Previously, the academic community considered malignant tumors with distant metastases as a contraindication to surgery and patients had no chance of surgery, however, many scholars have attempted these “off-limits” areas. In 1947, Alexamler and Haidit reviewed 24 patients with pulmonary metastases treated surgically and found that In 1965, Thomford et al. reported a 5-year survival rate of 31% in a group of patients with unilateral multiple pulmonary metastases. Subsequently, many domestic and foreign scholars have studied the surgical treatment of pulmonary metastases and expanded the indications for surgery; from ipsilateral solitary metastases to ipsilateral multiple pulmonary metastases, up to bilateral multiple pulmonary metastases can be treated surgically. Each time a new viewpoint is generated, some consensus is formed and some controversy is equally aroused. Theoretical basis for surgical treatment of pulmonary metastases There is a capillary network in the lungs with filtration function, plus all venous blood throughout the body must flow through the lungs. Therefore, when malignant tumor cells undergo hematogenous dissemination, they are most likely to reach the lungs and implant to form metastases. According to autopsy reports, almost 1/3 of patients who died of cancer had lung metastases, and these metastases were mostly isolated, with no metastases found in other parts of the body. Since the lung is often the site of initial metastasis of malignant tumors, if metastases are not removed, the tumors may continue to metastasize to other organs, so removal of lung metastases helps to prevent the spread of tumors. When certain tumors, especially sarcomas, develop pulmonary metastases after resection of the primary site, surgery becomes the treatment of choice because the histologic type of the primary site is less effective for radiotherapy or other treatments, and good results can be obtained. Many large case reports have shown that surgery for pulmonary metastases is effective and has a low complication and mortality rate, which is a realistic basis for the surgical treatment of pulmonary metastases. Surgical indications for the surgical treatment of pulmonary metastases (a) Consensus During the treatment of pulmonary metastases, a consensus on the surgical indications for the surgical treatment of pulmonary metastases has gradually been formed, including the following points: (l) the primary site must be completely controlled; (2) no tumor metastases are found elsewhere in the body; (3) the patient must be able to tolerate open-heart surgery and the lung function condition can tolerate partial or lobectomy of the lung; ( 4) the histological type of the primary tumor should be clear; (5) it is estimated that the lung metastases can be completely resected; and (6) there is a lack of effective non-surgical treatments. In contrast, the type of primary focus, number of pulmonary metastases, doubling time, and tumor-free interval are not surgical indications for the surgical treatment of pulmonary metastases, although they are an important basis for prognosis. It has also been suggested that for patients with tumor-free interval < 12 months and the number of lung metastases (5), they can be observed by CT for 3-6 months first, and if the lung metastases increase in size without change in number, surgical resection can be performed, while if the number of lung metastases increases, surgery is not suitable. The above criteria are derived from summarizing past experience in the treatment of lung metastases, but the indications for surgery may change as more clinical studies are conducted, depending on the results of more rigorous clinical studies. The current trend is that as chemotherapy and basal immunotherapy continue to advance, the indications for surgery for pulmonary metastases have expanded accordingly. (b) Indications for surgery for different pulmonary metastases Pulmonary metastases can be broadly classified into 4 dung: (1) pulmonary metastases from tumors of epithelial origin, such as colon cancer, rectal cancer, esophageal cancer, breast cancer, etc.; (2) pulmonary metastases from soft tissue sarcomas, such as smooth muscle sarcoma, liposarcoma, mixed sarcoma, osteogenic sarcoma, etc.; (3) pulmonary metastases from germ cell tumors, such as seminomatous cell tumors, testicular cancer, non-seminomatous germ cell tumors. (4) melanoma. There are differences in the treatment and prognosis of each category. In the last 20 years, chemotherapy has made rapid advances, so that 2 types of tumors sensitive to chemotherapy, namely breast cancer and germ cell tumors, are not suitable for surgery in the presence of pulmonary metastases. Previously, breast cancer lung metastases could be surgically removed, but with the development of chemotherapy and endocrine therapy, surgical resection of breast cancer lung metastases seems to be of little significance, but patients with solitary lung metastases can be considered for surgical resection; while patients with germ cell tumor lung metastases are generally not suitable for surgical treatment because chemotherapy is very effective and only a very small number of patients are suitable for surgery. The 5-year survival rate after resection of lung metastases from tumors of epithelial origin reaches 37%, the 5-year survival string after surgery for sarcoma reaches 29%, and the prognosis of melanoma is the worst; among them, the 5-year survival rate after surgery for lung metastases from osteosarcoma reaches 32%, while only 10% for soft tissue sarcoma; among tumors of epithelial origin, the effect of surgical resection of lung metastases from colorectal cancer is better, and the 5-year survival rate can reach 45. 