14 The Natural History of SRMs As stated previously, extirpative surgical series indicate that 20% to 30% of SRMs are benign entities2 and of the lesions that are RCC, 70% to 80% are low-grade, early-stage lesions believed to have little malignant

potential.3,4,7,15 Supporting the indolent nature of these tumors, several meta-analyses have demonstrated a slow interval growth rate for most tumors under surveillance, on the order of 0.2 to 0.3 cm/year with 23% Inhibitors,research,lifescience,medical to 33% of tumors demonstrating a zero growth rate while under observation.5–7 In addition, reports of metastases while on surveillance for SRMs are rare.7 Therefore, sufficient retrospective data suggest that most SRMs behave in an indolent fashion and can be safely observed. The remaining 20% to 30% of SRMs are malignant tumors with CI-1033 mw potentially aggressive features; 15% to 25% of SRM RCCs are high-grade lesions (Fuhrman grade 3–4). Locally advanced disease (≥ pT3) has been documented in 10% to 40% of SRMs, and 3% to 12% present with Inhibitors,research,lifescience,medical or will develop metastatic disease.3,15,16

Although a small proportion of patients may present with synchronous metastatic disease and an SRM, the existing literature implies that the risk of developing Inhibitors,research,lifescience,medical metastatic disease while undergoing AS for a SRM is even smaller—on the order of 1%.6,7 Consequently, synchronous and metachronous metastases may be different entities and patients who present without distant disease are more likely to have indolent tumors with little metastatic

potential. Therefore, an efficacious AS program should recognize the heterogeneity Inhibitors,research,lifescience,medical of SRM biology and seek to distinguish indolent lesions from aggressive tumors based on clinical parameters so that ideally, no patients Inhibitors,research,lifescience,medical die of RCC but rather of competing causes. Efficacy and Oncologic Outcomes for Patients Undergoing AS Despite a lack of Level I evidence, a number of robust, retrospective series demonstrate favorable outcomes for contemporary patients undergoing AS. More than 70 peer-reviewed articles appear within Medline on the topic of AS for SRM and a recent meta-analysis included 18 retrospective series comprising 880 patients.7 A number of retrospective AS cohorts demonstrated else a 0% to 5.7% risk of progression to metastasis while on surveillance with prospective studies and meta-analyses showing an overall rate of metastasis on the order of 1%.5–10 Although a direct comparison of AS to intervention is lacking, historic recurrence rates and cancer-specific survival following treatment (regardless of the intervention) are in the range of 90% to 95% and 95% to 99% at 5 years, respectively—indicating both the indolent nature of T1 lesions and the difficulty in comparing AS and primary treatment options.