With the availability of imaging, more than 50 renal cell carcinomas are now detected accidentally during the examination of non-specific abdominal symptoms or diseases of other organs. Imaging plays an important role at different stages of the diagnosis and management of renal cell carcinoma: for detection, localization, characterization, and staging of the primary tumor; as an aid to intraoperative localization; and as a follow-up tool during postoperative and non-operative treatment.

1. Chest x-ray

Patients with renal cell carcinoma should routinely have frontal and lateral chest X-rays. For patients with suspicious nodules on chest X-rays or those with clinical stage ≥ stage III, chest CT should be performed.

2. Ultrasound examination

Abdominal ultrasonography is the easiest and most commonly used method to detect renal tumors. Renal ultrasonography is useful to identify benign and malignant renal tumors and is indicated for the differential diagnosis of renal tumors in patients with chronic renal failure or iodine allergy that precludes enhanced CT scanning, as well as patients with complex renal cysts.

(1) Diagnosis of primary foci of renal cell carcinoma

.

(1) Diagnosis of primary renal cell carcinoma

.

1) Gray-scale and Doppler ultrasound

(1) Diagnosis of primary renal cell carcinoma

: Ultrasound is economical, simple, radiation-free, and has a high prevalence rate, and is the preferred method for clinical diagnosis of renal tumors. Most asymptomatic renal cell carcinomas are detected by ultrasonography. Gray-scale ultrasound can show the size, location and relationship between tumor and surrounding tissues. Color Doppler flow imaging
（Color Doppler flow imaging (CDFI) can provide information on the status of blood supply to the tumor, and can also provide a preliminary evaluation of venous tumor thrombosis. Gray-scale ultrasound and CDFI have a high sensitivity for the identification of cystic kidney tumors.

2) Ultrasonography

: For solid renal tumors, enhanced imaging is one of the most important tools to differentiate benign from malignant lesions. Real-time gray-scale ultrasonography
（The sensitivity and accuracy of blood flow examination can be improved with Contrast-enhanced ultrasound (CEUS), which provides more information on the early arterial perfusion and microcirculatory status of the mass and has high sensitivity and specificity for the detection and characterization of renal cell carcinoma.

(2) Preoperative staging of renal cell carcinoma

(2) Preoperative staging of renal cell carcinoma

: The scope of ultrasound examination is limited, and the accuracy of tumor staging is not as good as that of CT because of the influence of imaging resolution, patient conditions, and operator experience.

(3) Intraoperative diagnosis of renal cell carcinoma

Ultrasound is often used for intraoperative exploration to determine the extent of surgery, in addition to the routine use for guiding tumor puncture biopsies because of its non-radiation, flexible, and convenient nature. Intraoperative examinations can correctly visualize renal tumors and provide a clean determination of the relationship between the tumor and the renal pelvis and the extent of tumor thrombus in the renal vein, inferior vena cava, and right atrium.

3. CT examination

Abdominal CT is the most common test for preoperative diagnosis and postoperative follow-up of renal cell carcinoma. CT scans can diagnose most renal tumors qualitatively and with high diagnostic sensitivity and specificity. On CT scan, renal clear cell carcinoma has a typical contrast “fast-in-fast-out” appearance: a heterogeneous iso-/low-density round-like mass on plain scan, and a medium- to highly-enhanced tumor in the dermal medullary phase and a low-density mass in the parenchymal phase. The tumor density in the parenchymal stage is lower than that in the renal parenchyma. However, it should be noted that it is still difficult to differentiate some rare types of renal cell carcinoma from benign tumors such as eosinophilic adenoma and lipid-depleted vascular smooth muscle lipoma.
In addition to qualitative diagnosis, CT examination can provide patients with additional diagnostic information preoperatively, including: the extent of tumor invasion, including whether the venous system is invaded (T stage), whether regional lymph nodes are metastatic (N stage), whether there are metastases in organs adjacent to the scan area (M stage), whether there are metastatic vessels (CTA) and a rough assessment of the morphology and function of both kidneys.
Bosniak classification of renal cystic masses: Renal cystic masses are a group of diseases with a predominantly cystic presentation, which can be congenital, infectious, secondary or neoplastic (benign and malignant) disease. The imaging presentation can range from simple cysts, slightly complex cystic lesions to complex cystic solid masses. bosniak classifies renal cystic masses into 4 categories based on CT presentation and provides clinical management according to the different levels (see Table 6 for details). The diagnostic criteria of Bosniak’s categories I, II, and IV are clearer and the management advice is more appropriate, but the diagnostic sensitivity and specificity of some patients in categories IIF and III are still relatively low and need to be supplemented by further studies. MRI or CEUS may be helpful for the diagnosis of these lesions.
Table 6 Bosniak classification and management of renal cystic masses

4. MRI examination

The sensitivity and specificity of MRI for the diagnosis of renal cell carcinoma are equal to or slightly higher than those of CT. MRI is more accurate than CT for the diagnosis of renal vein and inferior vena cava tumor thrombosis, and it is clearer than CT for the visualization of structures within the renal cystic lesion. MRI is more accurate than CT in the diagnosis of renal vein and inferior vena cava thrombosis, and it also shows the structures in cystic renal lesions more clearly than CT. The differential diagnosis of renal cell carcinoma and hemorrhagic renal cysts is also superior to CT, so MRI may be a better choice than CT for these lesions.

