Growth kinetics of renal masses: Analysis of a prospective cohort of patients undergoing active surveillance

Background: Active surveillance (AS) represents a treatment option for renal masses in patients who are not surgical candidates either because of existing comorbidities or patient choice. Among renal masses undergoing AS, some grow rapidly and require treatment or progress to metastatic disease. Patient and tumour characteristics related to this more aggressive behaviour have been poorly studied. Objective: To report the analysis of a multi-institutional cohort of patients undergoing AS for small renal masses. Design, setting, and participants: This prospective study included 82 patients with 84 renal masses who underwent AS in three Canadian institutions between July 2001 and June 2009. Intervention: All patients underwent AS for renal masses presumed to be renal cell carcinoma (RCC) as based on diagnostic imaging. Measurements: Age, sex, symptoms at presentation, maximum diameter at diagnosis (cm), tumour location (central/peripheral), degree of endophytic component (1-100%), and tumour consistency (solid/cystic) were used to develop a predictive model of the tumour growth rate using binary recursive partitioning analysis with a repeated measures outcome. Results and limitations: With a median follow-up of 36 mo (range: 6-96), the mean annual renal mass growth rate for the entire cohort was 0.25 cm/yr (standard deviation [SD]: 0.49 cm/yr). Only one patient (1.2%) developed metastatic RCC. Amongst all variables, maximum diameter at diagnosis was the only predictor of tumour growth rate, and two distinct growth rates were identified. Masses that are ≥2.45 cm in largest diameter at diagnosis grow faster than smaller masses. This series was limited by its moderate sample size, although it is the largest published prospective series to date. Conclusions: We confirm that most renal masses grow slowly and carry a low metastatic potential. Tumour size is a predictor of tumour growth rate, with renal masses <2.45 cm growing more slowly than masses >2.45 cm.