The natural history of observed enhancing renal masses: Meta-analysis and review of the world literature

Purpose: Standard therapy for an enhancing renal mass is surgical. However, operative treatment may not be plausible in all clinical circumstances. Data on the natural history of untreated enhancing renal lesions is limited but could serve as a decision making resource for patients and physicians. We examined available data on the natural history of observed solid renal masses. Materials and Methods: A Medline review of the literature was performed from 1966 to the present regarding untreated, observed, localized solid renal masses. To these data we added our institutional experience with a total of 61 lesions observed in 49 patients for a minimum of 1 year. Variables examined were initial lesion size at presentation, growth rate, duration of followup, pathological findings and progression to metastatic disease. Overall weighted mean estimates were calculated for lesion size at presentation, growth rate and followup based upon combining single institutional series with complete information. Results: We identified 10 reports from 9 single institutional series in the world literature regarding the natural history of untreated solid localized renal lesions. The series included 6 to 40 patients (mean 25) with a mean followup of 30 months (range 25 to 39). When combined with our institutional data, a total of 286 lesions were analyzed, of which 234 could be included in the meta-analysis. Mean lesion size at presentation was 2.60 cm (median 2.48, range 1.73 to 4.08). Meta-analysis revealed a mean growth rate of 0.28 cm yearly (median 0.28, range 0.09 to 0.86) at a mean followup of 34 months (median 32, range 26 to 39) in all series combined. Pathological confirmation was available in 46% of the cases (131 of 286) and it confirmed 92% (120 of 131) as RCC variants. Evaluable data in this subset of confirmed RCC demonstrated a mean growth rate of 0.40 cm yearly (median 0.35, range 0.42 to 1.6). Lesion size at presentation did not predict the overall growth rate (p = 0.46). Progression to metastatic disease was identified in only 1% of lesions (3 of 286) during followup. Conclusions: The majority of small enhancing renal masses grow at a slow rate when observed. Although metastatic and cancer specific death are low, serial radiographic data alone are insufficient to predict the true natural history of these lesions. Therefore, physicians and patients assume a calculated risk when following these tumors. Basic biological data are needed to assess the natural history of untreated renal masses.