5. Positron Emission Tomography

Currently, the most widely used imaging agent for positron emission tomography-computed tomography (PET-CT) examinations is fluorine-18-fluorodeoxyglucose (18F-fluorodeoxyglucose).
(18F-Fluorodeoxyglucose (18F-FDG)), which is directly excreted by kidney without metabolism after intravenous injection, affects the display of renal lesions; on the other hand, the expression of GLUT-1 in the cell membrane of grade I-II renal clear cell carcinoma is low, and the fluorodeoxyglucose-6-phosphate catabolic enzyme of renal cell carcinoma is too high, resulting in only about half of the primary foci of renal cell carcinoma showing fluorodeoxyglucose. Therefore, 18F-FDG PET-CT imaging has limited diagnostic value for primary foci of renal cell carcinoma and is not recommended for routine use. Other new imaging agents that have been studied more often are fluorine-18 or carbon-11 labeled acetate, which have good imaging effect on renal cell carcinoma with better differentiation and lower malignancy, and can make up for the shortage of single 18F-FDG imaging, but they are still in the research stage and are not routinely examined. However, several studies have shown that PET-CT imaging is better than conventional imaging for lymph node metastasis and distant metastasis of renal cell carcinoma, especially in determining bone metastasis or skeletal muscle metastasis of renal cell carcinoma, and can monitor the efficacy of treatment and predict the prognosis of patients through changes in glucose metabolism at an early stage.

6. Nuclear bone imaging

Bone metastases from renal cell carcinoma tend to occur in the middle and long bones, and can be solitary or multiple, showing swelling and osteolytic bone destruction, invading bone marrow tissue in the early stage and destroying bone trabeculae, bone cortex, and forming soft tissue masses in the surrounding area as the disease progresses. Nuclear bone imaging is the preferred screening method for bone metastasis of renal cell carcinoma, but the sensitivity is only about 50. Patients with bone pain and other bone-related symptoms or elevated serum alkaline phosphatase or clinical stage ≥III renal cell carcinoma should undergo bone scan to clarify whether there is bone metastasis. MRI is very sensitive to tumor tissue in bone marrow and its surrounding edema, and can detect metastases and surrounding soft tissues that have not yet caused obvious bone destruction; MRI is preferred when spinal metastases are suspected.

7. Renal dynamic imaging

Nuclear renal dynamic imaging can accurately evaluate preoperative bilateral and fractional kidney function in patients with renal cell carcinoma and can help guide decisions about surgical options.

8. Renal tumor puncture biopsy

Percutaneous renal aspiration biopsy, including hollow-needle biopsy and fine-needle aspiration (FNA), can provide a pathologic histologic basis for renal tumors that cannot be diagnosed by imaging. Empty-needle biopsy is more accurate than FNA for the diagnosis of malignant tumors. Hollow-core needle biopsy is preferred for renal tumors with solid components. The coaxial technique allows multiple biopsies to be taken through coaxial cannulae, avoiding the risk of potential tumor tract implantation and metastasis. At least two good quality tissue specimens should be obtained, avoiding necrotic areas. The diagnostic yield and accuracy of hollow-core needle biopsy for cystic renal tumors is low and is not recommended.
The risk of puncture and the potential risk of spread, although low, should not be ignored. Percutaneous renal puncture biopsy is not indicated in critically ill patients. For patients undergoing surgery, puncture biopsy is also not recommended due to the high diagnostic accuracy of enhanced abdominal imaging. For patients with renal cell carcinoma who are not suitable for surgical treatment (old and frail, or have contraindications to surgery), or patients with advanced renal cell carcinoma who cannot be treated surgically, a puncture biopsy of renal tumor before systemic therapy can clarify the pathological diagnosis (including the type of pathology) and help to select therapeutic drugs. Patients with renal cell carcinoma who choose ablation therapy should undergo renal tumor aspiration biopsy to obtain pathological diagnosis. Therefore, in practice, it is still necessary to consider the risk of puncture, the skill level of the operator, and whether it may affect the current treatment plan to make a comprehensive decision.