I. Introduction to functional imaging techniques Functional imaging of tumors refers to the non-invasive display of metabolic, biochemical, physiological, molecular, genotypic and phenotypic characteristics of tumors. Current functional imaging techniques include positron emission tomography (PET), single photon emission computed tomography (SPECT), magnetic resonance spectroscopy (MRS), and optical imaging (OI). resonance spectroscopy (MRS), optical imaging, etc. PET shows positron emission isotopes or radionuclide labeled tissue or chemically specific molecules that can be selectively absorbed by some specific tissues. The absorbed radionuclide emits positrons, resulting in the release of gamma rays that are detected by the scanner, producing an image of local radioactivity. The most commonly used radionuclides are fluorine-18, carbon-11 and oxygen-15. the most commonly used tracer is a glucose derivative, fluorodeoxyglucose (18F-fluorodeoxyglucose, FDG). SPECT uses a γ-camera rotated around the patient to produce 3D somatoscopic images of the isotopic distribution of the injected γ-rays. γ-cameras used for SPECT cannot detect simultaneous γ-rays as well as PET, and therefore the spatial resolution of their images is lower than that of PET. MRS shows greater potential for tumor imaging. MRS is essentially an extension of conventional MRI that can detect compounds that are biochemically more important than water and fat molecules, compare normal and tumor tissues at the level of cellular metabolites, and reflect the nuclear location within the molecular and surrounding chemical environment. Optical imaging allows real-time, non-invasive application of rotational light and molecularly specific contrast agents to display epithelial tissue for early detection of epithelial lesions and smaller superficial tumors, and also allows real-time evaluation of surgical boundaries at the time of surgery. Different optical imaging techniques apply different physiological parameters, taking into account the interaction of light and tissue. II. Functional imaging techniques in radiation oncology Sometimes anatomical information has difficulties in distinguishing tumors from surrounding tissues, and it cannot fully reveal the pathophysiological features of tumors or evaluate the early response to treatment. With the development of functional imaging, tumor-specific physiological or molecular information can be used in radiation therapy planning to precisely determine the gross tumor volume (GTV) and clinical target volume (CTV) and to improve the accuracy of irradiation dose. Functional imaging has been applied to many aspects of radiation oncology, including pre-treatment tumor diagnosis and characterization, radiotherapy planning, evaluation of efficacy, recurrence detection, etc.; it has advantages in many aspects compared with anatomical imaging. 1.Tumor diagnosis and characterization Traditional anatomical images mostly determine the benignity and malignancy by the size and shape of lesions. However, tumors of the same size can have different biological behaviors. For example, some enlarged lymph nodes may just be reactive, while some small lymph nodes may also have metastatic lesions. Therefore, the size of lymph nodes is not a reliable criterion for differentiation. Functional imaging can remove this uncertainty and non-invasively show the complete biological behavioral characteristics of the tumor to improve the correctness of diagnosis, staging and staging. The most studied functional imaging technique is FDG-PET, which has been used for the diagnosis and staging of head and neck tumors, esophageal cancer, lung cancer, colorectal cancer, lymphoma and melanoma, breast cancer, thyroid cancer, etc. Gambhir et al. meta-analyzed 14,264 patients and found that the average sensitivity and specificity ranges of FDG-PET in oncology were 84% to 87% and 88% to 93%, respectively. and 88% to 93%, respectively. Furthermore, the information provided by FDG-PET led to a change in treatment plan in 26% to 48% of oncology patients. Numerous prospective studies have shown that FDG-PET can evaluate mediastinal lymph node metastases and distant metastases more accurately than conventional imaging methods, and can provide more precise staging information. In addition to tumor location, size, and tumor content, known tumor-specific biological features can also be visualized by scientific molecular markers and modulation, i.e., molecular and biological features of the tumor such as tumor grade, cell proliferation, apoptosis, angiogenesis, hypoxia, and receptor status. Non-invasive molecular imaging can be used as a prognostic factor to predict clinical outcomes or to screen the right population for specific tumor-targeted therapies. 2.Radiotherapy planning In 3D conformal and intensity-modulated radiotherapy techniques, precise outlining of the tumor area is the key to optimize the treatment gain ratio, which requires neither missing tumor tissue nor maximizing the protection of normal tissue. Traditionally, anatomical images represented by CT and MRI are the basis for guiding radiotherapy. both CT and MRI have very good axial and radial resolution and can be used to outline the target area and endanger organs; meanwhile, CT can provide physical density information for dose calculation during planning. Therefore, CT is the most widely used, and MRI complements CT in radiotherapy planning and is better for outlining soft tissues, especially brain lesions and endangered organs. However, the usefulness of anatomical images is limited when the density and morphological changes of tissues or tumors are not obvious. In fact, if only anatomical images are used, some of the tumor tissue may be missed and some of the normal tissue may be unnecessarily irradiated. Functional imaging can add more important information to radiotherapy planning and better show the tumor microenvironment and the possibility of regional lymph nodes and distant metastases. This information allows us to more accurately map the radiotherapy target area and organs at risk, thus reducing borderline tumor leakage or over-irradiation of normal tissue. In addition, higher doses of irradiation or tumor-specific treatment can be given to specific sub-target areas within the tumor. Based on various anatomical and functional imaging techniques, Ling et al. proposed the concept of biological target volume (BTV) based on anatomical images and various physiological or molecular images. For example, FDG-PET can be used to map the metabolically active areas of tumors for dosing during radiotherapy; tumor hypoxia imaging can indicate the need for