 Abstract of the lecture presented at the 12th ASEAN Association of Radiology Congress, Sep 2004 by Professor Rodney J Hicks, Director, Centre for Molecular Imaging, the Peter MacCallum Cancer Centre, Melbourne, Australia
The most rapidly growing diagnostic imaging modality of the past few years has been combined PET/CT. In the United States, these hybrid devices are significantly outselling stand-alone PET scanners. In Australia, the last 4 PET devices installed have all been hybrid instruments. Although this growth has been largely based on perceived diagnostic and, to some extent, aesthetic advantages, there is little doubt that the capabilities of combined anatomical and metabolic imaging extend beyond the simple fusion of images and the greater diagnostic confidence that this can provide.
PET/CT offers the ability to obtain unique information regarding the nature and location of disease processes. One of the best examples of this is the use of PET/CT for anatomically precise but metabolically guided biopsy. While histopathological examination remains an important “gold standard” for diagnosis it can suffer from sampling errors, particularly for diseases that are characterised by tissue heterogeneity or where structural appearances don’t allow localisation of the specific site of disease.
Superimposition of metabolic imaging data on regional anatomy is also of great potential utility in the planning of locoregional therapies. By defining tissue relations of metabolic abnormalities, PET/CT can instruct surgeons regarding the best operative approach.
Similarly by differentiating disease sites from secondary phenomena like pulmonary collapse, PET/CT can better define radiation treatment volumes. As more sophisticated treatment delivery systems are developed, better delivery planning is also required. This is particularly true for highly conformal treatments like intensity-modulated radiotherapy (IMRT). The tighter the treatment margins, the more precise the definition of disease extent needs to be since geographic misses are likely to result in early local failure.
PET/CT is increasingly being incorporated into radiotherapy planning at our facility, particularly for diseases that are known to be sub-optimally staged by conventional imaging techniques. These include lung and oesophageal cancer. Furthermore, the ability to simultaneously evaluate morphological and metabolic response is also likely to significantly improve evaluation of response to therapy. There are already data suggesting that PET is a superior technique to CT for therapeutic monitoring but factors such as secondary peri-tumoral inflammation as a result of treatment can compromise assessment of response and could be aided by availability of structural correlation.
As the installed base of PET/CT scanners increases there is also a great opportunity to extend the potential applications of metabolic imaging beyond cancer evaluation with FDG. A range of other PET tracers exist that are already showing clinical promise and that could easily be produced in commercial quantities for routine diagnostic use. Examples include F-18 fluoride bone scans, and receptor imaging. Using a range of tracers with PET/CT would lend the potential sensitivity of molecular imaging to the specificity of structural imaging.
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