## 4 Discussion
The reason is that in the lung cancer tumor radiotherapy program, the therapeutic dose distribution is complicated because of the complex location of the physiological anatomical structure of the treatment area in the thoracic cavity and various organs. Although the situation can be improved by the definition of tumor volume, it may also cause differences in the dose distribution of radiation therapy because of the size of the lesion, its location, and the relative positional relationship with the surrounding organs. The patients collected in the study had inconsistent lung tumor locations and a certain degree of tumor volume between different patients. These reasons all led to a large standard deviation in the study results. We present herein the benefits of using the MF approach, which may improve the outcome of lung cancer treatment; however, there were still some limitations of the study. Although the use of a threshold of 40% of the SUVmax to delineate the target volume can directly define the metabolic tumor volume from data provided by PET images, which may avoid human error, this may still not be perfect in some cases, as skilled clinicians might define a better tumor volume for treatment based on their experience. The aim of this study was to investigate the effects of using the MF technique in respiratory-gated radiotherapy. The goal was to find an objective method, such as one that relies on PETMF images, to define the tumor volume without the need for manual or semi-manual delineation. In addition, because this study is a retrospective study, it is not possible to compare images of different reconstruction methods with other images reconstructed through the change of the respiratory gating parameter during 4D-PET angiography, in the definition of tumor volume in the treatment plan, and the effect of treatment dose distribution and OARs dose acceptance. Our results indicated that the MF technique was of clinical value for this application, although further study is still required in order to render it an ideal tool. In this study, we compared the radiation doses received by the tumor target and OARs, and found that the MF technique maintained treatment quality but reduced the radiation received by OARs using image data from patients. However, no practical application of treatment plans for patients has been performed as yet. Whether or not the application of this technique will affect the prognosis of patients or cause any side effects still needs to be studied, and long-term follow up is necessary to validate the value of this technique. Finally, we selected images from end-expiration phases (T45%, T55%, and T65%) to reconstruct PETMF images, when the target is at the window in which the tumor movement is the least for the longest duration. This helps to deliver radiation to target areas more precisely. However, in the real case of tumor treatment, the respiratory waveform is not always uniform, as seen a priori, and the respiratory waveform does not always exactly correspond with the tumor motion. These factors all may affect the delivery of radiation to the target and the surrounding tissues. Therefore, for clinical application of the MF-Plan, validation in different settings in phantoms is needed, or clinical trials are required to confirm that this technique can target lesions and benefit patients.