Abstract
Purpose: Positron emission tomography (PET) of lung tumors suffers from breathing‐motion induced blurring. Gated PET ameliorates motion blurring and enables visualization of lung tumor functional uptake throughout the breathing cycle but has achieved limited clinical use in radiotherapy planning. We propose a process for generating a gated PET maximum intensity projection (MIP), a 3D projection of the 4D image set comprising gated PET images, as a technique to efficiently and quantitatively incorporate respiratory‐correlated PET information into radiotherapy treatment planning. Methods: 4DCT and respiratory‐gated FDG‐PET list‐mode data were acquired of 3 patients with a total of 4 small (4–18cc), well‐encapsulated lower‐lobe lung tumors. Internal target volumes (ITVs) for the lung tumors were generated by threshold‐based segmentation of the PET‐MIP images and the un‐gated PET images, and by manual contouring of the tumor in a 4DCT‐MIP image. The normalized overlap volumes and relative volumes of ITV<sub>PET‐MIP</sub> and ITV<sub>UNGATED‐PET</sub> with respect to ITV<sub>CT‐MIP</sub> were compared. The images were also visually compared. ITV<sub>CT‐MIP</sub> was considered the gold standard for these tumors having CT‐visible morphology. Results: The mean and standard deviation normalized overlap and relative volumes between ITV<sub>PET‐MIP</sub> and ITV<sub>CT‐MIP</sub> were 0.68+−0.07 and 1.07+−0.42, respectively, averaged over all 4 tumors and 5 segmentation‐threshold values. The mean and standard deviation normalized overlap and relative volumes of ITV<sub>UNGATED‐PET</sub> and ITV<sub>CT‐MIP</sub> were 0.47+−0.12 and 0.69+−0.56 respectively. Conclusions: PET‐MIP images better match CT‐MIP images for this sample of four small CT‐visible tumors, as compared to un‐gated PET images, based on the metrics of volumetric overlap and relative volumes as well as on visual interpretation. The PET‐MIP is a way to incorporate 4DPET imaging into the process of lung tumor contouring that is time‐efficient for the radiation oncologist and involves minimal effort to implement in treatment planning software because it requires only a single PET image beyond CT‐alone contouring.
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