## 1 Introduction  
In order to reduce the uncertainty caused by respiratory motion during lung cancer radiotherapy, four-dimensional positron emission tomography/computed tomography (4D PET/CT) and respiratory-gated radiation therapy have been applied [1]. The use of respiratory-gated 4D PET/CT improves the quality of lung cancer radiotherapy, as it enables delivery of the correct dose during the optimal time window of the respiratory phase [2]. In addition to new acquisition and radiation therapy techniques, obtaining a better depiction of the primary tumor decreases uncertainty related to tumor delineation, which may contribute towards motion target volumes. Methods developed in recent years that use normal tissue dosimetry have been shown to further improve the definition of tumor contours based on 4D PET/CT. These methods identify the region of interest for radiation treatment planning using imaging information from a certain respiratory phase, and enable treatment to be focused on targets of small sizes, named planning target volumes (PTVs). Reduction in PTVs may result in reduced toxicity and may potentially improve tumor treatment outcomes if used in conjunction with dose escalation [3,4]. However, the use of these methods remains controversial, and it has been suggested that better tumor contours need to be obtained during treatment planning [5]. Therefore, studies have been performed to compare the accuracy of tumor delineation by 3D PET/CT and 4D PET/CT [6,7]. In recent years, incorporation of 4D PET/CT into radiotherapy planning has been demonstrated to contribute towards more conformal treatment plans, which only shape the radiation beams and escalate radiation dose to cover the tumor. Moreover, employment of 4D PET/CT with the Motion Freeze (MF) technique can provide a clearer definition of certain tumor subvolumes in a specific section of the respiratory cycle [8].  
In order to effectively utilize 4D PET/CT imaging, improve the image quality, and enhance the accuracy of target volume definition, our research team applied the MF approach in 4D-PET imaging to generate a single 3D effective PET image. By deformable image registration, this process used 100% of the counts of the PET scan data acquired from all phases to correct for respiratory tumor movement, and enabled creation of an MF-PET image corresponding to a static frame of a specific respiratory phase. The results of our study indicated that application of MF in 3D and 4D PET can not only reduce tumor mismatching, but also effectively increases the quantitative standardized uptake value (SUV) [8]. When 4D PET/CT is used in treatment planning for lung cancer, SUV measurement is often affected by signal dispersion and image noise, which reduce the accuracy of the measurement. Use of the MF technique successfully solves this problem and assists with an accurate definition of target volumes in clinical cancer treatment planning.  
In this study, we used MF technology to assemble MF-PET images, and applied the results in respiratory-gated radiotherapy. We compared this approach with routine respiratory-gated radiotherapy and conventional 3D intensity-modulated radiation therapy, and also investigated the impact of using MF-PET imaging on target delineation and dosimetry evaluation parameters in the lung cancer treatment plan.