## 4 Discussion
This phenomenon of volume reduction was also similar to the results of our previous study [8]. The PTV is the result of 5 mm expansion through the ITV center. This PTV reduction is derived from the image difference used in defining the ITV. Although the threshold method was used to delineate the internal target volume, the value of SUV\({}_{\text{max}}\) does not affect the target contouring (as the SUV\({}_{\text{max}}\) in 3D-PET, 4D-PET, and MF-PET may vary). The reduction in the ITV is a result of the noise reduction in MF-PET images. The MF technique used in this study was first collected from 4D-PET images. The signal value in the same position in different respiratory phase images for which using the statistical median effectively avoids the statistical effects of excessive height and excessively low regions that may be caused by image noise or insufficient signal in each single respiratory phase. Therefore, the MF-PET image produces an image of the difference in ITV definition in the definition of tumor volume. In addition to using PET\({}_{\text{MF}}\) images, this study also incorporated the MIP technique to integrate images within the same time window of set respiratory phases. The delineation of ITVs using the MIP technique to reconstruct PET images with a threshold of 40% of the SUV\({}_{\text{max}}\) reduces human error, and the ITV obtained by this method also accurately defines the volume covering the movement of the tumor affected by respiration [4; 17]. First, in the phantom experiment, we used the known target size as the gold standard to discuss PET\({}_{\text{3D}}\), PET\({}_{\text{4D}}\), PET\({}_{\text{MF}}\), and target in the PET\({}_{\text{Static}}\) image at rest. The tumor volume was defined by the threshold of 40% of the SUV\({}_{\text{max}}\). The percentage difference in standard. In our study, the tumor volume of PET\({}_{\text{MF}}\) and PET\({}_{\text{Static}}\) obtained by MF is closest to the gold standard, and the second is PET4D-T50%. Although the respiratory gating technique is used to eliminate the influence of respiratory motion, the reduced signal acquisition time results in a higher SNR (signal-to-noise ratio) that still affects the definition of tumor volume, while the most different is PET\({}_{\text{3D}}\) that is completely affected by respiratory motion. Our results showed that among the treatment volumes defined by different methods, PTV\({}_{\text{MF}}\) was the smallest (Tables 4 and 5). Reduction of PTVs by the MF technique indicated a better treatment outcome, as the radiation dose received by OARs was reduced while still maintaining the same therapeutic quality (Tables 6 and 7). The MF Plan resulted in significant reduction of the dose received by the lung (V\({}_{\text{5}}\), V\({}_{\text{10}}\), V\({}_{\text{20}}\), and mean dose), the heart (mean dose), and the spinal cord (maximum dose). As shown in Figure 8, the results demonstrated that the percentage difference in the PTV was correlated with the percentage difference in the V\({}_{\text{20}}\), indicating that respiratory-gated PET image reconstruction using the MF technique reduced the radiation dose of OARs and improved the outcome of treatment planning. The results of this study show that using respiratory-gated radiotherapy with a delineation method reduced the ITV to decrease the radiation dose received by normal tissues in the lung were in agreement with the findings of [PERSON] et al. [3]. This indicates that the application of PETMF for the definition of tumor volume can improve the accuracy of tumor definition for treatment planning during respiratory-gated radiotherapy, as it lowers the damage to OARs. Although there is a significant improvement in the mean dose of OARs, the lung (V5, V10, V20, and mean dose), the heart (mean dose), and the spinal cord (maximum dose) in the radiotherapy program. However, in our study, the standard deviation is also quite large.