From evolution to brave new world



DALE BAILEY PhD | Principal Physicist, Department of Nuclear Medicine | ROYAL NORTH SHORE HOSPITAL, St Leonards |  NSW |  2065

Professor in Medical Radiation Science, Faculty of Health Sciences, Hon.Affiliate, Faculty of Medicine & Health, Hon.Affiliate, School of Physics, THE UNIVERSITY OF SYDNEY


When the accompanying article was written 10 years ago, I was watching closely to see what impact the latest hybrid imaging technology of PET/MRI would have. To date, around 250 systems have been installed globally with only four in Australia:

  • two are in clinical practice; and
  • two are dedicated for research.

Contrast this with over 100 PET/CT cameras in Australia currently.

The push towards more sophisticated imaging with PET/MRI, with the myriad of different tissue contrast sequences available, has not been sustained. One reason is that conventional clinical PET/MRI is generally slower than a similar study on a PET/CT camera and, in cancer imaging, CT often provides the perfect complement to an FDG PET scan.

Another factor against PET/MRI is the larger upfront capital cost. In fact, the major development in PET in the past few years has been the introduction of very high sensitivity, long axial field of view PET cameras that can image a whole subject in a very short time and use a vastly reduced radiation dose.

When the previous article (click here for copy) was written, the maximum axial field of view for PET was around 20 cm. Today, it is possible to purchase a 2 m long PET camera combined with a conventional multi-detector CT. A large axial field of view PET system is expected to be installed at Royal North Shore Hospital, Sydney in the second half of this year.

However, this does not mean that the field of nuclear medicine and molecular imaging has in any way stalled. What we have seen is the widespread emergence of some radionuclide therapy treatments for metastatic cancer. The radiolabelling of peptides and antibodies can be used to both demonstrate the disease sites and their retention time in various tissues. Then, after substitution of the diagnostic imaging radionuclide with a therapeutic radionuclide (β– particles or even a2+ particles) on the same peptide or antibody, it can deliver treatment.

Figure 1. Current cancer control options are shown. The colours used in the wheel surrounding the subject are shown on the figure to demonstrate their application. Surgery is the most invasive approach but has the most limited coverage. If complete local resection of the cancerous tissue is achieved, however, it offers the best option for a complete cure. With a decreasing level of invasiveness follows directed targeted treatments such as interventional radiological procedures and topical applications. Radiotherapy is, in general, minimally invasive but can only treat a limited number of sites of disease. When the cancer is more widespread a systemic approach is required with medical therapies such as chemo-, immuno- and biological (inhibitors) therapies. Molecular radionuclide therapy is a systemic approach which offers the advantage of being able to assess targeting prior to treatment and can monitor therapy as it is delivered using whole body SPECT or PET imaging.

The 2012 article talks about the introduction of CT to SPECT from a multimodality imaging point of view. However, the tissue density CT data can be used to correct for photon scattering and attenuation in the SPECT images to produce quantitative results, as for Pet scanning. It turns out that the therapeutic radionuclides rarely emit positrons (β+) making PET imaging not possible.

However, many of the therapeutic radionuclides emit gamma photons and can be measured with the SPECT camera. We now have devices that can, not only demonstrate the targeting of the cancer prior to treatment but can also be used to assay the radiation dose (in grays) each tumourous tissue receives. By using the same peptide or antibody for both diagnostic imaging for treatment planning and for molecular radionuclide therapy is referred to as a “theranostic” technique (see Figure 1).

Theranostics isnow used clinically for treating castrate-resistant disseminated prostate cancer and neuroendocrine tumours, with some further theranostic approaches on the way for breast cancer, solid tumours, lung cancer and more. It’s anticipated that this approach will become a major treatment strategy for metastatic disease during the next decade. This is indeed a brave new world that we are entering with theranostics making major in-roads into the treatment of many cancers.


Dale Bailey PhD, February 2022