Scintimammography with a traditional gamma camera has limitations, including poor spatial resolution, excessive lesion-to-detector distance, and inability to image in multiple positions or positions comparable to mammography. Recent advances in technology, however, have led to the development of high-resolution breast-specific gamma cameras that easily fit into a breast imaging practice. Researchers have found this technology useful in evaluating indeterminate mammograms, particularly in women with dense breasts and a family history of breast cancer.
Dr. Deborah Rhodes and colleagues at the Mayo Clinic in Rochester, MN, tested a prototype dedicated breast gamma camera on 40 women scheduled for biopsy (Mayo Clinic Proc 2005;80[1]:24-30). Researchers dubbed the technique, which uses technetium-99m sestamibi, molecular breast imaging (MBI).
MBI detected 33 of the 36 malignant breast lesions, with three false negatives (one due to positioning error). Sensitivity was 92%. Histopathology revealed 23 invasive ductal carcinoma, four ductal carcinoma in situ, three invasive lobular carcinoma, four mixed invasive ductal and lobular carcinoma, one lobular carcinoma in situ, and one invasive papillary carcinoma. Two-thirds of the lesions were 1 cm or smaller, and MBI detected 19 of the 24 (86% sensitivity). MBI detected all larger malignancies and identified additional lesions not seen on mammography in four patients.
Dr. Leonard R. Coover at Hamot Medical Center in Erie, PA, evaluated 37 women with dense breasts and a personal history of breast cancer. Using a commercially available gamma camera (LumaGEM, Gamma Medica-Ideas), they found that MBI detected all five cancers, including three that were undetectable by clinical exam or mammography, even on retrospective review (J Nucl Med 2004;45:553-558). The LumaGEM attaches to existing upright mammography systems in place of the film bucky.
Dr. Rachel Brem at George Washington University tested another commercially available system (Dilon 6800, Dilon Technologies) and found a high sensitivity for detecting ductal carcinoma in situ and invasive lobular cancer. Brem reported the findings at the National Consortium of Breast Centers meeting in March in Las Vegas. Researchers evaluated 117 women with breast cancer scheduled for biopsy. MBI identified all five women with invasive lobular carcinomas, which ranged in size from 15 mm to 60 mm. There were no false-negative studies. MBI also identified 12 of 13 women with DCIS, resulting in 92% sensitivity. The pathologic size ranged from 1 mm to 22 mm.
"While MRI has a sensitivity of greater than 90% for invasive cancers, it has a sensitivity of only 64% for DCIS," Brem said.
At the 2005 RSNA meeting, Brem's group reported a study that compared MBI with MRI in 20 women with 29 indeterminate lesions on mammography. The sensitivity of MBI was 89%, compared with 100% for MRI, but MBI's 70% specificity was significantly better than MRI's 25%. The positive predictive values for the two modalities were 57% for MBI versus 38% for MRI, while both techniques showed similarly high negative predictive values.
When MRI was first touted for problem solving in mammography, Dr. William Boren of West Valley Imaging Center in Las Vegas was skeptical. He looked into breast-specific gamma imaging.
"I thought MRI was too expensive and time consuming, and it had an unreasonable false-positive rate," Boren said.
After purchasing the compact Dilon 6800 scanner, his positive biopsy rate, which had paralleled the national average of one in five, improved to one in two.
"That's a huge advantage," he said. "We are biopsying fewer cases that are negative."
The exam takes about 30 minutes and the scanner costs $30,000, compared with 45 minutes and $2 million for an MR exam and scanner. Boren noted that patients prefer the nuclear scan to MR because they don't have to lie on their stomach, they deal with the same mammography technologists, and the study is shorter. He has a 3T MR scanner with a dedicated breast coil and has begun a comparison of the two modalities. Initial results show that both MRI and MBI have similar high sensitivity rates, but MRI's specificity is about 25% compared with 66% for MBI.
Both commercial vendors are developing a gamma-guided biopsy system to help localize lesions found with MBI.
