Why use CT-PET for radiotherapy localization? The value of CT-PET in radiotherapy: It is well known that radiotherapy is one of the three basic means of treating malignant tumors. According to WHO, 45% of malignant tumors can be cured, of which 18% rely on radiotherapy. As a representative of molecular imaging, the clinical application value of PET/CT in the diagnosis and differential diagnosis of malignant tumors has gained peer consensus. Especially in the process of standardized radiation therapy for malignant tumors, PET/CT has a very important value in the accurate staging of tumors, precise outlining of target areas, optimization of radiation therapy plans, and evaluation of the efficacy of radiation therapy. Determination of tumor staging Accurate clinical staging of tumors is the prerequisite for choosing the best treatment plan. Traditional CT and MRI for TNM staging have certain errors and limitations. PET/CT, with the outstanding advantages of functional imaging and anatomical image co-location, can complete the whole body examination in one scan, avoiding the omission of lesions; exclude or correct the suspicious diagnosis by analyzing the degree of lesion concentration and delayed scanning changes displayed by PET/CT; locate metastatic lymph nodes accurately, and at the same time, provide information about lesions invading surrounding normal tissues. The accuracy of tumor staging is greatly improved by analyzing the degree of concentration of the lesion and the change of delayed scanning. Accurately outline the biological target area, effectively improve the control rate of local lesions PET/CT is able to accurately carry out tumor imaging and in vivo three-dimensional prediction of tumor radiotherapy sensitivity has more obvious advantages, and has very obvious clinical application advantages for the decision-making of tumor radiotherapy modality. Studies have shown that 18F-FDG PET provides the size range of metabolically active tumors and determines the tumor boundaries, which is of great significance for accurate planning of tumor biological target area (BTV) and reducing the proportion of inappropriate treatments: firstly, 18F-FDG can find more external invasion of tumors and distant metastases, thus expanding the treatment plan; secondly, 18F-FDG PET can help to reduce the risk of tumor damage by identifying the tumor periphery of the benign lesions (e.g., the tumor). benign lesions (e.g., lung atelectasis, tissue necrosis, etc.) while narrowing the GTV determined by CT, thus effectively increasing the possibility of controlling the tumor and reducing normal tissue damage. It has been reported that the application of PET/CT for biological target localization has resulted in a 33% to 70% change in target area compared with conventional CT localization [2].Erdi reported [3] that in a study of 11 tumor patients in which the planned target area (PTV) was determined by CT and PET respectively, 7 of the 11 PTVs determined by PET enlarged the CT-determined PTV (mean enlargement of 49%), and 4 cases narrowed the PTV (mean enlargement of 49%), and 4 cases narrowed the PTV (mean narrowing of 49%). cases narrowed the PTV (mean 18%); Schnilt et al. found in a retrospective study that the irradiation field determined by CT in 39 patients with non-small-cell lung cancer was altered in 15 cases after application of 18F-FDG PET guidance; Schmucking et al. [4] reported that in 27 oncologic radiation therapy patients with PTVs determined by information provided by 18F-FDG PET images, 3 out of 25 cases had their PTVs changed. PTV, 25 cases had varying degrees of shrinkage ranging from 3% to 21%, while the other two cases had an enlarged PTV. Determination of standardized uptake value (SUV) in tumors for effective control of radiation therapy dose to tumors Radiobiological studies have shown that the distribution of cancer cells within tumors is heterogeneous, and due to differences in blood supply and cellular heterogeneity, the sensitivity of different cancer cell nuclei to radiation therapy varies considerably. If the target is irradiated with a uniform dose, some cancer cells will inevitably survive due to insufficient dose, which will become the root cause of recurrence and metastasis; if the target dose is too high, it will lead to serious damage to the surrounding sensitive tissues.PET/CT can respond to the proliferative activity of tumor cells in each region of the tumor by measuring the SUV of each region of the tumor, and then give different irradiation doses according to the needs for biomedical intensity-modulated radiotherapy to achieve the optimal effect of radiotherapy. Achieve the best radiotherapy effect. Evaluate the effect of tumor radiotherapy, determine whether the tumor is residual, recurrence or scar necrotic tissue after radiotherapy Tumor tissues often form fibrosis, necrosis and scar tissue after radiotherapy, and it is difficult to identify with tumor residual or recurrence in terms of morphology and density relying on CT, MRI, etc. PET/CT can identify and evaluate the efficacy of the treatment better by using the metabolic contrast of 18F-FDG in tumors AKhurst et al. reported that After treatment of 365 non-small cell lung cancer patients, the positive predictive value of 18F-FDG PET to determine the residual or recurrence of the primary foci after treatment was 98%, and the diagnostic sensitivity for distant metastatic foci was 100%. Practice has shown that if PET/CT is performed immediately after radiotherapy in a short period of time, false positive results may occur due to the effect of radiation pneumonitis or macrophage glycoconjugation in tumor necrotic tissue. Even when early FDG PET/CT was performed at 1 month after radiotherapy, the positive predictive value of detecting residual tumor was close to 90%, while its negative predictive value was relatively low, and was close to the positive predictive value after 3-4 months of treatment. Therefore, it is suggested that PET/CT examination should be done once at 1 month after treatment, although