3%. The number of metastases, whether the metastases are located unilaterally or bilaterally, serum carcinoembryonic antigen level, presence of hilar and mediastinal lymph node metastases, and Dukes stage are all important prognostic factors. In patients with pulmonary metastases from melanoma, surgical resection of the metastases can prolong survival only when isolated metastases are present in the lung, with a median survival of 40 and 13 months in the non-surgical and surgical groups, respectively, and a 5-year survival rate of 33% in these patients after resection of pulmonary metastases has been shown. However, resection of pulmonary metastases in patients with effective systemic therapy did not improve the 5-year survival rate. Surgical resection is the most effective treatment for patients with soft tissue sarcoma who have developed lung metastases and for whom chemotherapy and radiation therapy are ineffective. For those patients with recurrence of lung metastases after resection, multiple resections can be repeated. Patients who can achieve eradication after multiple resections have a good prognosis, while those who still have residual tumors have a poor prognosis. In addition, the size and number of lung metastases and the degree of differentiation of the primary tumor are also important indicators that affect the prognosis. In conclusion, since chemotherapy is ineffective in soft tissue sarcoma with lung metastases, the best treatment is to completely remove the metastases, which is also the best way to prolong the patient's survival. Carcia Franco et al. concluded that the only indication for surgery of pulmonary metastases from osteosarcoma is complete resection of the tumor and pulmonary function, and even patients with malignant pleural effusion can have their pulmonary metastases surgically removed after control by chemotherapy. The only prognostic factor is whether the tumor can be completely resected. The 5-year survival rates of soft tissue sarcoma and osteosarcoma resected by multiple open-heart surgeries were 20% and 40%, respectively, while there was no significant difference in the survival rates of patients with different numbers of surgeries. In this regard, we believe that in clinical practice, the surgical treatment of lung metastases should be individualized and needs to be developed with the participation of various disciplines, especially chemotherapy departments. Factors affecting prognosis The following factors affect the prognosis of surgical treatment of pulmonary metastases: (1) histological type of the primary focus: germ cell tumors have the best prognosis, followed by tumors of epithelial origin; among tumors of epithelial origin breast cancer has a better prognosis, and melanoma and soft tissue sarcoma have the worst prognosis; Todd summarized the 5-year survival rate after surgical resection of patients with pulmonary metastases, 20% to 50% for osteosarcoma, 42.0% to 53.3% for gynecologic tumors . 0%~53. 3%, soft tissue sarcoma 18%~28%, renal tumor 24. 0%~53. 8%, head and neck tumor 40.9%~47.0%, colon cancer 21.0%-38.6%, testicular nine cancer (3-year survival rate) 51%~71%, breast cancer 31.0%~49. 5%, and melanoma 0%~33%. (2) Whether the metastases are completely resected: Koong et al. conducted a study on total pneumonectomy for pulmonary metastases. 133 patients who underwent total pneumonectomy for the first surgery for pulmonary metastases had a 5-year survival rate of 20% in the radical group, while those who did not undergo complete resection had a postoperative survival time of less than 2 years; while patients who underwent total pneumonectomy for recurrence of pulmonary metastases after surgery had a 5-year survival rate of 30%, and those who did not achieve radical treatment had a survival rate of 0. Therefore, it is believed that the long-term survival of such patients depends mainly on whether the metastases can be completely resected, and is not closely related to the histological type of the primary focus, the number of metastases, or the length of the tumor-free interval, and that total pneumonectomy is suitable for those patients who can achieve radical treatment, while patients who cannot achieve radical treatment are not suitable for total pneumonectomy. The prognosis of patients with complete resection of lung metastases is significantly better than that of those who cannot be completely resected. As long as complete resection can be achieved, it is worthwhile to perform lobectomy or total pneumonectomy, whether it is a single or multiple open-heart resection, or even bilateral open-heart resection. (3) The number of lung metastases: the fewer the lung metastases, the better the postoperative results, and it is generally believed that the prognosis of lung metastases more than 4 is obviously poor. (4) Tumor-free interrogation period: Monteiro et al. studied the prognosis of 78 patients with lung metastases from tumors of epithelial origin, excluding the influence of histological type on prognosis, and concluded that the most important prognostic factor was the length of the tumor-free interval. (5) Others: time of tumor multiplication, presence or absence of mediastinal lymph node metastasis, etc. In general, most scholars believe that the most important prognostic indicator is the type of pathology, and the good prognostic factors are: complete resection of lung metastases, tumor-free interval > 24 months, and resection of a single lung metastasis. We share the same view. Surgical incisions and resection methods For