hypoxia-targeted therapy or high-dose irradiation by intensity-modulated radiotherapy techniques to overcome hypoxia resistance, the feasibility of which has been demonstrated, but caution is needed because the hypoxic areas can change during fractionated radiotherapy. Functional imaging can influence radiation treatment planning in three ways. First, it may detect lesions that are not detected by CT and MRI . Second, it may detect lesions outside the area of the lesion detected by CT and MRI. Third, it may reveal subregions or lesions with increased biological activity within the area of the lesion detected by CT and MRI. The impact of PET on radiotherapy planning for brain tumors, head and neck tumors, and non-small cell lung cancer (NSCLC) has been studied more frequently. Based on the information provided by FDG-PET, changes in the target area (size and/or shape) of radiotherapy have been reported in 27% to 100% of patients with brain tumors, 10% to 100% of patients with head and neck tumors, and 27% to 83% of patients with NSCLC. It is important to keep in mind that although PET provides unique information on tumor metabolism, PET has a lower spatial resolution compared to CT and MRI. Therefore, at present, PET and other functional images are generally used as a supplement to CT when used for radiotherapy planning. 3.Efficacy evaluation (1) Early efficacy evaluation It is of great significance to predict the efficacy of treatment at the early stage of treatment, so that patients with ineffective treatment can be identified at an early stage, and the ineffective treatment can be terminated and the treatment plan can be changed in advance, thus avoiding over-treatment and under-treatment. The traditional approach to evaluate the efficacy of radiotherapy is to observe the change in tumor size through physical examination and anatomical imaging. However, changes in lesion size can only be demonstrated a long time after treatment. Moreover, when tissue fibrosis, edema or necrosis is present, it is difficult to apply anatomical imaging to identify with tumor residual or recurrence. For example, patchy tissue takes 6 months to mature, before which time it can be considered tumor remnant. Metabolic changes precede anatomical changes in tumor volume; moreover, predicting tumor status and treatment outcome by molecular and physiological changes is theoretically more accurate than anatomical imaging by CT and MRI. Therefore, functional imaging can compensate for the innate deficiencies of anatomical imaging by providing early information on the efficacy of radiotherapy or chemotherapy regimens. Functional imaging for this purpose should be performed early in treatment, for example after 1 cycle of chemotherapy or just after treatment. FDG-PET has been reported to be more accurate than CT in detecting early treatment response in patients with lymphoma, breast cancer, and cervical cancer. It is also used to evaluate treatment response in other tumors, such as head and neck tumors, NSCLC, and brain tumors, but the results are less uniform. There are other tracers and imaging methods. 11C-MET is a good tracer in evaluating the efficacy of radiotherapy in patients with brain tumors, head and neck tumors, and NSCLC. 1H-choline, 1H C lactate, or 31P are used as tracers in MRS to monitor the efficacy of radiotherapy or chemotherapy. Decreased 1H-choline levels in patients with brain tumors and lymphomas are positively correlated with the efficacy of radiotherapy and negatively correlated with disease progression. (2) Detection of recurrence Because treatment leads to changes in normal anatomy and adjoining relationships and scar tissue formation, anatomical imaging is often difficult to determine tumor recurrence after surgery and radiotherapy unless there are very obvious anatomical volume changes. In some cases, the diagnosis is not made until the tumor is significantly enlarged. Identifying recurrence with functional imaging can increase the chances of successful remedial treatment. FDG-PET has been shown to be of great value in detecting recurrence in colon cancer, breast cancer, and NSCLC. For example, Staib reported that the sensitivity and specificity of FDG-PET for detecting local recurrence and distant metastases in colon cancer were as high as 98% and 90%, respectively, significantly better than 91% and 72% for CT anatomical imaging; FDG-PET testing is especially necessary for patients with elevated serum cancer S antigen after treatment or negative and indeterminate conventional imaging. FDG-PET is particularly useful in detecting head and neck tumors, cervical cancer, and melanoma have also shown some promise in detecting recurrence. For example, Greven et al. reported that PET is better than clinical examination and CT and MRI in detecting recurrence of head and neck tumors, and suggested that biopsy should be postponed and closely monitored when CT and MRI are suspected and PET is negative. The biological target area refers to the area with different radiosensitivity within the treatment target determined by a series of tumor biological factors. These factors include: lack of oxygen and blood supply; proliferation, apoptosis and cell cycle regulation; oncogene and oncogene alteration; infiltration and metastasis characteristics. These factors include both sensitivity differences within the tumor area as well as normal tissue. Biological conformal intensity modulated radiation therapy refers to the use of advanced physical intensity modulated radiation therapy technology to give different doses of irradiation to different biological targets and to maximize the protection of normal tissues, which is the hope to improve the efficacy of tumor treatment significantly. However, a single functional molecular imaging technique cannot fully reflect the biological characteristics of tumors. Therefore, it is more feasible to select several functional molecular imaging techniques to jointly construct biological targets and guide the implementation of biologically conformal intensity modulated radiotherapy. In fact, the metabolism, lack of oxygen and blood supply, proliferation, receptor expression, apoptosis, oncogene, infiltration and metastasis characteristics of the biological target area are intrinsically related to each other. It is expected that the combined detection of several functional molecular images can reflect the characteristics of the biological target area more comprehensively and realize the individualized biomimetic intensity modulated radiotherapy. This will be an important development direction for the application of functional molecular imaging in radiation oncology.