only 50% of the residual tumor can be detected, the residual tumor can be treated in time; and the second review should be conducted at an interval of 3-4 months, and if PET/CT positive images are found, targeted radiotherapy can be used to improve the medium- and long-term efficacy of radiotherapy. In summary, PET/CT provides the distribution of biologically active tumor tissues in tumor radiotherapy, improves the accuracy of tumor diagnosis and the precision of localization, and at the same time optimizes the radiation dose of radiotherapy, improves the therapeutic effect, and reduces the recurrence of the lesion; moreover, it also plays a monitoring role in the evaluation and recurrence of malignant tumor treatment, and becomes an indispensable clinical modern oncology diagnostic and therapeutic It has become an indispensable tool for clinical diagnosis and treatment of modern oncology. Influence of PET/CT on the treatment decision of head and neck tumors: The key to the decision of tumor treatment is accurate diagnosis and staging, and the traditional clinical staging methods use anatomical imaging techniques such as CT, MRI, etc., which have certain limitations. 18F-FDG PET/CT has obvious advantages compared with it, and PET/CT can not only accurately locate the tumors, but also correctly distinguish abnormalities and other abnormalities, but also provide the best results in the clinical diagnosis and treatment. PET/CT can not only accurately locate the tumor and correctly distinguish the abnormal 18F-FDG uptake of fat and muscle tissues, but also well display the anatomical relationship between the tumor and the surrounding tissues. It has been reported in the literature that PET/CT can influence the treatment plan of 18% of patients, 27% of radiotherapy decisions were changed, 9% were changed to other treatments, 10% were changed to palliative radiotherapy, 6% were changed to irradiation target area, and 12% were adjusted the dose. Some scholars performed FDG PET/CT before radiotherapy and found that 16% of the patients had suspected distant metastases, which led to the change of radical radiotherapy to palliative radiotherapy. Some scholars prospectively evaluated the effect of PET/CT on the external irradiation of tumor patients: PET/CT changed the treatment plan from radical radiotherapy to palliative radiotherapy in 20% of the patients, changed the irradiation dose in 30% of the patients, and changed the volume of target area in 40% of the patients. In the treatment of head and neck malignant tumors, radiation therapy is an important means, standardized and precise irradiation can effectively improve the effect of radiation therapy and reduce the complications. one-stop whole-body scanning examination of PET/CT can provide the accurate staging of tumors, which is clinically important for the selection and development of treatment plans and determination of the efficacy of the treatment, and at the same time PET/CT in the development of head and neck tumors radiotherapy plans, can avoid normal tissues, and can be used in the development of radiotherapy. At the same time, PET/CT can increase the dose to the tumor target area as much as possible under the condition of avoiding normal tissues, effectively reduce the toxicity reaction of vital organs, and decide whether to continue radiotherapy, change radiotherapy plan or terminate radiotherapy according to the treatment reaction, so as to make the patient’s treatment plan more scientific and reasonable. The value of PET/CT in evaluating the therapeutic efficacy of head and neck tumors and detecting residual and recurrent lesions: 18F-FDG PET/CT can be used to detect residual or recurrent lesions after treatment, accurately evaluating the therapeutic efficacy, which is of great value for the early evaluation of the therapeutic efficacy of radiotherapy and chemotherapy, and is superior to structural imaging tests such as CT and MRI, etc. With its outstanding advantages of the integration of functional and anatomical images, PET/CT may become an accurate and precise imaging method for head and neck tumors. PET/CT has the outstanding advantage of integrating functional and anatomical images in the same machine, which may become a new platform for accurate radiotherapy simulation and localization. When the information from PET/CT is applied to formulate the scope of radiotherapy, the determination of GTV will be more accurate, which has been confirmed in many studies. Since the amount of 18F-FDG concentrated in tumor cells during treatment is linearly related to the growth rate, the change of metabolic level during treatment precedes the change of tumor volume, and the increase of 18F-FDG uptake in the focal area means the failure of treatment, while the rapid decrease of metabolic activity during treatment indicates a good response to the treatment, so it can be used to distinguish between effective and ineffective response to the treatment, and for the ineffective cases, it can be used to switch to other treatment methods as early as possible, and then to determine the GTV of the tumor. The negative and positive predictive values of PET/CT for recurrence were 100% and 80%, respectively. Other authors have reported that PET/CT is more useful for the diagnosis of residual lymph nodes than for the diagnosis of residual primary foci. Anatomical changes and scarring caused by surgery, as well as edema and fibrosis caused by radiotherapy, will make it difficult to detect the presence of active head and neck tumors on clinical examination and conventional imaging. PET/CT has been reported to accurately localize approximately 2/3 of the abnormal FDG uptake lesions in the head and neck region, and has a significantly higher detection rate, especially in patients previously treated with surgery or radiation therapy, than in patients with untreated neoplastic tumors. As a non-invasive PET/CT examination, it has become the preferred diagnostic method for clinical consideration of recurrence of head and neck malignant tumors after