A Gamma Camera for Molecular Mammography

Speaker Michael O'Connor cited a study by the National Cancer Institute which shows that in women with dense breast tissue,  traditional mammogram successfully identified cancerous tumors only 40% of the time, and ultrasound 43%. But MRI successfully spots cancerous tumors at 80%. Women with dense breast tissue have a significantly increased risk of developing breast cancer, yet the sensitivity of mammography drops with tissue density. It's a frustrating contrast especially because offering these women MRI's isn't a fiscal option, as they cost about $3,000. Even with health insurance, this might not be an option for many women.

For the past six years, O'Connor and his colleagues at the Mayo Clinic have been investigating different molecular imaging techniques for screening for breast cancer, in the hope of finding a cheaper, equally reliable method to MRI. They've focused many of their efforts on scintigraphy, which images the body by catching gamma rays emitted from the patient (thanks to an injected radioactive tracer), rather than passing X-rays through them. But this technique has been difficult to apply to breast imaging because it's tricky to get the breast into the field of view of the camera without missing segments or getting signals from the rest of the body as well (which washes out the resolution). Thus, many groups are working on small field of view gamma cameras, with only one currently commercially available.

The gamma camera itself contains crystals that respond to the gamma rays by emitting a little pop of light. Collectively they create an image. O'Connor and his colleagues are putting their money on Cadium Zinc Telluride (CZT) crystals, which are currently a bit pricey, but they believe the cost will drop when they're made commercially available. These crystals can be operated at room temperature (some detectors had to be cooled to liquid nitrogen temperatures which would be a bit chilly for breast imaging!), and they have no "dead space" so you can get very close to the breast tissue.

The resolution on the scintimammography (so called when imaging the breast) using CZT crystals is striking. O'Connor showed mammography images of a breast that appeared to have no tumors or other build up, and then showed the image of the same breast imaged under the gamma camera. The tumors appeared as clear as day in the gamma image; but were completely invisible to the mammogram. In a clinical trial of dense breast tissue the gamma camera caught 10 tumors out of twelve while the mammogram only caught three. Its resolution is comparable, but not better than MRI.

Two vendors are working on making the technology commercially available, and O'Connor estimates the cost of the procedure will be about $400.

But there are some key drawbacks that O'Connor and his colleagues are still working on. First, there are some false positives from the gamma camera, identifying non-cancerous objects like pampillomas as tumors. Secondly, while the radioactive tracer is FDA approved, the treatment delivers a dose of radiation 6-7 times larger than a mammogram. While this is still below the level of natural background radiation, the group is "very sensitive" to the size of the doze and are trying to reduce it for both the patients and the radiologists who deliver the treatment.