New TNM Staging System Predicts Progression of Small Renal Masses Under Active Surveillance: A Retrospective Analysis of a Single-center Prospective Series

: We validated the newly proposed cT1 classification (new cT1a -tumor size ≤3 cm - vs new cT1b -tumor size >3 cm) for renal masses treated with active surveillance (AS). A prospective AS cohort was retrospectively assessed for growth rate and progression (defined as cTNM upstage, volume doubling time [VDT] <12 mo, onset of metastasis, and/or tumor-related death). Out of 130 patients, 22 (17%) had new cT1b tumors. These patients were older (80 vs 69 yr) and with a lower estimated glomerular filtration rate (46 vs 63 ml/min/m2). New cT1b masses were more likely to undergo a renal mass biopsy (46% vs 23%), which showed low-grade renal cell carcinoma in most cases. Growth rates were significantly faster for new cT1b masses (median 0.24 vs 0.12 cm/yr, p < 0.001). Overall, progression defined as cTNM upstage or VDT >12 mo occurred in 27 patients (11 new cT1b vs 16 new cT1a tumors). Progression-free survival rates at 24, 36, and 60 mo was 73%, 60%, and 40% for new cT1b versus 95%, 88%, and 79% for new cT1a masses, respectively (p < 0.001). In multivariable Cox regression models at a landmark of 12 mo, after adjusting for growth rate, the new TNM classification independently predicted progression (hazard ratio = 4.63, 95% confidence interval: 1.91, 11.2, p < 0.001). The major limitations are the retrospective nature and the lack of renal mass biopsies in a proportion of cases. Our results validated the new cT1a classification system, which may better select patients eligible for AS.