Global disease score (GDS) is the name of the game!

Abstract

For some decennia, CT and MRI have with beautiful images predominated medical in vivo imaging. However, because of their limited sensitivity and structural nature, they have not provided quite the key to improved understanding of disease or better solution to clinical problems that their impressive images promised that they could. Therefore, it is time for molecular imaging in the shape of PET/CT and PET/MRI to utilize the unique advantages of PET and hybrid imaging to accomplish these very important challenges. This requires easily accessible and reliable quantification procedures, which can assess the entire disease burden in the body and provide a simple score expressing the extent and activity of disease within minutes. In this editorial, we argue that the global disease score (GDS) is this measure, and must replace the maximal standardized value (SUVmax), which is an easily accessible, but often misleading, oversimplification. PET was conceived in the late 1950s and came to birth in the 1970s. So did CT and MRI. They shared the same tomographic principle, but because of their higher spatial resolution, CT and MRI for some time put PETon the sideline, until a change began with the emergence of hybrid PET/CT at the turn of the millennium and was reinforced with the advent of PET/MRI approximately 10 years later. The wonderfully sharp images one can achieve with CT and MRI have helped countless patients and thousands of doctors around the world to form an impression of what was wrong and what to rectify. However, the enthusiasm has gradually been damped by the realization that these modalities are hampered by nonnegligible limitations, meaning that the tissue changes they depict are typically late-occurring events that seldom appear until cure is no longer an option or lesions no longer represent active disease. Together with their suboptimal sensitivity, these are the main reasons why CT and MRI have not delivered what they were supposed to in the understanding and management of a number of serious disorders including cancer and cerebral, cardiovascular, and musculoskeletal diseases. Thus, it is time for molecular imagingwith PET to leave the sideline and step into play. Besides being molecular and possessing a thousand times higher sensitivity than CT and MRI, PET is an inherently quantitative modality. Achieving optimal utilization of this potential is not straightforward and is something that has still not reached a stage where measurements at different institutions are directly comparable. For PET to become a clinical success inmanymore diseases, it is necessary to replace easily accessible measures like SUVmax, SUVpeak, and SULmax [1] with conceptually more correct indices of disease. By representing tracer uptake in only a few of the body’s diseased cells, these values are not true indicators of any disease. They disregard the amount, extension, and activity of disease and the fact that many, if not most, diseases are heterogeneous. However, to replace them with other, more comprehensive and more accurate quantities is a challenge, as it requires great computing power and fast, user-friendly, and reproducible computer programs effectively providing proper segmentation and correction for background uptake and partial volume effect, of which the latter plays a major role especially in small lesions [2, 3]. We suggest instead a single number, the GDS, which expresses the amount and extent of disease and its activity. The concept was introduced already in 1993 for FDG-PET imaging of the brain [4], and can be used for the entire body, tumor tissue, or specific organs such as the heart [5–7]. Semi-automated programs enabling the calculation of GDS and other parameters have for some years been used to This article is part of the Topical Collection onAdvanced Image Analyses (Radiomics and Artificial Intelligence)