the surgical treatment of pulmonary metastases, we usually use surgical incisions including postero-lateral incision, anterior incision, bilateral chest opening, median sternal incision, and transsternal clamshell chest opening. We believe that regardless of the open-chest method, the key is to be able to completely remove the metastases. Thoracoscopy is generally not advocated because the hand cannot completely touch the lung tissue during thoracoscopic exploration and metastases are easily missed. Surgical methods include: (l) partial lung resection: the most commonly used procedure, for a single metastasis located in the periphery of the lobe, wedge resection is sufficient. (2) Segmental lung resection: If the metastases are located in an easily separable segment of the lung (such as the dorsal segment of the lower lobe, the lingual segment of the left upper lobe, etc.), segmental lung resection is also feasible. (3) Lobectomy: If the metastases are large and located in the root of the lobe or multiple metastases are located in the same lobe, or if the lobe where the metastases are located has already undergone partial lung resection, lobectomy is feasible if it is estimated that the patient’s lung function can tolerate a more complete removal of the metastases. (4) Total pneumonectomy: Jungraithmayr et al. concluded that wedge resection of the lung can be performed for the first occurrence of pulmonary metastases, and for those with recurrent pulmonary metastases after surgery, resection can be performed again, or even multiple times, which is beneficial to prolong the long-term survival of the patient, but total pneumonectomy is only used as a last resort after multiple surgical recurrences of pulmonary metastases; total pneumonectomy does not prolong the long-term survival of the patient, and these patients Such patients often eventually die from contralateral lung metastases and extrapulmonary metastases. Total pneumonectomy of one side of a lung metastasis needs to be performed with great caution, and is still an option for only a very small number of patients after weighing the pros and cons. (5) Tumor enucleation: For multiple metastases on one or both sides, which are estimated to be difficult to be completely resected, enucleation of metastases can be considered, and the metastases that can be reached intraoperatively can be enucleated one by one; enucleation of metastases on the other side can be performed simultaneously or electively. This procedure is suitable for some patients whose primary focus is soft tissue sarcoma. When other treatments are not effective and the patient is generally in good condition, this type of tumor reduction can be considered with the aim of reducing the tumor load and creating conditions for other treatments. Patients with pulmonary metastases with mediastinal and hilar lymph node metastases can often be found clinically, but the percentage is not high; Loehe et al. suggested that systematic lymph node dissection could help to remove pulmonary metastases more completely and improve the survival rate of patients. Controversies and Prospects Many useful attempts have been made by domestic and foreign scholars on the surgical treatment of pulmonary metastases, but there are still some problems that deserve to be studied. 1, Only a few literatures have directly compared the effects of surgical and non-surgical treatments, pointing out that surgery can benefit patients with pulmonary metastases from melanoma; while many other studies failed to exclude the effects of combined treatments such as chemotherapy and immunotherapy. Therefore, the place of surgery in the treatment of patients with pulmonary metastases still needs to be further validated in prospective randomized controlled studies. 2. The indications for surgery for pulmonary metastases are still debated. If the number of lung metastases exceeds 4, bilateral lung metastases and multiple recurrent lung metastases, whether surgery is beneficial to patients is still controversial. 3. Postoperative chemotherapy for some primary tumors can improve the survival time of patients, which means that adjuvant therapy should be considered after surgery for lung metastases, and its effectiveness needs to be evaluated in clinical trials. For example, whether adjuvant chemotherapy is needed after resection of lung metastases from soft tissue sarcoma is still controversial. Most scholars believe that chemotherapy is ineffective and costly, and advocate that surgical resection or even repeated resection of metastases is still more effective than chemotherapy alone; however, there are only retrospective studies on this issue, and there is no prospective study to support the results. Among the many factors affecting prognosis, there is still disagreement as to which indicators can better determine prognosis. 5.It is still controversial whether thoracoscopic techniques are suitable for resection of pulmonary metastases. It is generally believed that metastases that cannot be detected by conventional examination can often be palpated during open-heart surgery, but some scholars believe that there is no difference between the results of thoracoscopy and open-heart surgery. In addition whether lymph nodes should be routinely cleared has also been debated. With the progress of tumor treatment, patients with pulmonary metastases have more and more chances of survival. Starting from the above aspects, to improve the treatment strategy of pulmonary metastases so that more patients can benefit from it, the joint efforts of colleagues in thoracic surgery and oncology-related disciplines are needed.