The patient is a 65-year-old male with a history of well-controlled hypertension and a small mass in the right kidney found incidentally on CT. The mass was located on the ventral side of the kidney, solid, heterogeneous, with a maximum diameter of 3.2 cm. It was located in the right renal hilum adjacent to the renal artery and ureter. The patient had no complaints of discomfort, the examination was negative, and the left kidney was normal. The blood creatinine was 106 μmol/L. Question: How should this patient be further evaluated for management? Clinical problem With the widespread use of imaging such as CT, incidental renal masses are becoming more common. Renal masses are found incidentally in approximately 13-27% of abdominal imaging examinations. Most of them are small, purely cystic, benign masses without contrast enhancement and do not require management. A small proportion are solid or complex cystic masses that can be contrast-enhanced and considered malignant. Such masses will be enhanced on CT by 15 HU or more. In this article, we define a small renal mass as a renal mass with enhancement on imaging and a maximum diameter of 4 cm or less. From 1988 to 2003, the incidence of small renal masses increased relative to other renal tumors, accounting for 48%-66% of diagnosed renal tumors and 38% of resected renal tumors, and these patients are often asymptomatic. Of small renal masses, approximately 80% are malignant and 20% are benign. When incidental small renal masses are identified, differentiating between benign and malignant masses and the manner in which malignant masses are managed can be quite challenging. The Bosniak grading system can be used to clarify the nature of the cyst. The diagnosis of vascular smooth muscle lipoma can be confirmed if there is a fatty component in the renal mass on CT or MRI, but if there is calcification, this is often indicative of malignancy. 1. Benign renal masses Plain and enhanced CT show simple cysts with thin walls and watery densities within them. The Bosniak classification system is a method of classifying cystic masses based on imaging features.Bosniak grade I masses are benign, non-enhancing, with no enhancement. thin walled, without septa, calcifications, or solid components. bosniak grade II masses are benign cysts with thin septa and may have enhancement, fine calcifications, or slightly coarse calcifications that are not long. High-density, well-defined cysts without enhancement and up to 3 cm in diameter (also called high-density cysts) are also included in this category and do not require further evaluation. grade IIF cysts with multiple thin linear partitions or small smooth thickened walls or septa that may contain thick nodular calcifications cannot be measured with contrast enhancement. Cysts located entirely within the kidney, without enhancement, with high density, and up to 3 cm in diameter are included in this category. Bosniak grade III cysts have irregular or smoothly thickened walls or compartments that are enhanced by contrast, and some of these grade III cysts are malignant. bosniak grade IV masses are malignant and have all the features of a grade III cyst, but also include an enhancing soft tissue component (independent of the compartments or walls). Surgical resection is recommended for Bosniak grade IV masses. A small renal mass is shown on CT plain and on enhanced CT as a vascular smooth muscle lipoma containing fatty density. When a renal mass is solid or complexly cystic, the mass can be evaluated for possible malignancy by features such as size, shape, contour, and the presence of enhancement. Renal masses are best evaluated by specialized renal CT or MRI with scan thicknesses ranging from 3 mm to 5 mm. Renal masses can be evaluated as solid or complex cystic (Bosniak grade III or IV) by enhanced CT or MRI. Most enhancing masses are malignant; therefore, solid, enhancing renal masses are often managed as malignant when patients have a long life expectancy. Various imaging features of small renal masses (e.g., tumor size, location, depth of invasion, relationship to the hilum, condition of the contralateral kidney) influence management decisions. The imaging in this case showed an enhancing small mass in the right hilum, the middle pole of the kidney. The tumor in the hilum was directly adjacent to the renal artery, making it technically challenging for partial nephrectomy. In this case, laparoscopic partial nephrectomy was successfully performed. Figure B shows an enhancing cystic solid mass of the left kidney (Bosniak grade IV), which was pathologically confirmed as cystic renal cell carcinoma after partial nephrectomy. Figure C shows a completely solid and enhancing right renal mid-polar mass, 5.5 cm in diameter, in a 72-year-old patient with stage III chronic kidney disease and a functioning isolated kidney on the affected side. The contralateral kidney had atrophied and lost function. The patient underwent a successful partial nephrectomy. Figure