Breast Specific Gamma Imaging (BSGI) is a unique FDA-approved approach for detecting breast cancer based on molecular imaging that detects the physiological difference in cancer tissue as opposed to traditional imaging techniques that rely on physical differences in tissue structures, according to study data presented on Wednesday at the 2008 meeting of the Radiological Society of North America (RSNA). There exist significant limitations to mammographic detection of breast cancer in women with dense breasts: more than 50% of women under 50 have mammographically dense breasts, more than 33% of women over 50 have dense breasts. Breast density is a strong independent risk factor for the development of breast cancer, with a 4-6 fold increase risk of breast cancer. "In women with dense breasts, cancers which are detected with mammography are larger and later stage, and also in women with dense breasts, 35% of breast tumors are not mammographically visible," explained Rachel F. Brem, MD, Director, Breast Imaging and Intervention, Department of Radiology, George Washington University, Washington, DC, who conducted the study. "The challenge we face is how we can improve breast cancer detection for women with dense breasts."

Mammography and ultrasound are anatomic approaches to imaging breasts, asking ‘what does breast cancer look like?’ BSGI is fundamentally different from, and complimentary to, mammography and tomosynthesis because BSGI looks at the physiology and metabolic activity of the breast tumor. This novel approach examines how breast cancer tissue functions differently than surrounding normal tissues and harnesses that information for the improvement of breast cancer detection. BSGI is a form of molecular breast imaging in which the findings are based on the physiologic characteristics of the tumor compared to the normal surrounding breast tissue. In a BSGI procedure, the woman is seated comfortably in a chair with a device that can be imaged much like mammography. "During the BSGI, a very low dose tracer that is a safe molecular imaging compound that has been used for decades, Technitium Sestamibi, is injected into a vein in the arm and requires no additional shielding." Dr. Brem stated, adding, "The study takes about 40 minutes to complete and the images can be read immediately afterwards."

"Findings are based on the physiologic characteristics of the tumor compared to the normal surrounding breast tissue," explained Dr. Brem. (Read the abstract here.) The principle is that the uptake of the tracer is proportional to blood flow and mitochondrial activity, which is a function of the overall metabolic activity of the breast, and the uptake rate is much higher is cancer tissue than in normal tissue. Dr. Brem provided an example, "In one 48 year old patient, with a known breast tumor, a second unsuspected tumor was found using the BSGI procedure." Previous studies using BSGI have shown that it improves cancer detection in high risk women with normal mammograms and physical examinations, that BSGI can detect breast cancer as often as MRI, also that BSGI can detect the earliest breast cancers, even those as small as 1 mm. "Our preliminary data suggested that BSGI can detect unsuspected additional cancers in women with newly diagnosed breast cancer," said Dr. Brem. "This is very important because the surgical management is going to be dependent on the number of tumors in each breast."

Since BSGI was FDA-approved in 1999, over 80,000 US women have been imaged and BSG is currently available at over 50 US sites. With this increased use of BSGI, comes the continued need to study detection of breast cancer in women with suspicious breast lesions or newly diagnosed breast cancer. "The purpose of this study then was to determine how often BSGI identifies additional unsuspected areas of breast cancer in women with one breast tumor," explained Dr. Brem. All women were imaged with a high resolution breast specific gamma camera (Dilon 6800) comfortably in a seated position. The images obtained in the same view as those obtained with mammograms: CC and MLO view with any additional images obtained as deemed necessary by interpreting radiologist. "BSGI-detected lesions were considered to be additional sites if in the breast opposite the known cancer, in a different area of the same breast, or in the same quadrant but separate from the index lesion," said Dr. Brem.

The study reported on 159 women who were aged 29 to 93 years, mean 54 years, with a single biopsy proven cancer. Of these women, 39% were premenopausal and 61% were postmenopausal. Regarding history of breast cancer: 12% personal history of breast cancer, 43% breast cancer in first degree relative, and 5% family history unknown because they were adopted. Dr. Brem noted "Interestingly, 73% of the women included had dense breasts."

In this study, 56 additional areas were detected with BSGI. Of those, 45/56 (80%) of additional foci underwent biopsy; however 3 did not because they decided to have mastectomies, and 8 decided to have follow up. Occult cancers were found in 14/45 (31%) of lesions that underwent biopsy were malignant, 14/56 (25%) of additional foci were malignant, and in 14/159 (9%) of women had occult foci of cancer detected with BSGI. "It is important to remember that prior to this examination, these women had no indication at all that there was a second foci," Dr. Brem told MedicExchange. As in many cancers, the size of the tumor at the time of breast cancer diagnosis is very important. The mean size of the tumors detected in this study was 1.1cm and the smallest tumor detected was 1 mm. Of those, 7 were less than 1cm, the smallest size of a early diagnosed breast tumor, so 50% of occult cancers detected with BSGI were less than 1 cm. The total unsuspected cancers were 9/14 (64%) in the same breast, and 5/14 (36%) unsuspected cancers in the opposite breast.

In conclusion, BSGI detected occult cancer in 36% of women who underwent biopsy, in 29% of women with additional suspicious areas, and in 9% of women with unsuspected cancer. At least 50% of occult cancers detected were less than 1 cm. BSGI is an excellent complementary examination to mammography for women at increased risk for breast cancer with normal mammograms and physical examination. The equipment costs about $250,000 and the injected tracer costs about $75, relatively cost effective compared to breast PET, with BSGI requiring no shielding and administering half the radiation. Also, BSGI could be used to screen for breast cancer, while PET is used only for those women who already have breast cancer. BSGI will set off a radiation detection device for merely one day after the procedure due to the low dose of radiation. Patients can carry a note with them for the next day that they can show to a security guard. "BSGI is a comfortable test for the patient, which is commercially available and which approaches breast cancer detection in a fundamentally different way of looking at breast cancer than mammography and ultrasound," Dr. Brem concluded.

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