Renal cancer (RCC) accounts for 2%-3% of all malignant tumors in adults and 85%-90% of primary malignant tumors of the kidney. 20% have metastases at the time of initial diagnosis and 30% have metastases after surgery, making it the second most common malignant tumor in the urinary tract. The preoperative diagnosis and clinical staging of kidney cancer depend on imaging examination, and the treatment choice depends on the tumor stage and patient condition. The progress of imaging technology in recent decade, especially the widespread use of B-ultrasound and the upgrading of CT, has greatly contributed to the improvement of kidney cancer diagnosis and treatment. With the development of imaging technology, the diagnostic methods and concepts of kidney cancer have been changing quietly: (1) ultrasound is widely used to diagnose more and more asymptomatic incidental carcinoma and small kidney cancer less than 3 cm; (2) ultrasound is the first choice for kidney cancer diagnosis, and CT scan and enhancement scan are recognized as the best imaging tests for clinical diagnosis and staging of kidney cancer; (3) intravenous urography (IVU) is no longer used as the first choice for kidney cancer diagnosis. (3) intravenous urography (IVU) is no longer a mandatory imaging test for renal tumors because of its low diagnostic value, while CT-enhanced scan can also replace it for satisfactory evaluation of contralateral renal function; (4) renal arteriography, as an invasive test, has been replaced by other non-invasive imaging techniques such as renal acoustic imaging, spiral CT, MRI and three-dimensional image reconstruction, which are less invasive and have a higher diagnostic rate; (5) for small renal tumors that are difficult to be diagnosed by imaging, preoperative puncture biopsy is no longer recommended. (6) Ultrasound, color Doppler ultrasound, CT and MRI scan can accurately understand the status of inferior vena cava tumor; (7) Emission tomography (PET) or PET?CT can detect distant metastases or evaluate the efficacy of chemotherapy and radiotherapy; (8) Nuclear renogram can evaluate the kidney function of the affected side and the contralateral side, and nuclear bone scan can detect bone metastases. Nuclear bone scan can detect bone metastasis. The biggest problem in the diagnosis of kidney cancer is the differentiation of small kidney tumors. Among small renal tumors, renal carcinoma accounts for 68%-87%, as well as eosinophilic tumor, vascular smooth muscle lipoma (AML), smooth muscle tumor, fibroma and lymphoma. Most of the small renal tumors can be distinguished by modern imaging techniques, but sometimes multiple imaging examinations are needed to confirm each other. 1.1 B-ultrasound examination With the continuous development of grayscale image and color Doppler technology, the diagnostic level of small kidney cancer by ultrasound has been improved significantly. The presence of small fluid echogenicity (necrotic foci) within the lesion is beneficial to the diagnosis of small kidney cancer; the hypoechoic border of small kidney cancer (caused by pseudo-envelope or cystic lesion) is helpful to differentiate it from AML. Ultrasound angiography can enhance the blood flow display and compensate for the shortcomings of conventional color ultrasound in showing low velocity blood flow in tumors or deeper or smaller tumors, which can significantly enhance tumors with little or no blood supply and help the differential diagnosis of small kidney tumors. tammela [1] reported that the diagnostic accuracy of ultrasound for kidney cancer was 85% and the staging accuracy was 70%-74%. The accuracy of color Doppler ultrasound in diagnosing renal vein and inferior vena cava tumor thrombus is 93%, the sensitivity is 81%, and the specificity is 98%. disadvantages of B ultrasound examination: multiple tumor foci are easy to be missed; it is difficult to characterize the histological subtypes of renal occupying lesions and renal cancer pathology. 1.2 CT examination Currently, CT scan is the best method to detect and qualitatively diagnose small kidney cancer [2], and thin layer scanning is the key to improve the detection rate. Conventional CT is limited by the scanning speed, and the enhanced scan level is mostly in the renal parenchymal stage. Spiral CT (SCT) has accelerated scanning speed and can scan the whole kidney in multiple consecutive phases with one breath hold, avoiding the level omission caused by different breathing amplitude, reducing the influence of partial volume effect, more accurate CT value determination, increased detection rate of small cancer foci, and more improved display of malignant features such as separation, nodules and tumor invasion of blood vessels in cystic kidney cancer. The multilayer spiral CT (MSCT) introduced in recent years further makes up for the shortcomings of single-layer spiral CT, with faster sampling speed, it only takes 0.5s for the bulb to rotate for one week and acquire 4 or more layers of images, and it only takes a few seconds to complete the whole kidney scan, and the scan time difference between the upper and lower pole of the kidney cancer foci is basically negligible. It can not only improve the density resolution, correctly localize and show tumors of 0.5-1.0 cm or even smaller in diameter; but also accurately reflect the enhancement characteristics of the lesions and characterize the pathological histological subtypes of most renal cancers. It has been reported that the CT value of clear cell carcinoma in the