the need to obtain histologic confirmation of suspicious MRI findings before changing surgical treatment. This was not done in all of the studies included in the meta-analysis. Given a PPV of 75%, one in four patients with suspicious findings will, in fact, have benign disease. Additionally, given the imperfect specificity of MRI and limited data regarding the appropriate use of the American College of Radiology Breast Imaging Reporting and Data System (BI-RADS) category 3 in MRI, it is generally not acceptable to recommend short-term follow-up MRI imaging for suspicious lesions in the ipsilateral breast that may alter surgical treatment. Proceeding with an image-guided biopsy is often prudent in these cases.

Several studies have shown that diffusion-weighted imaging (DWI) and spectroscopy have shown promise in increasing the PPV of MRI. Further studies on these techniques are warranted.

False negatives, in which additional tumor is present but not detected, are also known to occur with MRI. Background diffuse parenchymal enhancement of normal tissue obscuring lesion enhancement is a common reason for false-negative imaging. Moderate and marked background enhancement can reduce the sensitivity for multifocal and multicentric lesions that are small and lack distinguishing morphologic features. Even extensive nonmasslike enhancement, which occurs with DCIS, is difficult to detect in the presence of moderate/marked background enhancement. Background parenchymal enhancement is hormonally mediated and does not correlate with mammographic density. Scheduling the patient’s MRI study during the first half of her menstrual cycle and discontinuing exogenous hormone therapy for several months before the study decrease the risk of excessive background enhancement. However, this is often not possible in patients with newly diagnosed malignancy, who need to avoid treatment delays.

Pectoral Muscle and Chest Wall Involvement

Tumors located posteriorly in the breast may invade the pectoral muscles and/or chest wall. Chest wall invasion by breast cancer is defined as tumor infiltrating the ribs, intercostal muscles, and/or serratus anterior muscle ( Figure 9.5 ). Tumors with chest wall invasion are classified as T4a, regardless of size (see Table 9.1 ). This results in a minimum TNM classification of at least stage IIIb disease (see Table 9.4 ), which correlates with a worse prognosis compared with the prognosis for patients who have tumors of comparable size without chest wall invasion. Preoperative treatment with chemotherapy and/or chest wall radiation, followed by more extensive surgery, including chest wall resection, is typically performed. Alternatively, a tumor that invades only the pectoralis muscle does not change the disease stage; however, it alters the surgical treatment. When a tumor superficially invades the pectoralis muscle, a portion of the muscle may be resected, or when deep muscle invasion is present, a radical mastectomy with removal of the entire muscle may be required ( Figures 9.6 and 9.7 ).

Mammography and ultrasound are typically limited in making the diagnosis of muscle or chest wall invasion. Posterior tumors are often difficult to include in the mammographic image, particularly when they are affixed to the muscle. Acoustic shadowing caused by many tumors interferes with the sonographic visualization of the posterior border of the mass and underlying muscle. Even when tumors are palpable and clinically suspected to invade the underlying muscle, the ability to preoperatively determine the extent of invasion is limited. Breast MRI is superior in making this diagnosis. The pectoral muscle and chest wall are well visualized on breast MRI. It has been shown that contrast enhancement of the pectoral muscles or chest wall muscles is the only reliable finding to predict invasion. The enhancement may appear infiltrative with preservation of the muscular architecture or it may be masslike. It should be noted that muscle enhancement related to a recent biopsy can result in a false-positive MRI appearance. Loss of fat planes or close proximity of the tumor mass to the muscle are not reliable indicators of invasion. Also, vessels feeding the tumor, which course through the muscle, are commonly visualized and do not indicate invasion.

Skin Involvement

Breast cancers invading the skin are classified as T4b (see Table 9.1 ), also resulting in at least stage IIIb disease (see Table 9.4 ). Skin invasion is defined as skin ulceration or tumor nodules within the skin. Involvement of the dermis alone does not meet the criteria for skin invasion to increase the tumor stage. Inflammatory breast carcinoma is a separate entity in which peau d’orange changes involve greater than one third of the breast, resulting from tumor micrometastases within the dermal lymphatics (to be discussed later). However, peau d’orange changes that do not involve more than one third of the breast—which would meet the criteria for inflammatory breast carcinoma—are also classified as T4b. Locally advanced breast cancer is at times associated with direct skin invasion, which is typically contiguous with the underlying mass and localized to the skin overlying the mass ( Figure 9.8 ). Skin enhancement demonstrated on MRI with this pattern suggests skin involvement. Skin edema and skin thickening (>3mm) may also occur as the result of lymphatic obstruction with or without malignant involvement.