This study supported that CNB was high yield in diagnosing SRMs. CNBs achieved specific histologic diagnosis in 86.9% (146/168) of SRMs in this study, similar to previous reports (80 − 95%) [3, 15]. CNBs not only provided histologic evidence of malignancy versus benignity, but also provided prognostic information such as tumor subtypes and nuclear grading. The accuracy of tumor typing (100%), subtyping (97.3 -100%) and nuclear grading for CCRCC (83.8%) in this study was similar to data from other groups [15, 16, 21,22,23].

Core needle biopsy had a great impact on the outcomes of this cohort. A significant number of patients (24/168, 14.3%) with suspicious small renal masses were reassured of benign diagnosis by core needle biopsies. As a result, the great majority (over 90%) of these patients chose active surveillance instead of partial nephrectomy or ablation, which would have been recommended to these patients at other institutions where CNB is not routinely performed. Clearly, biopsy allows for incorporation of histopathology into the decision-making process. As such, the benign resection rate has been reduced to a minimal level (3.2%) at our institution compared to national average (over 30%) [11, 24]. A systemic review of surgical series and United States population level burden estimate found that benign histology accounted for 40.4% of resected renal masses measuring less than 1 cm, and that misclassified benign lesions remained high (17.2%) for renal masses measuring 3–4 cm [9]. On the other hand, this study demonstrated that routine application of biopsy into the management of small renal masses was transformative and almost completely eliminated benign pathology on surgery. Similarly, other studies have also shown renal tumor biopsy reduces surgery for benign tumors [24, 25]. An analysis of 106,258 patients with small renal masses from the National Cancer Data Base (NCDB) from 2004 to 2015 showed increased use of renal biopsy (from 8.0 to 15.3%) and an associated increase in non-surgical management (from 11.7 to 15.6%). Altogether, these data suggest that SRM CNB has the potential to alter patient management and reduce the risk of overtreatment and mistreatment. In addition, a small number of patients were confirmed to have metastatic carcinomas or lymphoma, for which systemic chemotherapy was given rather than surgery or ablation. Therefore, CNBs should be considered before invasive interventions for any SRMs.

A common concern, however, for renal mass biopsy is the non-diagnostic results [23, 26]. Based on a previous review of 20 studies including 2,979 patients and 3,113 biopsies of localized renal masses, the non-diagnostic rate of renal mass core biopsy was as high as 13.9%, among which, 90.4% were diagnosed as malignant lesions upon resection [17]. In the current study, among the 16 negative/non-diagnostic CNBs, 2 cases proved to be malignant in resection, leading to a false negative rate of at least 12.5%. Another patient underwent partial nephrectomy for AML due to non-diagnostic CNB result. These results underscore the importance of clinical and radiological correlation when the biopsy is clearly non-diagnostic or yields only normal kidney parenchyma. To improve the diagnostic rate, repeat biopsy may be considered in patients with high clinical suspicion [15, 22, 23]. In addition, rapid onsite evaluation by touch preparation may have some merit in improving the diagnostic yield of small biopsies.

It’s also noted that nuclear grading by CNB was less than optimal given that 16.2% of CCRCCs were eventually upgraded to higher nuclear grade on resection. The discrepancy of nuclear grade was mostly due to intra tumoral heterogeneity as previously discussed [1, 27,28,29]. Therefore, cautions should be exercised when determine the treatment options to avoid undertreatment of focally aggressive tumors.

Overall, CNB diagnosis had a significant impact on the treatment for patients with small renal masses. Most RCCs were managed by resection or ablation while surgery was rarely applied to benign lesions and non-RCC type malignant tumors. Additionally, the management of the atypical and negative/non-diagnostic cases was variable, which in turn highlighted the challenge for managing patients without clear pathologic diagnosis.

The limitation of this study is the relatively small size of the cohort. Also, the retrospective chart review may not capture all the nuances and factors that might also have contributed to the management decisions for all patients. Additional studies focusing on radiological-pathological correlation may provide insight on how to further stratify malignant risks based on radiologic features.