cortical phase of enhancement scan generally rises above 100 Hu, while granular cell carcinoma has a lower degree of enhancement with CT value below 100 Hu. MSCT plain scan: small renal carcinoma has uniform density, lower than or equal to renal parenchyma, and a few exhibit high density. Hemorrhage, necrosis, cystic change and calcification are rare. multi-stage enhancement scan of MSCT: ①Cortical stage (arterial stage) can show the characteristics of tumor tissue blood supply and adjacent cortical infiltration. Small renal carcinoma with multiple blood supply shows very significant enhancement, and the CT value increases by 50-120 Hu compared with plain scan, reaching or even exceeding the degree of renal cortical enhancement, but the enhancement is transient, transient and heterogeneous, followed by a significant decrease in density; whereas small renal carcinoma with less blood supply has insignificant enhancement, which is often obscured by the normal renal parenchyma. The parenchymal stage can show the density contrast between normal tissues and tumor foci, which is helpful to determine the demarcation between tumor and normal tissues, and most of the tumors show different degrees of enhancement, with the CT value rising more than 20 Hu. It can provide a reliable basis for the correct diagnosis of small kidney cancer. CT three-dimensional reconstruction: It can apply multiple image post-processing, integrate arteriogram, venogram and conventional CT examination together, accurately display the relationship between tumor and renal vascular system, collecting system and normal renal parenchyma, reduce the application of invasive examination, and provide a more accurate diagnosis of small renal cancer. SCT: the specificity of diagnosing renal cancer is 95%, the accuracy is 95%, and the accuracy of staging is 91%[4] ; the sensitivity of diagnosing renal vein and inferior vena cava aneurysm embolism is 85%, the specificity is 98%, and the accuracy is 96%[5] . For renal cancer larger than 3 cm in diameter, the sensitivity of SCT in detecting clear cell type is 80.2%, and the sensitivity in detecting non-clear cell is 80.7% [6]. disadvantages of CT examination: there is X-ray radiation, there is a possibility of contrast allergy, some adenomas or eosinophilic tumors or pseudo-enhanced small renal cysts may be misdiagnosed as small renal cancer, and showing intravenous cancer emboli is not as good as magnetic resonance examination. 1.3 Magnetic resonance imaging (MRI) is not superior to CT for the diagnosis of small kidney cancer, but it has no X-ray radiation and contrast allergy, has high resolution of soft tissue, can distinguish high-density cysts from kidney cancer, and has certain advantages for the characterization of pathological histological subtypes, so it is an alternative diagnostic imaging tool for patients with renal insufficiency and contrast allergy. For small renal masses that are difficult to diagnose with enhanced CT, the application of the more sensitive fat-suppressed dynamic-enhanced MRI has differential diagnostic significance[7] . MRI changes of small kidney cancer: T1-weighted image shows more uniform low signal or isosignal, with high signal for intra-tumor hemorrhage and low signal for cystic necrosis; T2-weighted image shows slightly high signal or mixed high and low signal, with pseudo-envelope as a low signal band around the mass. MRI is superior to CT in showing renal vein or inferior vena cava involvement, surrounding organ invasion, and tumor hemorrhage, necrosis, and cystic degeneration.MRI of normal vessels is low signal with flow space, and cancer thrombi are isosignal. MRI angiography after enhancement is the best method to show renal vein cancer thrombus, which can accurately show the cancer thrombus in renal vein and determine the existence and extent of cancer thrombus with an accuracy similar to that of digital subtraction angiography.The sensitivity of MRI in evaluating renal cancer staging and lymph node metastasis is similar to that of CT [9], and it is superior to CT in determining whether there is tumor infiltration in adjacent organs with an accuracy of 97%-100% [10]. MRI has the disadvantage of lower spatial resolution and higher cost, so it is generally used when CT examination is difficult to diagnose. 2. Progress in surgical treatment of kidney cancer In recent years, the surgical treatment of kidney cancer has made great progress. On the one hand, modern medical imaging technology has significantly improved the detection of early kidney cancer, which has triggered the impact of kidney unit preservation surgery (including open surgery and laparoscopic surgery) on traditional radical kidney cancer surgery. On the other hand, the development of medical equipment and technology has led to the challenge of minimally invasive surgery for kidney cancer (including radiofrequency, microwave, high-energy focused ultrasound, cryoablation, intra-tissue irradiation, ethanol injection therapy, etc.) to nephrectomy surgery, showing a broad application prospect. 