TH-D-213A-01: An Evaluation of FDG-PET Uptake Thresholds for Head & Neck Target Definition Based On Local Regions of High Inter-Observer Concordance

Purpose: Thresholding of 18 F fluorodeoxyglucose (FDG) uptake imaged with positron emission tomography(PET) has been proposed for radiotherapy target delineation. However, there exists no consensus on the threshold best corresponding to disease extent, due to a lack of truth for validation. The purpose of this investigation was to establish criteria under which a fixed FDG‐PET threshold may be useful for target delineation. An image‐based surrogate of target definition ‘truth’ is proposed, derived from a measure of the concordance between multiple expert observers defining the gross tumor volume (GTV) with FDG‐PET and CT. A method was developed that can automatically localize these regions and was used to evaluate the coincident FDG uptake threshold. Methods and Materials: 10 patients with head and neck (H&N) cancers underwent FDG‐PET‐CT imaging using a hybrid scanner (Biograph 16, Siemens Medical Systems). For each patient, 8 observers specializing in H&N cancers delineated the GTV on concurrently‐displayed FDG‐PET‐CT. Regions of high target definition concordance were localized by forming the union of observer target definitions, applying a gradient filter and selecting an intensity threshold equivalent to 6 of 8 observers. At regions of high concordance not attributable exclusively to CT information, the FDG uptake threshold relative to maximum uptake in the GTV was measured. Results: In 4 patients, the mean FDG uptake threshold was 40.9%. In regions of concordance not attributable to CT, the mean threshold in 6 patients was 32.8%. Conclusion: A method was developed for localizing regions of high inter‐observer concordance. These regions served as a surrogate truth for disease extent against which FDG‐PET uptake thresholds were evaluated. For primary H&N cancers, a threshold of approximately 30–40% was able to delineate the tumor at regions of the target boundary where CT information was not definitive. The application of uniform thresholds for H&N target delineation is not recommended.