SUVmax of 2.5 should not be embraced as a magic threshold for separating benign from malignant lesions

Abstract

Lesions of an indeterminate nature are often encountered in images such as those from CT and MRI. Examples include solitary pulmonary nodules and various incidentalomas that may be found in any body region. FDG PET can be useful in differentiating benign from malignant lesions, given the fact that malignant lesions generally have a higher glycolytic rate and consequently higher FDG uptake. Both visual image interpretation and (semiquantitative) standardized uptake value (SUV) measurements may be used for this purpose. Some clinicians advocate the use of a maximum SUV (SUVmax) threshold of 2.5 to separate benign from malignant lesions, which is based on the results of several old studies that were published more than 10 years ago [1, 2]. However, this approach suffers from major shortcomings, which may lead to patient mismanagement. This communication aims to clarify several important issues on the validity of using an SUVmax threshold of 2.5 to differentiate benign frommalignant lesions. First, SUVmeasurements are affected bymany parameters, including the equipment used, the physics, and biological factors. Partial volume and spillover effects, attenuation correction, the reconstruction method and parameters for scanner type, the count noise bias effect, radiotracer distribution time (i.e. the time between radiotracer injection and imaging), competing transport effects, and body size all affect SUV measurements considerably [3]. Because of the variability in PET acquisition in different institutions [4], interinstitutional SUV measurements are likely to vary too. Consequently, thresholds such as the SUVmax of 2.5 that have been reported as diagnostically useful by research groups in some institutions [1, 2], may not be useful at all in other institutions if different FDG PET protocols are used. In this context, initiatives such as the “EANM procedure guidelines for tumour PET imaging”, which have been shown to significantly reduce variability in SUV across different centres [5], are of crucial importance. One important factor that should not be overlooked is the effect of partial volume effects on SUV measurements [6, 7]. Because of the relatively low spatial resolution of PET, significant averaging of pixel intensities of lesions with the surrounding tissues occurs. Motion blurring (e.g. due to patient, cardiac and respiratory motion, and peristalsis) further leads to undesired averaging of pixel intensities of lesions with the surrounding tissues. If not corrected for, partial volume effects may lead to inaccurate (underestimated) measures of the true FDG activity, especially in small lesions [8–10]. Several methods can be used for partial volume correction, and these can be divided into methods applied at the regional level (e.g. use of recovery coefficients, geometric transfer matrix approach, and deconvolution) and methods applied at the pixel level (e.g. partition-based correction, multiresolution approach, fitting method, the so-called “maximum a posteriori” approach, and kinetic modelling) [9]. Partial volume correction has been shown to improve accuracy of SUV without decreasing (clinical) test–retest variability significantly, and it has a small but significant effect on observed tumour responses [10]. Unfortunately, no general, widely accepted solution to the partial volume effect problem has yet been found. There is an urgent need for a standard widely adopted method to deal with partial volume effects, because this will T. C. Kwee :M. G. E. H. Lam Department of Radiology and Nuclear Medicine, University Medical Center Utrecht, Utrecht, The Netherlands