Segmentation-free direct tumor volume and metabolic activity estimation from PET scans

Tumor volume and metabolic activity are two robust imaging biomarkers for predicting early therapy response in F-fluorodeoxyglucose (FDG) positron emission tomography (PET), which is a modality to image the distribution of radiotracers and thereby observe functional processes in the body. To date, estimation of these two biomarkers requires a lesion segmentation step. While the segmentation methods requiring extensive user interaction have obvious limitations in terms of time and reproducibility, automatically estimating activity from segmentation, which involves integrating intensity values over the volume is also suboptimal, since PET is an inherently noisy modality. Although many semi-automatic segmentation based methods have been developed, in this paper, we introduce a method which completely eliminates the segmentation step and directly estimates the volume and activity of the lesions. We trained two parallel ensemble models using locally extracted 3D patches from phantom images to estimate the activity and volume, which are derivatives of other important quantification metrics such as standardized uptake value (SUV) and total lesion glycolysis (TLG). For validation, we used 54 clinical images from the QIN Head and Neck collection on The Cancer Imaging Archive, as well as a set of 55 PET scans of the Elliptical Lung-Spine Body Phantom™with different levels of noise, four different reconstruction methods, and three different background activities, namely; air, water, and hot background. In the validation on phantom images, we achieved relative absolute error (RAE) of 5.11 % ±3.5% and 5.7 % ±5.25% for volume and activity estimation, respectively, which represents improvements of over 20% and 6% respectively, compared with the best competing methods. From the validation performed using clinical images, we found that the proposed method is capable of obtaining almost the same level of agreement with a group of trained experts, as a single trained expert is, indicating that the method has the potential to be a useful tool in clinical practice.