Multiple growth periods predict unfavourable pathology in patients with small renal masses

Objective: To use the number of positive growth periods as a characterization of the growth of small renal masses in order to determine potential predictors of malignancy. Patients and Methods: Patients who underwent axial imaging at multiple time points prior to surgical resection for a small renal mass were queried. Patients were categorized based on their pathological tumour grade and stage: favourable (benign, chromophobe and low-grade pT1–2 renal cell carcinoma [RCC]) vs unfavourable (high-grade of any stage and low-grade pT3–4 RCC). A positive growth period was counted each time the difference in greatest tumour diameters between two images was positive. The Cochran–Armitage trend test and Somers' D association were used to determine if the number of positive growth periods was correlated with unfavourable pathology. Results: Of the 124 patients, 86 (69.4%) had favourable pathology and 38 (30.6%) had unfavourable pathology. Those who had favourable pathology were younger than those who had unfavourable pathology: median (interquartile range [IQR]) 61.0 (52.2–66.0) vs 68.5 (61.5–77.0); P < 0.001. The overall growth rate was higher in the unfavourable group, but was not statistically significant: mean (sd) 0.7 (1.7) vs 1.6 (2.8) cm/year; P = 0.07. There was a significant trend difference in the number of positive growth periods between favourability groups (P = 0.02). An association between increased number of positive growth periods and unfavourable pathology was observed: 0.15 (95% confidence interval 0.02, 0.29). The ratios of favourable to unfavourable pathology were 1.8, 1.0, 0.66, 0.59 and 0 as the number of positive growth periods increased from 0 to 4, respectively. Conclusion: While overall growth rate was not predictive of pathology favourability, there was a positive association between the number of positive growth periods and unfavourable pathology. The number of positive growth periods may be a potential parameter for malignant potential in patients undergoing active surveillance for small renal masses.