E16. Breast cancer: recent advances in PET imaging

Abstract

The assessment and management of breast disease involves an integrated multidisciplinary approach, with the imaging component often being multifaceted and incorporating multiple modalities. Conventional mammography and ultrasound have major roles in both diagnostic evaluation and image-guided sampling/intervention in primary breast disease. Conventional computed tomography (CT) and magnetic resonance imaging (MRI) rely on anatomical changes for diagnosis, staging, and followup of cancer. There are evolving applications for functional imaging investigations, including positron emission tomography/computed tomography (PET/CT) [1−3]. The advent of combined PET/CT over a decade ago enabled more precise localisation of isotope activity to anatomical sites, resulting in increased sensitivity and specificity when compared with stand-alone PET imaging; diagnostic accuracy was increased [1]. PET is based on the detection of photons released when radionuclides emit positrons (positively charged electrons). A positron undergoes annihilation with an electron within milliseconds of emission, releasing two photons moving in opposite directions; these are detected by coincidence imaging as they strike scintillation crystals. Spatial resolution is inherently limited by the mean positron range, with the annihilation reaction occurring up to 5 mm from the event. Respiratory movement results in limitation in resolution within lung parenchyma, with nodules 10 mm in size regarded as often below the resolution of PET. Developments in respiratory gating during acquisition − 4D scanning with ‘bin allocation’ of data − increases the sensitivity of assessing lung lesions [4]. Detector technology is evolving with the development of digital detectors. In most centres the CT component of an integrated PET/CT study is currently low-dose and of relatively low resolution, enabling anatomical placement of lesions but not evaluation of inherent tissue characteristics or very clear discrimination between adjacent structures. Newer PET/CT systems incorporate multi-slice (up to 128) CT, enabling very high resolution, multiplanar and volume rendered images; in some institutions there is a gradual shift towards undertaking ‘diagnostic’ intravenous contrast-enhanced CT as part of the PET/CT study. This enables higher resolution and delineation of structures, with increased conspicuity and identification of lesions, particularly in organs such as the liver where uptake of tracer maybe heterogeneous. The utilisation of contrastenhanced ‘diagnostic’ CT as part of PET/CT may simplify the patient pathway with a single investigation being undertaken rather than a number of potential alternative studies. The true potential benefit of this approach may be established if robust cost/benefit analysis studies are undertaken. The main isotope utilised in current clinical oncological practice is 2-deoxy-2-( 18 F)fluoro-D-glucose (FDG), a glucose analogue, with the positron-emitting radioactive isotope fluorine-18 substituted for the normal hydroxyl group at the 2 � position in the glucose molecule [1].