D shows an enhancing small 0.9 cm diameter left renal mass. It was recommended to wait for observation and management. However, due to the patient’s youth, laparoscopic partial nephrectomy was finally chosen, although the tumor was small and the final pathological findings were G3 clear cell carcinoma with renal peritoneal invasion. Figure E shows an enhancing 4 cm renal mass on the ventral side of the left kidney. Partial resection pathology confirmed a pheochromocytoma, which was benign. Figure F shows small renal masses bilaterally, both of which underwent laparoscopic partial nephrectomy. The smaller the renal mass, the more likely it is to be benign. Of the 2770 surgically resected specimens with solid renal masses, 46% of the masses less than 1 cm in diameter were benign, 22% of the masses 1-2.9 cm in diameter were benign, and 20% of the masses 3-3.9 cm in diameter were benign. In malignant tumors, the larger the diameter, the higher the pathological stage. The growth rate of small renal masses is usually slow (2-4 mm/year). At short-term follow-up (within 3 years), there was no significant difference in the growth rate of benign and malignant masses. In a meta-analysis, 30% of small renal masses did not grow during the 23-29 month follow-up period. Of these, 83% of the masses that did not grow were considered malignant and 89% of those that did grow were considered malignant. There were no clinical or imaging features including initial tumor size and tumor pathology diagnosis that could effectively predict the rate of tumor growth. The majority of resected small kidney cancers were pathologically low-grade. However, in three studies, 14%-26% of kidney cancers 3-4 cm in diameter were high-grade (grade 3 or 4) and 12%-36% invaded the perirenal fat (defined as stage pT3a). Patients with symptomatic (e.g., low back discomfort or hematuria) small renal masses often have a worse prognosis than patients with incidental ones. Metastases are already present in 1-8% of patients with 3-4 cm diameter kidney cancer when first diagnosed. Analysis of the SEER database (Surveillance, Epidemiology, and End Results Program database) from 1998 to 2003 showed that metastases occurred in 5.2% of 8792 patients with small renal masses (less than or equal to 4 cm in diameter). The likelihood of metastasis increased by 3.5% for every 1 cm increase in the primary tumor. Puncture biopsy Puncture biopsy under CT guidance appears to be safe (there is a very small possibility of bleeding or puncture needle tract implantation). Its diagnostic sensitivity is 80-92% and specificity is 83-100%. Smaller tumors (<3 cm) have a higher false-negative rate, which can be reduced by repeated punctures and improved experience of the operator and pathologist. The fact that most punctures are benign does not exclude that the rest of the tumor is malignant. However, in some cases of renal vascular smooth muscle lipoma, posterior renal adenoma, or infection, the diagnosis can be confirmed if the puncture is benign. The diagnosis of benign is often suggestive of eosinophilic tumor, although it is difficult to differentiate pathologically from smallpigmented cell carcinoma. In the absence of clear evidence supporting the benign or malignant nature of the tumor and when puncture results are inconclusive, caution should be exercised and imaging follow-up, repeat puncture or surgical management is required. Combining techniques from histology or molecular biology may increase the sensitivity of puncture biopsy. The addition of molecular biology diagnostics including RNA extraction and polymerase chain reaction significantly increases the sensitivity of differentiating renal cancer subtypes (100% vs. 87%) and the negative predictive value for the diagnosis of renal clear cell carcinoma (100% vs. 87.5%) compared to histologic analysis alone. However, these findings need to be confirmed at other centers, and molecular diagnostic methods are not routinely used in clinical applications. Recommendations for management The clinical management of small renal masses is controversial, and there is a lack of randomized controlled data to compare outcomes of wait-and-see, surgical versus ablative treatments in the management of small renal cancers. Therefore, treatment options should be determined by the patient's tumor status, survival life expectancy, physician experience, and medical condition. Wait and see Wait and see uses ultrasound, CT or MRI to monitor the size of the tumor continuously. Despite the lack of evidence, CT or MRI is usually the preferred option due to the high level of clarity and reproducibility. An interval of 8-12 months between examinations is usually recommended. However, the cost of the test and the radiation dose of CT (30-90 mSv per test) need to be taken into account. Occasional metastases from small renal masses are relatively rare, making wait-and-see an attractive option for patients of advanced age and frailty with a short life