High-intensity focused ultrasound (HIFU) is an advanced means of non-invasive tumor treatment developed in recent years. It is a method of focusing high-energy ultrasound emitted from outside the body through soft tissues on target tissues in the body, which causes the temperature in the local tissues to rise above 60°C slowly or within a short period of time, resulting in coagulative necrosis of tumor cells. Currently, the tumors that can be treated with HIFU include prostate cancer, pancreatic cancer, liver cancer, uterine fibroids, soft tissue sarcomas, retroperitoneal metastases and other benign and malignant tumors. The basic principle of focused ultrasound therapy is to use the good directionality, penetration and focusing of ultrasound in biological tissues to focus ultrasound on the target area in the organism, forming a high-intensity ultrasound convergence zone. “The high-intensity ultrasound used in HIFU treatment is different from diagnostic ultrasound, the average intensity of diagnostic ultrasound is 100 mW/cm2, while the ultrasound intensity of HIFU can reach 10 kW/cm2, and the intensity and duration of action of ultrasound can be measured in the image. The intensity and duration of action of HIFU can be adjusted at any time under the positioning and monitoring of imaging technology, and the whole tumor can be gradually ablated by controlling the ablation range in the target tissue. Compared with other ablative treatments, focused ultrasound ablation therapy has obvious features: 1. it is a non-invasive ablation therapy, no puncture or incision is needed during HIFU treatment, which is the only non-invasive ablation technology used in clinical practice, avoiding the risk of bleeding and tumor implantation and metastasis caused by puncture or incision surgery, and without any radiation damage (because ultrasound itself has no radiation). 2. 2. In the process of focused ultrasound treatment, the scope of ablation is controlled under the surveillance of imaging technology, and the ablation of larger target tissues is finally completed by using the three-dimensional combination of ablation area and motion control, so its ablation scope is less restricted by the size and shape of target tissues, which is a kind of conformal treatment. In general, the ablation range of puncture ablation treatment is mostly 3~5 cm, but the ablation range of ultrasonic ablation treatment can be more than 10 or even 20 cm through the three-dimensional combination of ablation area. Focused ultrasound ablation therapy in clinical application should pay attention to the following issues: 1. Strictly grasp the scope of application of focused ultrasound ablation: To achieve safe and effective ultrasound ablation therapy for target tissues in vivo from outside the body, certain conditions must be met. First of all, there should be an acoustic channel for ultrasound to penetrate and focus, so that the target area can form a good focus area for ultrasound convergence, thus requiring the treated area to be well displayed in the ultrasound image, and there should not be a strong acoustic reflection interface or obvious tissues that absorb acoustic energy, such as large bones, calcified foci, gas-containing tissues and obvious scar tissues, etc. The local skin that ultrasound passes through should not be infected. The local skin crossed by ultrasound should not have infection, ulceration and significant fibrosis. Secondly, the target tissue can produce good thermal effect on high intensity ultrasound, which is not only related to the ultrasound intensity, frequency and time of action, but also depends on the acoustic properties of the target tissue itself, including the acoustic impedance, sound absorption properties, water content and blood perfusion of the tissue. Currently, ultrasound ablation therapy is mainly applied to solid tumors of parenchymal organs and bone tumors with good ultrasound access. Because of the risk of organ perforation, tumors with cavernous organs are not suitable for ablation therapy. Cystic, mucinous and plasma tumors with significant fluid content are not suitable for ultrasound ablation therapy because of poor sound absorption performance and insignificant temperature rise effect, and tumors with gas-containing cavernous organs are contraindicated for focused ultrasound therapy. As an emerging local treatment, the exact indications of focused ultrasound therapy for different diseases are yet to be confirmed by a large number of clinical trial studies and evidence-based medicine. Theoretically, ultrasound ablation should achieve efficacy equal to or even better than invasive treatment. Currently, focused ultrasound therapy is more often used for patients with benign tumors (uterine fibroids) that are not suitable for surgical treatment (pancreatic cancer, prostate cancer, advanced tumors) or strongly require non-surgical treatment. 