In contrast to mammography and ultrasound which use anatomic approaches, molecular or functional imaging characterizes and measures the metabolic activity at the cellular level. Molecular imaging includes contrast-enhanced MRI, MR spectroscopy (MRS) and nuclear medicine imaging such as Breast–specific gamma imaging (BSGI) and Positron emission mammography (PEM). Functional positron emission tomography (PET) with fluorodeoxyglucose (FDG) combined with computed tomography (PET-CT) can be used for diagnosis, staging and monitoring therapy for breast cancer. However, whole-body PET or PET-CT is of minimal value in patients with early stage disease.
Breast MRI has become an important tool for the detection and characterization of breast cancer. It has been reported to have diagnostic sensitivities of 94-99% for invasive breast malignancies and 58-89% for DCIS. Applications of breast MRI include: evaluation of the extent of ipsilateral malignancy, screening of the contralateral breast in patients with newly diagnosed breast cancer, screening women who are at high risk, for the evaluation of patients with metastatic axillary adenocarcinoma and an unknown primary cancer site and for the assessment of silicone implant integrity. However, MRI costs approximately ten times more than mammography, is not widely available and can not be used in women with implanted medical devices or who suffer from claustrophobia.
In addition, the overlap in the MRI features of certain benign and malignant lesions decrease the specificity and positive predictive value which results in an increase in the biopsy rate. If a suspicious lesion is clinically and mammographically occult and cannot be found on a targeted ultrasound (which can detect 46% of these lesions), the biopsy or needle localization needs to be done with MRI guidance. MRI guided procedures are time consuming and costly.
Currently it is the standard of care to perform an MRI of both breasts for preoperative evaluation of patients with newly diagnosed breast cancer. Since MRI is able to detect unsuspected disease in the ipsilateral breast and in 3% of patients in the contralateral breast as well, it can change in the management plan. In patients with negative surgical margins after breast conservation surgery the 10 year recurrence rate is low. After the initial lumpectomy, positive margins are seen in 40% of cases, requiring additional surgery, cost and anxiety. MRI can improve the surgical outcome by better identifying the extent of the disease resulting in a decrease in the positive margins rates. MRI can also better visualize the presence of adenopathy, occult contralateral disease, pectoralis or chest wall involvement and therefore guide in the surgical and radiation planning. It can also help in identifying patients who would be eligible for partial breast radiation.
However, there are certain surgeons who believe that unsuspected disease detected by MRI prior to breast conserving therapy may be of little clinical consequence since it is controllable by radiation therapy and may cause an increase in the rate of mastectomies and delays in surgery.
