Molecular Breast Imaging (MBI) and Breast Specific Gamma Imaging (BSGI):
What is it? Molecular Breast Imaging (MBI) and Breast Specific Gamma Imaging (BSGI) are both specialized nuclear medicine breast imaging techniques that require intravenous injection of a radioactive agent. Due to differences in equipment, BSGI requires a higher radiation dose than MBI.
Some centers are using MBI or BSGI for supplemental screening for women with dense breasts and also for problem solving.
How it works: The short-lived radioactive agent 99mTc-sestamibi accumulates in cancer cells more than normal cells, allowing cancer to be seen on the basis of differences in metabolism. Starting about 5 minutes after intravenous injection of the radiotracer, each breast is gently stabilized between two detectors (Figure 15. MBI), or between one detector and a compression paddle (Figure 16. BSGI), for about 10 minutes per view (for a total of 40 minutes for a routine examination), with positioning otherwise similar to mammography.
Figure 15. MBI has 2 detectors Figure 16. BSGI has 1 detector
Sometimes additional images are needed to fully include all the breast tissue. This technology does not look at the anatomy of the breast as a mammogram or breast ultrasound does; rather, it examines the functional behavior of the breast tissue (by showing differences in cellular uptake of the radioactive agent) (see Fig. 17). The radioactive agent emits invisible gamma rays, and a gamma camera is used to detect these gamma rays.
BSGI employs sodium iodide scintillation crystals and requires a higher amount of 99mTc-sestamibi activity to be administered, in the range of 15 to 30 millicuries (mCi), which delivers an effective radiation dose of 4.5 to 9 mSv. New MBI systems make use of a pair of cadmium zinc telluride (CZT) digital detectors with specialized collimators, both of which improve detection of gamma rays, allowing imaging to be performed using a lower amount of 99mTc-sestamibi, typically 6 to 8 mCi (an “off-label” dose), which delivers an effective radiation dose of 1.8 to 2.4 mSv. This is comparable to background radiation and is about three- to four-fold the effective radiation dose from standard digital mammography (0.5 mSv) or tomosynthesis (0.6 to 0.7 mSv).
Figure 17. Use of molecular breast imaging (MBI) for screening. This 65-year-old woman has heterogeneously dense breasts, with no abnormality seen on mammography (left image, MLO view – meaning image taken from a side angle). MLO MBI image (right) obtained after i.v. injection of 8 mCi (300 MBq) 99mTc-sestamibi shows intense uptake of radiotracer (arrow) in a 1.9 cm grade 2 invasive ductal cancer with negative axillary node biopsy.
Benefits: MBI, performed with a low-radiation-dose protocol, detects an additional 7 to 8 cancers per thousand women screened compared to mammography alone, and is being used at the Mayo Clinici,ii in screening research trials and now in usual clinical practice. Similar results have been observed in community practice.iii,iv
MBI and BSGI can be helpful for some women who need but cannot tolerate MRI for reasons such as kidney failure, claustrophobia or who have pacemakers or some other metallic implants.
Considerations: The radiation from this test is to the whole body, unlike mammography which is a low dose to just the breasts. Uptake of radiotracer in normal breast tissue increases in the luteal phase of the menstrual cycle, which may complicate interpretation; when performed, screening studies are typically scheduled in days 7 to 14 of the cycle in premenopausal women. The ACR Practice Parameter in 2017v suggests MBI is a potential option in supplemental screening for high-risk women and those with dense breasts who cannot undergo MRI, but cautions that the technique involves ionizing radiation to the whole body. Chart 1 below compares the radiation dose to other common imaging studies. Most of the radiation dose from sestamibi is deposited in the gastrointestinal track (gall bladder, colon) and bladder, and not the breasts. Data from atomic bomb survivors and other historical data have failed to show any increased risk of cancer from exposure to these low doses of radiation. As such, many national and international organizations recommend against associating any risk with the radiation exposure from such medical testing.
MBI and BSGI are never used in women who are pregnant. Facilities offering MBI or BSGI should have direct biopsy capability; otherwise, MRI may be needed to clarify or biopsy an abnormality detected on MBI or BSGI.
Chart 1. Graph compares the effective radiation dose (mSv) to the whole body from common medical exams (CT = computed tomography; PET = positron emission tomography). Annual background radiation is between 2 and 10 mSv (greater at higher elevations such as Denver, CO). HPS and AAPM guidelines suggest a threshold of 100 mSv below which there is no expected risk; the limit for annual exposure of radiation workers is 50 mSv. Nevertheless, radiation exposure should always be minimized (except when undergoing treatment of a known cancer).
iRhodes DJ, Hruska, CB, Phillips, SW, et al. Dedicated dual-head gamma imaging for breast cancer screening in women with mammographically dense breasts. Radiology 2011;258:106-118. Retrieved from http://pubs.rsna.org/doi/citedby/10.1148/radiol.10100625
iiRhodes DJ, Hruska CB, Conners AL, et al. JOURNAL CLUB: Molecular Breast Imaging at Reduced Radiation Dose for Supplemental Screening in Mammographically Dense Breasts. AJR American Journal of Roentgenology 2015; 204:241-251
iiiShermis RB, Wilson KD, Doyle MT, et al. Supplemental Breast Cancer Screening With Molecular Breast Imaging for Women With Dense Breast Tissue. AJR AM J Roentgenol 2016; 10.2214/AJR.15.15924;1-8.
ivHruska CB, O'Connor MK. Curies, and Grays, and Sieverts, Oh My: A Guide for Discussing Radiation Dose and Risk of Molecular Breast Imaging. J Am Coll Radiol. 2015;12(10):1103-5.
Note that the Mayo Clinic and several of its investigators receive royalties through licensing agreements for MBI.