Measurement of metabolic tumour volume: static versus dynamic FDG scans

Background: Metabolic tumour volume assessment using positron emission tomography (PET) may be of interest for both target volume definition in radiotherapy and monitoring response to therapy. It has been reported, however, that metabolic volumes derived from images of metabolic rate of glucose (generated using Patlak analysis) are smaller than those derived from standardized uptake value (SUV) images. The purpose of this study was to systematically compare metabolic tumour volume assessments derived from SUV and Patlak images using a variety of (semi-)automatic tumour delineation methods in order to identify methods that can be used reliably on (whole body) SUV images. Methods: Dynamic FDG PET data from 10 lung and 8 gastrointestinal cancer patients were analysed retrospectively. Metabolic tumour volumes were derived from both Patlak and SUV images using five different types of tumour delineation methods, based on various thresholds or on a gradient. Results: In general, most tumour delineation methods provided more outliers when metabolic volumes were derived from SUV images rather than Patlak images. Only gradient-based methods showed more outliers for Patlak based tumour delineation. Median measured metabolic volumes derived from SUV images were larger than those derived from Patlak images (up to 59% difference) when using a fixed percentage threshold method. Tumour volumes agreed reasonably well (<26% difference) when applying methods that take local signal to background ratio (SBR) into account. Conclusion: Large differences may exist in metabolic volumes derived from static and dynamic FDG image data. These differences depend strongly on the delineation method used. Delineation methods that correct for local SBR provide the most consistent results between SUV and Patlak images.