expectancy. Also, wait-and-see may be considered for any patient with a mass less than 1 cm, despite the lack of evidence for an imaging interval. The surveillance process when the tumor progresses then requires surgical intervention, and this delayed intervention does not affect the future treatment profile. Wait-and-see should not usually be used in young, healthy patients due to the lack of sufficient evidence to support it and the inability of imaging to confirm the diagnosis of benignity or malignancy and the prediction of its biology. The current outcome of wait-and-see demonstrates that surgical intervention is not urgent. Preserved renal unit surgery Radical nephrectomy has been the standard of care for kidney cancer for many years; however, preserved renal unit surgery (partial nephrectomy) is gradually becoming the surgical option of choice for small renal masses. Partial nephrectomy can be performed both open and laparoscopically and involves removal of the mass and sufficient kidney tissue around it to preserve part of the kidney. Chronic kidney disease is becoming increasingly common (one study showed that a quarter of patients with small renal masses had chronic kidney disease), so preservation of kidney function is important. The only randomized controlled trial available comparing partial nephrectomy with radical nephrectomy in renal tumors less than 5 cm in diameter confirmed that partial nephrectomy is safe, except for a slightly higher rate of complications, including severe bleeding (3.1% vs 1.2%), urinary fistula (4.4% vs 0%), and secondary surgery (4.4% vs 2.4%). Tumor-specific mortality rates at 5 and 10 years after open partial nephrectomy were low (2.4% and 5.5%), similar to the results of radical nephrectomy. In one study, renal insufficiency (12% vs 22%) and proteinuria (35% vs 55%) were significantly lower at 10-year follow-up after partial nephrectomy than after radical nephrectomy. In another study, the risk of chronic renal insufficiency beyond stage 3 after partial nephrectomy (20%) was much lower than after radical nephrectomy (65%) (p<0.001). The value of preserving the renal unit is further affirmed by reports suggesting that 4-10% of contralateral kidneys may develop heterochronic tumors. Open partial nephrectomy often requires a lumbar incision of approximately 15 cm or even larger, sometimes requiring removal of the ribs. more than 50% of patients have ongoing incisional complications such as lumbar bulge, lumbar discomfort, abnormal sensation or herniation. Laparoscopic partial nephrectomy Minimally invasive procedures to preserve the renal unit include laparoscopic or robotic-assisted partial nephrectomy and imaging-guided ablation (Figure 3). In a large, retrospective multicenter clinical trial comparing laparoscopic and open surgery for partial nephrectomy for stage T1 tumors (78% of small renal masses), both groups had similar rates of intraoperative complications (<1.8%) and positive incision margins (<1.6%). After 3 years of follow-up, tumor-specific survival and renal function were similar in both groups. However, laparoscopic partial nephrectomy had a longer time to block blood flow than open partial nephrectomy (30 min vs 20 min) and a slightly higher incidence of postoperative bleeding (4.2% vs 2%). Our recently described laparoscopic "early opening of renal blood flow" technique resulted in a shorter renal vascular block time (mean 14 points) and a lower incidence of postoperative bleeding, similar to open surgery. A study showed that the overall mortality rate at 7 years for laparoscopic and open partial nephrectomy (16.9% vs 16.5%) was similar to the tumor-specific mortality rate (3.1% vs 2.3%). Preservation of the renal unit approach Preservation of the renal unit surgery (partial nephrectomy) is the preferred approach for small renal masses. Partial nephrectomy can be done laparoscopically (Figure A), open surgery, or robotically. The procedure usually involves blocking the renal blood supply to create a bloodless surgical field and removing the tumor, which includes a portion of normal renal tissue. After removal of the tumor, the vascular dissection and collecting system are tightly sutured to ensure hemostasis and no urinary leakage. To minimize ischemic renal injury, the block time is minimized to less than 20-30 minutes. Ablation includes cryoablation (Figure B; requires freezing of the entire tumor to ? 20 to ? 