MacManus explained that for many cancers, disease staging using 18F-FDG PET is more accurate and can predict survival better than conventional staging methods. In lung cancer, for example, PET frequently changes lymph node staging compared with CT-based techniques.
"Another evident benefit is patient selection," he said, noting that PET excludes about 30% of patients with incurable disease from futile radiotherapy treatment. "If PET/CT is used, the likelihood that patients make it through is reduced even further."
One area that's been studied in some depth is the relevance of PET to non-small cell lung cancer (NSCLC). In research comparing CT-based treatment planning with PET or PET/CT planning for NSCLC, all studies showed that patients undergoing PET had different plans with and without PET. Here, PET provided valuable information for defining radiotherapy target volumes and for contouring lymph nodes that are not picked up by CT scans.
It's important to note, however, that the role of PET differs between tumour types. In head-and-neck cancers, for example, PET increases the accuracy of lymph node staging but is not as suitable for contouring the primary tumour. What's more, MacManus notes that there's still a distinct lack of studies directly demonstrating the patient benefit of PET-based radiotherapy for other malignancies.
Despite the lack of high-quality evidence of its clinical benefit, MacManus reckons that for many cancers, PET-based radiotherapy planning should be performed. "For me, the evidence of superior staging is sufficient to use PET routinely for radiotherapy planning in those cases," he concluded. "But carefully thought out procedures for such planning are imperative."
Targeted tracer
One PET variant that shows great promise for both patient selection and predicting treatment response is that of hypoxia imaging. Here, enhanced tracer uptake can help predict radioresistance, enabling the use of confounding strategies such as dose painting to boost the radiation to hypoxic subvolumes.
And while 18F-FDG is the mainstay of clinical PET, there are many other radiotracers emerging that can image specific aspects of tumour biology. Speaking in the same symposium, Morand Piert, of the University of Michigan Medical Center (Ann Arbor, MI), cited tracers such as 18F-MISO and 18F-FAZA as showing promise for hypoxia imaging.
Another study highlighted by Piert examined the radiotracer Cu-ATSM. This work showed that uptake of Cu-ASTM was not positive in responders but was high in non-responders. Meanwhile, FDG uptake was positive in both responders and non-responders had a high. "This means that hypoxia tracers are able to differentiate prior to treatment," he explained. "Therefore it's possible to change treatment based on hypoxia imaging."
Piert also detailed some other PET tracers that have been proposed for radiotherapy treatment planning. These include amino acid tracers such as 11C-methionine, which shows particular promise for use in the brain. MET has demonstrated the ability to differentiate brain tumour versus radiation-induced necrosis with a sensitivity and specificity of 80-90%. Another example is 11C-choline, a membrane synthesis tracer that's proving useful for monitoring the treatment response of prostate and bladder cancers.
"We can use these radiotracers for radiotherapy by including metabolic information into treatment plan," he said, noting that "we have to validate them further first."