Development of Positron Emission Mammography

The development of PET (Positron Emission Tomography) technology applied to the detection of breast cancer is the objective of a project being carried out by the Consortium PET-Mammography. The high incidence of breast cancer and the relative inefficiency of the conventional detection methods (X-ray and ultrasound mammography) suggest the need for better imaging techniques and improved equipment. The project answers this need by developing new equipment with higher sensitivity for beast cancer detection.

Project Description

Breast cancer is the most frequent malign neoplasm in women. According to the American Cancer Society, 1 in 9 women will develop invasive breast cancer during their life. Conventional X-ray mammography has detection limitations especially in dense breast tissues. According to published results, the overall sensitivity for cancer tumor detection is around 80%, depending on the type of breast. For dense breasts the sensitivity drops to 70% with a lower limit in size for a detectable tumor of 10 to 20 mm. The same limitations occur with complementary approaches as ultrasound imaging.  On the other hand, current methods have a high rate of false positive results: 60 to 85% of the biopsies following an imaging indication obtained with X-rays or ultrasounds do no correspond to malign pathology.

The Positron Emission Mammograph (PEM) prototype developed in this project is intended to evaluate PET technology in the diagnosis of malign neoplasm in the breast and of ganglion loco-regional invasion. PET (Positron Emission Tomography) is an image technique used in the detection and characterization of malign carcinoma. It consists in the injection of a substance marked with a positron emitter (usually ¹⁸F-fluoro-deoxy-glucose, FDG) and in the detection of photons emitted in opposite directions. Since there is an increase in glucose consumption in cancer cells, FDG is an indirect marker of cell proliferation. The detection of the emitted photons allows the reconstruction of an image revealing the dimension and position of the tumor.

In what concerns breast cancer diagnosis, results obtained so far with whole body PET equipments and the FDG tracer are very positive despite of poor statistics. For localized breast cancer, the sensitivity varies between 77% and  100%, and the specificity between 88% and 100%, independently of breast density. However whole body PET systems are expensive and bulky and not adapted to a systematic screening. Because of their open geometry they are also very sensitive to the background emitted from the chest which reduces the detection sensitivity in the breast. Moreover, the spatial resolution and rate performance are still poor.

Relative to whole body PET, the dedicated Positron Emission Mammograph is expected to improve significantly the sensitivity to breast cancer in particular to small tumors. The new equipment is expected to have an image resolution of the order of 1-2 mm and to achieve a very high data acquisition rate, improving the detectability of small (~ 1 mm) tumors by more than two orders of magnitude. The expected performance of the device will allow also to reduce the injected doses and to shorten considerably the examination time, when compared to the present PET exams. Furthermore, the Positron Emission Mammograph is a compact equipment, more economic that the present whole body PET systems.

Technological developments in various complementary areas are underway. High density and fast scintillation crystals coupled to special photosensors are exploited to achieve the desired performance. Dedicated low-noise electronics and high-rate data acquisition systems allow boosting the sensitivity in a compact mechanical construction. Digital techniques are investigated to improve the time resolution and the rejection of uncorrelated photons. New image reconstruction algorithms adapted to the PEM geometry are being developed. Validation of the PEM technology will be obtained with clinical tests carried out with the new equipment.

Future Prospects

The technology developed in the project may be used in the future to build other dedicated PET systems or to build multi-modality devices combining PET with other medical imaging techniques. Especially interesting is the ability of the technology to stand very high magnetic fields making it suitable for combination with MRI.