40°C) and radiofrequency ablation (requires heating to 60-100°C). This can be performed percutaneously (under imaging guidance) or laparoscopically, with a probe inserted into the tumor to deliver the ablative energy. The ablated tumor remains in the body after ablation. Laparoscopic partial nephrectomy can now be applied even to technically challenging small masses in the hilar region, central, complete intrarenal or isolated kidney. Clinical observations show that postoperative recovery time is shorter for laparoscopic surgery than for open surgery. It is important to note that laparoscopic surgery requires skilled surgical technique and studies have shown that the results of this procedure are better in specialized tertiary care hospitals. If experience with laparoscopy is lacking, open surgery may be preferable. Ablation therapy Ablation therapy uses a probe inserted into the tumor to produce a temperature sufficient to kill the cells. Cryoablation and radiofrequency ablation are the most commonly used treatments after a puncture biopsy has confirmed a tumor. In a group of 80 patients treated with laparoscopic cryoablation, a median follow-up of 8 years showed a mean reduction in tumor volume of 57% in the first year, 72% in the third year, and 89% in the fifth year. Seventy-three percent of the tumors were not detectable by MRI at year 5. At year 10, the overall and tumor-specific mortality rates were 49% and 17%, respectively (31% of patients had previous surgery for heterochronic contralateral renal cell carcinoma). With the development of probe technology, imaging-guided percutaneous ablation may be more effective and have a lower complication rate than laparoscopic ablation. Percutaneous radiofrequency ablation has also shown good results in short-term studies, although long-term follow-up is lacking. In three clinical trials involving 286 patients, patients were followed for an average of 1.2 to 2.3 years, with a 90% tumor control rate. Tumor control was defined as the absence of tumor enhancement on CT or MRI. Complications (1% bleeding, 1% reoperation, 5% pulmonary complications due to coexisting disease, and 1% congestive heart failure due to coexisting disease) occurred in approximately 10% of patients undergoing cryoablation. Complications after RF ablation occur in approximately 10% of patients (1-5% bleeding, 2% urethral injury or stricture, 1.6% severe neurogenic pain). After ablation, MRI or CT is recommended 6-12 months later based on experience. Residual tumor with enhancement or continued tumor growth is evidence for repeat treatment, including repeat ablation. Uncertainties There is no way to predict what kind of small renal masses will become problematic if left untreated for an extended period of time. Although more "preclinical" tumors (which can be considered cured) are being treated aggressively than in the past, the increasing mortality rate from kidney cancer may suggest that at least some small renal tumors are biologically "inert" and may not need to be Treatment. Approximately 1/3 of elderly patients die from unrelated coexisting disease 5 years after curative renal surgery, so the risk of surgery due to coexisting disease needs to be weighed when managing elderly patients. The interval between repeat imaging of small renal masses with stable presentation during the follow-up period is uncertain, and more studies are needed to define a reliable diagnostic basis to determine the prognosis of the tumor. The predictive value of molecular biological markers of pathology after puncture biopsy needs more studies. Randomized controlled data on the prognosis of corresponding wait-and-see, surgical and ablative treatments are lacking, and long-term follow-up data are still needed for ablative treatment, including a comparison of cryoablation and radiofrequency ablation. Recommended management process for small renal masses Puncture biopsy should be considered if the patient is young (<70 years) and healthy, and although wait-and-see is not recommended, wait-and-see and ablative treatment options still need to be discussed with the patient for their choice. The size of the tumor is an important factor to consider when developing a treatment plan. For example, a small spherical tumor of 1 cm has a volume of 0.5 ml, which becomes 33.5 ml for a 4 cm tumor, which implies a larger tumor load. Guidelines The recommendations in this article are informed by the 2007 EAU guidelines for the treatment of kidney cancer and the 2009 AUA guidelines for the treatment of small renal masses. Most of these guidelines are based on expert experience due to the lack of randomized controlled data. Conclusions and recommendations The patient in this case had a solid enhancing renal small mass. The patient should be aware of the risk of malignancy and the possibility that the tumor is benign or "lazy". A gross needle aspiration biopsy may be considered. Possible management tools need to be brought out and discussed. Although the current natural course of renal masses indicates a low risk of metastasis and growth to inoperable, we need to be aware of the benefits of surgery (especially partial nephrectomy) for the patient. Tumors near the hilum may not be suitable for ablation because it may cause damage to the nearby renal vessels and ureters. If partial nephrectomy is feasible and the pathology is confirmed to be malignant, reliable data suggest that patients have a greater than 90% chance of surviving beyond 10 years without local recurrence and with normal renal function.