The metastatic potential of renal tumors: Influence of histologic subtypes on definition of small renal masses, risk stratification, and future active surveillance protocols

Objective The influence of histology in metastatic potential is often overlooked when discussing the management options of small renal masses (SRM), with size or growth rate often serving as the triggers for the intervention. We aim to re-examine the definition of a SRM by evaluating the metastatic potential of renal masses incorporating tumor size and histology to create metastatic risk tables. Materials and methods Surveillance Epidemiology and End Results (SEER)-18 registries database was queried for all cases of clear cell, papillary, and chromophobe renal cell carcinoma (RCC) diagnosed between 2004 and 2012. There were 55,478 cases identified that included 43,783, 8,587, and 3,208 cases of clear cell, papillary, and chromophobe, respectively. Tumors were stratified using 1-cm increments to determine the metastatic potential by calculating the metastatic rate at presentation for different size intervals in histologic categories. Results For all 3 histologies, tumors measuring 5 cm or less had a rate of metastatic RCC at presentation of less than 4%. The metastatic potential was highest for clear cell, followed by papillary and then chromophobe tumors. Setting a cutoff of no more than 3% for metastatic potential to be called a SRM, makes clear cell carcinoma and papillary carcinoma a SRM up to 4 cm, whereas the chromophobe RCC would be considered a SRM up to 7 cm. Conclusion Although clinical staging and tumor size have been the key determinants in decision-making of patients with solid renal tumors, the histology-specific risks of metastatic potential are different for each mass. The definition of a SRM should be based on the metastatic potential and not on tumor size alone. This information could be helpful for counseling and managing patients with SRMs as well as for modifying active surveillance protocols.