2.1 Kidney unit preservation surgery (NSS) mainly includes renal tumor enucleation, partial nephrectomy, “bench surgery” + autologous kidney transplantation. Uzzo et al [11 ] summarized the experience of 1833 cases of kidney preserving surgery reported in the literature in the past 20 years, the local recurrence rate was 0%-10%, the 5-year survival rate was 72%-100%, and the incidence of multicenter tumor foci was 15%. The incidence of multicentric tumors was 15%. The size of the tumor directly affects the prognosis, for example, the local recurrence rate is 0%-3% for renal cancer ≤4cm in diameter, and the local recurrence and distant metastasis rate is 16% for >4cm in diameter. (2) Laparoscopic NSS: It is the result of the joint development of laparoscopic equipment and technology, and has evolved from treating small superficially convex tumors to treating renal cancer adjacent to the collecting system or renal sinus, near the hilum or completely intrarenal type, or isolated kidney, and has achieved the same efficacy as open surgery [12], while blood loss, hospital stay, and intraoperative and postoperative complications are significantly better than those of open surgery. However, this technique is difficult and requires good and complete laparoscopic instruments and skilled laparoscopic operation techniques. indications for NSS: (i) absolute indications include bilateral renal cancer or renal cancer requiring long-term hemodialysis after kidney resection due to functional or anatomical reasons; (ii) relative indications include renal cancer with impaired renal function due to congenital malformation or renal disease or with hypertension, diabetes mellitus, kidney stones, nephritis, etc.; (iii) The advantages and disadvantages of NSS and controversies: the advantages of NSS are that it can preserve kidney function to the maximum extent, avoid hemodialysis and improve patients’ quality of life; the disadvantage is that there is about 2%-16% local recurrence after NSS, and those with recurrence need to operate again, which may delay treatment. There is a possibility of delayed treatment. There is a consensus to use NSS for kidney cancer patients with absolute or relative indications, while it is controversial to perform NSS for early kidney cancer with selective indications. Proponents of NSS believe that after nephrectomy on one side, it is difficult to deal with tumors or other diseases in the opposite kidney, so the kidney should be preserved as much as possible. Those who oppose NSS believe that since 7%-25% of kidney cancers are multifocal and exist as satellite cancers, even more than 2 cm away from the primary cancer, the local recurrence rate of kidney preservation surgery can reach 10%, while the possibility of kidney cancer appearing in both kidneys is only 1%-2%, so there is no need to risk the local recurrence of the tumor. Despite the controversy, NSS is still an elective treatment for early kidney cancer. Precautions for the use of NSS: ①The first principle is to ensure tumor-free survival of patients. The multicentric lesions of renal cancer and extra-pseudomembranous cancer infiltration (including renal parenchymal infiltration and small venous cancer thrombi) are the main causes of tumor residual and local recurrence, and the safe resection range usually adopted at home and abroad includes the normal renal parenchyma 1 cm outside the tumor pseudomembrane [13]. For those who need to preserve renal function, a safe resection margin of more than 5 mm outside the pseudo-envelope can also be taken [14], but simple tumor enucleation is not recommended. (②Elective NSS must strictly grasp the clinical stage of the tumor (i.e., early renal cancer ≤4 cm) and explain the advantages and disadvantages of various treatment options to patients and families to obtain full understanding. ③NSS should leave enough renal function to maintain the survival of the organism, otherwise total nephrectomy plus blood purification should be performed. 2.2 Minimally invasive surgery for kidney cancer With the development of minimally invasive technology and improvement of probe design and delivery system, a variety of minimally invasive surgeries for kidney cancer have emerged in recent years, including radiofrequency, microwave, high-energy focused ultrasound, cryoablation, intra-tissue irradiation, ethanol injection therapy and so on. The advantages are that more normal kidney units can be preserved, less complications, faster recovery and shorter hospital stay. The disadvantage is that the tumor is not removed and there is a possibility of recurrence, and postoperative CT is needed to observe the contrast changes of the tumor foci and judge the efficacy. There is still controversy about its clinical efficacy. In addition, some equipment is very expensive. Cryoablation: In the past decade, local cryotherapy of small renal carcinoma has been carried out under B ultrasound, MRI-guided percutaneous, trans-laparoscopic or open surgery, and most of the results are satisfactory, but long-term follow-up results are lacking. Among them, Gill et al [15] reported 32 patients treated with laparoscopic cryotherapy for small renal carcinoma with a mean hospitalization of 1.8 d, complete recovery time of 2 weeks, and no tumor recurrence at a mean follow-up of 16 months (7-23 months), among which 23 cases were biopsied for residual masses after surgery and the results were negative. At present, it is believed that the freezing technique and its temperature regulation still need further research to improve. Radiofrequency ablation (RFA): Under the guidance of ultrasound, CT, etc., needle electrodes are placed into the tumor by percutaneous puncture or laparoscopy, and radiofrequency energy is used to generate high temperature, resulting in coagulation and necrosis of tumor cells. At present, the study of RFA for small kidney cancer has achieved certain effect, such as Pavlovich et al [16] applied RFA to treat 24 cases of diameter.