Tumor volume delineation using segmentation by thresholding on PET images: influence of source-to-background ratio, size and lesion motion

Abstract

PET imaging is increasingly used in radiotherapy department to assist in volume tumour contouring. In this work several factors affecting the volume of the lesion determined on PET images are investigated: lesion size, background concentration and lesion motion. The measurements are carried out on a phantom with different sizes spheres filled with FDG. Threshold values were determined as a percentage of the maximal activity in the inserts. It is found that the optimal threshold value is around 40% for most of the objects, with a slight influence of background radiation. For the smaller objects, less than 17 mm in diameter, the threshold increases up to 20% for objects of 10 mm in diameter. When applying the movement, there is not a optimal threshold value. Difference between large and small objects becomes more significant when applying motion. Thus, objects less than 13 mm are on the detection limit for some background values. The recovery coefficient curves obtained in each case are shown for discussion.