Tumor metastasis is when malignant tumor cells reach distant parts of the body from the primary tumor and grow to form new lesions. It is the main reason why cancer is difficult to be cured. Since metastases are often multiple and may exist in multiple organs at the same time, it is difficult to eradicate them completely by surgery, radiotherapy or drugs, so the treatment of metastatic cancer is a major clinical problem. However, the appearance of metastases does not mean that there is no therapeutic value. For metastases with good response to drug treatment or relatively isolated metastases, comprehensive treatment based on surgical resection can also achieve better results. The liver is a common organ for tumor metastasis, and some issues related to surgical treatment of metastatic liver cancer will be discussed below. Why are tumors prone to liver metastasis? For most tumors, the presence of liver metastases means that the disease is no longer at an early stage. There are many reasons why the liver becomes a site of metastasis for many distant organ tumors, especially gastrointestinal tumors. For gastrointestinal tumors, it may be because the liver is the first visceral organ to be encountered when tumor cells of gastrointestinal origin are released into the capillaries, small post-capillary veins and subsequent portal circulation. However, the interaction and residence of circulating tumor cells in the organ alone is not sufficient to form metastases, but also requires the survival and progressive growth of metastatic tumor cells into tumors under the interaction between the host organ such as the liver and the metastatic tumor cells under the regulation of a series of genes. The phenomenon of organ-specific metastasis to the liver is also seen in other primary tumors. For example, the incidence of liver metastases is significantly higher in primary melanoma and breast melanoma than in other individual organs. These tumors do not drain directly into the liver via the portal circulation, and the susceptibility to liver metastases may be due to the presence of an organ-specific microenvironment in the liver suitable for the growth of these tumor cells, the exact mechanism of which is not yet fully understood. How to detect liver metastatic lesions early? When a malignant tumor of an organ, such as colorectal cancer or breast cancer, is temporarily cured by surgical resection, clinicians often ask the patient to have regular review or chemotherapy, etc. according to the intraoperative tumor stage and postoperative pathology of the patient. However, while reviewing the primary tumor sites such as rectum, colon and breast, it is important not to neglect the examination of the whole body condition, especially the liver. When there is a suspicious lesion, CT and MRI of liver can be performed, combined with tumor-specific markers in blood such as CEA, CA19-9 and CA125, most of them can make a clear diagnosis. After the diagnosis of liver metastasis, one should also be alert to whether there are other organs such as bone, lung and brain metastasis at the same time, especially when accompanied with symptoms such as back and bone pain, chest discomfort and cough. It is sometimes necessary to have relevant examinations of these organs such as lung and brain CT, whole body bone nuclear scan or even whole body PET-CT scan, because if there are other metastases in addition to the liver metastases, the treatment plan may be completely different. When liver metastasis occurs in colorectal cancer patients after surgery, what is the appropriate situation for surgical resection? Colorectal cancer is a participating malignant tumor, and liver metastases may occur in up to 50% of patients in the period after surgical removal of the primary tumor. Among them, 10-25% of patients are suitable for surgical resection of liver metastases, and effective chemotherapy will also increase the resection rate of liver tumors. To date, surgical resection is the most effective means of treating liver metastases from colorectal cancer. Generally speaking, good liver function, single tumor or no more than 3 tumors, and the remaining liver is expected to be compensated after resection, such cases are suitable for resection treatment. Of course, a detailed evaluation is needed before surgery, and the treatment plan is finally formulated by liver surgeons and oncologists together. If colorectal cancer liver metastasis can be removed surgically, do I still need chemotherapy before and after surgery? Whether colorectal liver metastases need chemotherapy before surgery (neoadjuvant chemotherapy) or adjuvant chemotherapy after surgery if they are suitable for surgical resection is also a matter of concern. Although the results of different clinical studies are inconsistent, the general opinion is that adjuvant chemotherapy is more effective before and after surgical resection. There are also some unresectable patients who have been treated with chemotherapy and obtained complete resection after tumor shrinkage. There are relatively mature chemotherapy regimens, and the commonly used chemotherapy drugs include 5-fluorouracil, oxaliplatin and irinotecan. However, if there has been chemotherapy before surgical resection, it is usually necessary to rest for 3-4 weeks before surgery. All of these chemotherapy drugs have varying degrees of liver toxicity. For example, 5-fluorouracil can cause hepatic steatosis, oxaliplatin can cause hepatic sinusoidal obstruction, and irinotecan can cause steatohepatitis. In these cases, if liver damage is not recovered without hepatoprotective treatment, premature surgery is very risky. Is there any other local treatment suitable for metastatic liver cancer other than surgical resection? The purpose of treatment for colorectal cancer liver metastasis is to destroy the lesions in the liver as much as possible. In addition to surgical resection, local treatment such as radiofrequency ablation and microwave ablation can also be considered. Some clinical studies have compared the effects of surgical resection and radiofrequency ablation for liver metastases, and found that the effects of surgery and radiofrequency ablation for tumors less than 3 cm are similar, but the effects of radiofrequency treatment for tumors larger than 3 cm are relatively poor, because the recurrence rate is high after radiofrequency treatment, which is related to the limited scope of ablation treatment. Local treatment can also be combined with surgical resection, which is less traumatic and safer than surgical resection of both parts, and better than intraoperative radiofrequency ablation for small lesions deep in the liver. If liver metastasis is combined with other organ metastasis after colorectal cancer surgery, is there any need to operate on liver metastases? If liver metastases occur after colorectal cancer surgery and metastases from other organs are also found, this is often a very advanced stage. In this case, surgical resection is usually no longer considered. Only when there is only one site of metastasis outside the liver and the lesion is relatively limited, and the lesion inside the liver is more suitable for surgical resection, can we consider removing the liver metastases, and then adjuvant chemotherapy or radiotherapy to deal with other sites of metastases to control the tumor development as much as possible.