This article reviews active surveillance (AS) in the management of small renal masses (SRM), the role of renal tumour biopsy (RTB), patient selection, tumour growth kinetics, and outcomes. SRMs which are defined as masses ≤4 cm in diameter and enhance on triphasic CT characteristic of renal cell carcinoma (RCC) are increasing in incidence and AS is increasingly recommended. The stage migration and increased abdominal imaging have improved the five-year DSS, but overall mortality is unchanged. 46.3% of SRM <1cm are benign and 30% of incidental SRMs are benign. AS is defined as: initial monitoring of tumour size by serial abdominal imaging (US, CT, or MRI) and delayed intervention for those that progress. RTB plays an important role with >90% accuracy, safety, and concordance with final histology (systematic review and meta-analysis). Tumour size, solid vs. cystic appearance, and exophytic location are significant predictors of a successful biopsy. Von Hippel Lindau patients with clear cell renal cell carcinoma (ccRCC) are routinely followed until tumours are >3cm before treatment. Growth is slow and metastases are not described in these ccRCCs <3cm. SRM size correlates with the risk of malignancy and metastasis. Every 1cm increase in tumour size is associated with a 17% increase in malignancy risk. Overall risk of metastasis in new tumours <3cm is 2.4% and 8.4% for tumours 3.1-4cm. Currently, local tumour growth is the only clinically useful prognostic factor for risk of metastasis. Growth rate (serial axial diameters) or tumour volume is most commonly used for measuring tumour progression. Average tumour growth rate was 0.1-0.4cm per year and was not predictable by either initial size or type. In SRMs, the risk of a potentially aggressive RCC is roughly 20%, >50% are indolent, and about 30% are benign. Patients >70 with SRMs, benefit the least from surgery. Indications for AS include: diminished life expectancy, co-morbidities with high risk, severe renal dysfunction, informed younger patients who refuse active treatment; AS is contraindicated in healthy younger patients and non-compliance to follow-up.
Delayed Intervention and Surveillance for Small Renal Masses (DISSRM) registry was used to develop a scoring system from the existing literature. Presently, local progression and metastasis risk with AS is not accurately predictable. In a meta-analysis (n-6471), no statistical differences were noted in metastatic progression whether managed by surgical excision, ablation, or surveillance. Delaying intervention for SRMs did not compromise the feasibility of nephron sparing or laparoscopic surgery nor increase risk of progression.
There is currently no clear consensus about the best imaging technique or optimal follow-up schedule in AS protocols. The American Urological Association recommends CT or MRI within six months after the start of AS followed by CT, MRI, or US at least annually thereafter. Experts suggest serial imaging at three monthly intervals for one year, and then every six months up to three years and yearly thereafter if there is minimal growth or no growth. In conclusion, lesions that progress on AS are usually >3cm and have rapid growth rates and with intermediate follow-up, progression usually occurs more than three years after diagnosis.