At this week's European Congress of Radiology (ECR 2009) in Vienna, Austria, researchers examined some of the imaging modalities currently being employed for treatment assessment, in a special focus session entitled "Assessment of tumour response".
"Morphology works okay; we use it every day, but we need to recognise that it doesn't do everything it was meant to," said Anwar Padhani, radiologist and head of imaging research at the Paul Strickland Scanner Centre in the UK. "Some radiologists are just shrugging their shoulders and saying: 'I can't do any better than this', but they could if they knew how."
Currently, FDG-PET is the most widely used technique for acquiring functional information from tumours treated with chemotherapy or chemo-radiotherapy. The increased popularity of this technique has been boosted by the addition of CT to PET scanners, says Rodney Reznek, professor of diagnostic imaging at Saint Bartholomew's Hospital, UK.
Reznek - who introduced and chaired the ECR session - predicts that FDG-PET/CT will ultimately replace CT as the oncology imaging "workhorse" for many tumours. He also believes that PET will be added to the RECIST (response evaluation criteria in solid tumours) guidelines, a set of published rules that define whether cancer patients improve, stay the same or worsen during treatments.
FDG-PET works particularly well, for example, for monitoring patients treated with the cancer drug Imatinib. "The problem with assessing a response to Imatinib using traditional methods is that you wouldn't see a change in size for some time," Padhani explained. "With PET/CT, you can see a drop in FDG uptake within a week. If you see that change, you can predict that this patient is going to do quite well, that the therapy is working."
However, FDG-PET isn't suitable for monitoring pathophysiological changes induced in all types of cancers or by all types of therapy. Nor is FDG uptake specific enough in assessing the effects of drugs that attack more specific physiological processes such as hypoxia, angiogenesis and cellular hyperproliferation.
Researchers are investigating other potential PET radiotracers to image this kind of activity. These include 18F-fluorothymidine for imaging cellular proliferation, 18F-fluoromisonidazole for assessing tumour hypoxia and 18F-Annexin V for apoptosis imaging. Dynamic MRI and functional CT, both of which can monitor blood flow, may also be ideal for measuring the efficacy of anti-angiogenic drugs.
MRI methods
Padhani told the ECR delegates about some of the functional MRI methods that, while currently less prevalent than FDG-PET, are being employed more and more for evaluating tumour response. He noted that such techniques are particularly suited for monitoring response to novel anti-cancer therapies, many of which are cytostatic (inhibitors of tumour growth).
Several common MRI techniques can be used to monitor response to therapy. For example, altered metabolism can be monitored using MR spectroscopy, hypoxia via blood-oxygenation-dependent MRI, cell proliferation using diffusion MRI and angiogenesis using dynamic contrast enhancement.
Padhani points out that the use of functional imaging techniques to assess tumour response is going to be a more difficult task than just looking at lesion size. A good understanding of the technique being deployed and the biological processes being depicted will be crucial, he emphasized.