treatment. Third, the value of CT-PET in lung cancer radiotherapy PET/CT will have a significant impact on radiation therapy. Radiation oncologists have used PET/CT information directly in their radiation treatment planning (RTP). The fundamental goal of radiation therapy is to deliver a sufficiently high curative dose to the tumor area while minimizing the dose to its surrounding tissues and organs. Three-dimensional conformal intensity-modulated radiation therapy is currently the most advanced radiation therapy technique, and the precise localization of the radiotherapy target area is the key to this technique.CT plays an important role in tumor localization for radiation therapy, but has certain limitations. Because CT is to identify the relationship between the lesion and the surrounding tissues through the change of tissue density, it is difficult to accurately outline the boundary of the tumor if the density of the lesion and the surrounding tissues are similar.PET/CT technology combines the tumor volume, metabolically active degree of the tumor, the surrounding tissues of the tumor and the anatomical structure of the body surface, and at the same time, provides the staging and classification of the tumor, which can be used for the design of the radiotherapy plan, the design of the irradiation field and the calculation of the dose distribution of the tumor target area, and the calculation of the dose distribution of the tumor target area. Calculate the dose distribution in the tumor target area to achieve the best dose distribution in the tumor target area with the least damage to the surrounding normal tissues. It has been shown that 18F-FDG PET examination can change the biological target area of radiation therapy in 30% to 60% of non-small cell lung cancer patients. Among the 96 malignant tumor patients we analyzed, 33 patients applied PET/CT to guide the localization of MM50 radiation therapy, all of which were able to outline the boundary of the lesion relatively accurately, and the results of clinical follow-up and PET/CT review were satisfactory. 23 of the 33 patients (70%) had some difficulties if localized by CT plain scanning. If the central lung cancer with lung atelectasis, it is difficult to determine the boundary between the tumor and the atelectasis on PET or CT alone, and the whole area may receive radiation therapy, and the tissues that caused the atelectasis received unnecessary irradiation.PET/CT fusion images show the boundary between the tumor and the surrounding tissues very clearly by the change of the color gradient, and therefore he will play an important role in three-dimensional shape-adapted intensity-modulated radiation therapy. MM50 before treatment MM50 after treatment Highly differentiated squamous carcinoma of the right lung. It is difficult to outline the exact boundary of the tumor with the undifferentiated lung tissue on the CT image. the PET/CT fusion image shows clear boundary of the tumor, accurate localization, and satisfactory treatment effect. Role of ET/CT in surgical oncology. PET/CT can provide local and systemic information about the tumor, determine the anatomical location of the lesion and its interrelationship with normal tissues, and surgeons can clarify whether the tumor can be resected or not before surgery in order to avoid unnecessary surgical exploration. For patients who are suitable for surgery, PET/CT fusion images are utilized to guide the determination of the surgical approach and the extent of lesion resection. In our hospital, 25 patients had their surgical approach and extent of resection determined based on the PET/CT fusion images showing the size of the tumor, its borders, and its relationship with the surrounding tissue structures. Postoperative pathology confirmed that no cancer cell infiltration was seen at the margin of the surgical incision, and the resected lymph nodes were consistent with the PET/CT findings except for the hilar lymph nodes in one case of lung cancer, which was inconsistent with the PET/CT findings. The fusion image shows the size, boundary and relationship with the surrounding tissue structure of the tumor, and determines the surgical method and the scope of resection However, with the deepening of clinical practice and the accumulation of cases, some limitations of FDG for tumor diagnosis have gradually appeared, mainly false-positive and false-negative. In a group of 54 cases in China, 9 out of 28 cases of tuberculosis (32%) had FDG uptake, 5 out of 8 cases of tuberculosis had positive FDG imaging in the active phase, and 6 out of 18 cases of lung infection (33.3%) had positive FDG uptake, including cryptococcus, Mycoplasma pneumoniae, mycobacteriosis, lung abscesses, bacterial pneumonia, and pseudotumor pneumonitis (mechanized), each with a positive FDG uptake. For false negatives, renal clear cell carcinoma, bronchioloalveolar carcinoma, gastric mucinous cell carcinoma, imprinted cell carcinoma, and some hepatocellular hepatocellular carcinomas have been seen. Therefore, the use of positronic radiopharmaceuticals that reflect one metabolic state alone cannot reflect the real situation of the human body. For this reason, we suggest that PET/CT imaging should be combined with the use of positronic radiopharmaceuticals reflecting different metabolic processes, so as to improve the sensitivity and specificity of early tumor diagnosis, and to give full play to the characteristics of PET/CT equipment, so as to achieve the real purpose of early diagnosis of tumors. To sum up, PET/CT has outstanding advantages in tumor diagnosis, staging, efficacy and prognosis judgment and assisting in the formulation of treatment plan and determining the positioning of radiotherapy biological target area. However, due to the limitation of tracer development, PET/CT examination still has certain false positives and false negatives. With the emergence of new radiopharmaceuticals and the introduction of specific molecular probes, PET/CT will promote the diagnosis and treatment of tumors to a new level.