Radiotherapy response evaluation using FDG PET-CT - established and emerging applications.

Radiation therapy is a common component of curative cancer treatment. However, there is a significant incidence of treatment failure. In these cases salvage surgical options are sometimes appropriate. Accurate assessment of response and early recognition of treatment success or failure is therefore critical to guide treatment decisions and impacts on survival and the morbidity of treatment. Traditionally, treatment response has depended upon anatomical measurement of disease. However, this may not correlate well with the presence of disease, especially after radiotherapy. Combined positron emission tomography and computed tomography (PET-CT) imaging employs radioactive tracers to identify molecular characteristics of tissue. PET imaging exploits the fact that malignancies have characteristic molecular profiles which differ compared to surrounding tissues. The complementary anatomical and functional information facilitates accurate non-invasive assessment of surrogate biomarkers of disease activity. This article reviews the rationale for PET-CT response assessment in radiation oncology, describing current uses of 2-[(18)F]-fluoro-2-deoxy-d-glucose (FDG) PET-CT in treatment response following radiotherapy in head and neck, oesophageal, rectal and brain tumours. Emerging applications of FDG PET-CT in cervical and lung carcinomas and hepato-pancreatico-biliary (HPB) tumours particularly pancreatic carcinoma and liver metastases (post selective internal radiotherapy treatment (SIRT)) are reviewed. Finally, the limitations of FDG PET-CT are considered, highlighting areas for future development.