Several alternative breast imaging techniques are currently in use in the evaluation of breast abnormalities. Two of these rely on the use of radioactive materials that are injected into the bloodstream. These techniques are a form of functional imaging because they reflect differences in metabolism between cancer cells and non cancerous cells.
Breast-Specific Gamma Imaging- BSGI (Dilon)- is a FDA approved nuclear medicine-based diagnostic breast imaging technique using radiopharmaceuticals to reveal images of tumors of the breast. This molecular imaging technique involves a single intravenous injection of 20 mCi of technetium Tc 99m (99mTc) sestamibi. Five minutes after injection of radiotracer, the patient is imaged in a sitting position with each breast compressed. A dedicated gamma camera and standard mammographic projections (both craniocaudal and mediolateral oblique positions with 5-10 minutes per image) are obtained using light compression to limit patient motion.
As dense breast tissue often pose challenges in mammographic detection of breast cancer, the ability to take advantage of the functional differences between tumor and normal cells with different uptake of radiotracer allows for the detection of small tumors independent of the surrounding breast tissue.
Several studies have determined that combining BSGI and mammography greatly improves the detection of breast cancer over the use of either imaging study alone. BSGI also demonstrates similar sensitivity and greater specificity to breast MRI in detecting invasive lobular carcinoma. In addition, this test is an alternative to patients who cannot undergo breast MRI due to various issues including obesity, ferromagnetic implants, claustrophobia and reduced kidney function. BSGI is currently approved for evaluation of breast lesions in patients with probable or known breast malignancy, monitoring of response to chemotherapy and suspected recurrence, indeterminate breast abnormalities and technically difficult breast imaging. It is currently not indicated for breast cancer screening as BSGI involves intravenous injections of radioisotopes, which exposes all organs of the body to gamma ray emissions. In comparison, mammography with lower-energy x-rays exposes only the breasts to the radiation. A recent study proposes that the lifetime radiation induced cancer death risk involved in a single BSGI study is 20–30 times greater than that from a single digital mammography study in women aged 40 years, with this risk ratio increasing to 132–198 times greater at age 80 years. However, this study is based on BSGI from Dilon Technologies, a single detector unit with 20 mCi dosage. A dual gamma camera system (LumaGEM from Gamma Medica-Ideas) with solid state digital detectors and special collimators is available and research is being conducted to bring dose down to 4 mCi. This may allow for a potential role in screening in the future.
A second FDA approved molecular breast imaging test is Positron Emission Mammography- PEM (Naviscan), which also involves the injection of a radionuclide and exploits the differences in metabolic activity of benign versus malignant breast lesions. After injection of 10mCi of fluorine 18 fluorodeoxyglucose (a positron-emitting radionuclide used in PET-CT examination for the detection of metastatic cancer), a pair of dedicated gamma radiation detectors are placed above and below the breast with mild breast compression to detect gamma rays. Similar to BSGI, patients are imaged in a sitting position. However, the patient must fast for about 6 hours prior to the exam, and diabetic patients must have a blood glucose check prior to FDG injection. A quiet wait time of 1 hour after injection is required for proper FDG metabolism. Each breast is then imaged for about 10 min. Afterwards the patient is advised to maintain distance from others for about 45 min because of the radioactivity that was injected. Results from a recent prospective multicenter NIH-sponsored trial showed that PEM demonstrates increased specificity at comparably high sensitivity and 26% higher PPV than Breast MR, which translates to reducing the number of unnecessary biopsies. Like BSGI, PEM may serve as alternative imaging tool for patients who may not tolerate MRI. However, unlike BSGI, PEM is currently only reimbursed for patients with known breast cancer including presurgical planning and staging, monitoring for recurrence/response to neoadjuvant therapy, and equivocal exams following a diagnostic mammographic workup.
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