BALTIMORE, Md--Both thallium-201 and technetium-99m are under study as agents for use in examining breast lesions for malignancy, Alan Waxman, MD, of Cedars-Sinai Medical Center, Los Angeles, said at a meeting on nuclear medicine sponsored by Johns Hopkins University.
Thallium-201 has shown good specificity and sensitivity in detecting palpable breast lesions down to about 10 mm in size, the usual limit for palpability. However, Dr. Waxman said, "while thallium is useful in this range, it is just not good enough."
Dr. Waxman's study of technetium-99m methyoxyisobutyl isonitrile (MIBI) has shown some improvement in detecting abnormalities not visible on thallium scans. Improved target-to-background ratio, he said, gives a slightly better sensitivity but also a slightly lower specificity. Nonetheless, lesions as small as 7 mm may be detected with MIBI.
Magnetic resonance (MR) imaging can locate tumors down to 4 to 5 mm in size, he said, but with less specificity (36% for MR imaging versus 81% for MIBI) due to a higher number of false positives.
Not a Screening Test
When using MIBI, Dr. Waxman employs a triple-headed camera to shorten imaging time to about 10 minutes. He suggests beginning imaging 5 minutes following MIBI injection. Tests on more than 40 patients have shown little difference between a prone or supine position of the patient, he said.
MIBI's sensitivity for nonpalpable lesions is dependent on size, and its specificity is fair to good for these lesions. For palpable lesions, MIBI has a negative predictive value of 98%, but, Dr. Waxman said, "I would still want to biopsy a woman with a palpable mass who was MIBI negative. It's a political and moral issue: You have to act on what is right in the medical world you're living in."