2. The basic principles to be followed in focused ultrasound ablation therapy: Focused ultrasound ablation therapy is mostly used for patients who cannot be treated by surgical resection in the middle and late stages, but it should still follow the basic principles of comprehensive tumor treatment. Multiple disciplinary team (MDT) should be implemented to develop a comprehensive treatment plan and a reasonable treatment sequence for different types of tumors locally and systemically. For benign tumors, such as uterine fibroids, in principle, the ablation range should be more than 80%, and the main purpose is to control the growth of the tumor and improve the clinical symptoms; for palliative treatment of larger tumors, in order to ensure the safety of the patient, the ablation range should be more than 80%. The non-invasive advantages of focused ultrasound therapy are particularly important. Regardless of the treatment, it is important to reduce the adverse stimulation to the patient, and to improve the comfort of the treatment with the existing means under the premise of ensuring the safety. Focused ultrasound therapy is a non-invasive treatment, but not non-invasive and painless. To achieve ablative treatment of tumor tissues in vivo is to cause trauma to tumor tissues, and the accumulation of heat in the acoustic channel will also affect the normal tissues on the acoustic channel, which will inevitably produce therapeutic pain, so different degrees of anesthesia or sedation and analgesic treatment should be implemented during the treatment process is also necessary. 3. Timely assessment of the efficacy of focused ultrasound therapy: To evaluate the efficacy of ultrasound ablation therapy, it should be determined in a timely manner: whether the lesion is ablated (i.e. coagulative necrosis occurs), the effective ablation range, and whether the ablation range contains the whole tumor lesion. Since it is not possible to obtain surgical resection specimens for pathological examination as in conventional surgery, nor is it possible to obtain anatomical imaging information on the complete resection of the lesion, the evaluation of ultrasound ablation therapy cannot rely on purely anatomical images at present, but rather on functional imaging techniques to obtain evaluation information by comparing pre- and post-treatment changes in blood perfusion and tissue metabolism of the ablated lesion. Currently, the most accurate and reliable means of assessing the efficacy of ultrasound ablation is dynamic enhanced magnetic resonance imaging. Low mechanical index microbubble ultrasonography can be used to assess the extent of ablation immediately after ablation treatment, which plays an important role in evaluating the efficacy and guiding the adjustment of ablation dose during the treatment. Since the assessment of efficacy requires the comparison of functional images before and after treatment, the imaging parameters used before and after treatment and the anatomical level of comparison should be consistent. In addition, the changes of patients’ symptoms, signs and tumor markers, the puncture results of local lesions and the improvement of survival quality before and after ablation treatment are also auxiliary indicators to assess the efficacy of ablation treatment. Safety of focused ultrasound ablation therapy: Focused ultrasound therapy is a non-invasive treatment, but it is by no means a non-invasive treatment. The risk of ultrasound ablation therapy is reflected in the following aspects: 1. Ultrasound ablation therapy is to produce a strong bio-thermal effect on the target tissue within a short period of time with high-energy ultrasound to achieve a higher temperature for tissue inactivation. Once the treatment is off-target, it will produce strong damage to the tissues outside the target area, especially when there are cavernous organs and nerve tissue around the target area, which is more likely to lead to a series of serious complications. The way to control this risk is to clarify the advantages and disadvantages brought by ablation treatment, so that the lesions suitable for ablation can be ablated, and the lesions that are prone to more serious complications after ablation can be treated with low-dose thermal injury. The safety of ultrasound ablation treatment depends on the real-time monitoring of imaging technology. The existing monitoring technology, whether MRI or ultrasound imaging, cannot meet the requirements of real-time monitoring, and the existing monitoring indicators cannot meet the requirements of safe and precise monitoring of ablation treatment in terms of sensitivity and specificity, which objectively reduces the accuracy and precision of ablation treatment. The dose of ultrasonic ablation treatment is affected by various factors such as the tissue in the acoustic channel, the depth of the lesion, the reflection of sound energy in the back field, the type of tumor tissue, etc., and it varies with time. At present, ultrasound ablation therapy is mostly applied to patients with advanced tumors, whose conditions have progressed to advanced stages, with complicated conditions and large tumors to be treated, and have already experienced numerous other forms of treatment and their side effects, which have seriously damaged their body and mind, all of which bring great difficulties and risks to the treatment. After more than ten years of development and clinical research, on the one hand, a large number of patients who received focused ultrasound therapy have benefited from it, proving that focused ultrasound technology can safely and effectively ablate and inactivate tumors in vivo from outside the body, and through comprehensive treatment based on ultrasound ablation therapy, long-term tumor-free survival has been achieved, with the longest being more than ten years, and its treatment technology of preserving organs is recognized and welcomed by patients; on the other hand, ultrasound This new tumor treatment technology needs a process of gradual development, maturity, perfection and standardization, and its clinical application value is yet to be strongly supported by evidence-based medical evidence, and it is believed that focused ultrasound treatment technology will definitely occupy a place in tumor treatment.