Alessandro Volpe and Michael A S Jewett

Division of Urology, Department of Surgical Oncology, Princess Margaret Hospital and the University Health Network, University of Toronto, Toronto, Ontario, Canada

Malignant tumors of the kidney cause about 2% of cancer incidence and mortality in the United States. It was estimated that 31,900 new cases of kidney cancer were diagnosed in 2003 and there were 11,900 deaths (1,2).

Since 1950, there has been a 126% increase in the incidence of renal cell carcinoma in the United States (3). Between 1975 and 1995, the incidence has been increasing by 2.3% annually among white and 3.9% among black men (4). A raising trend has been observed worldwide and is due in part to the widespread use of new and improved noninvasive abdominal imaging modalities, such as ultrasonography, computed tomography, and magnetic resonance imaging (3-14). The number of abdominal radiological examinations has been increasing steadily in the last two decades and almost doubled between 1986 and 1994 (4).

The increasing incidence of renal cell carcinoma has occurred in all age groups and in all clinical stages, but the greatest increase has been observed in localized tumors, which increased by 3.7% per year from 1973 to 1998 (4,7).

Most renal cell carcinoma are now incidentally detected by imaging as small renal masses in asymptomatic patients, while historically almost 80% of renal tumors presented with flank pain and/or hematuria. In the early 1970s, the reported incidental detection rate was 7% to 13%, which has increased to 48% to 66% in the recent years (15-23). Tumor size at diagnosis has also substantially decreased over time. Series from the Memorial Sloan Kettering Cancer Center and the Mayo Clinic report that the mean size of resected renal tumors has dropped from 7.8 to 5.3 cm from 1989 to 1998, and that there was a 32% decrease in mean tumor size at the time of diagnosis, respectively (24,25).

There are numerous reports that the incidentally detected lesions are on average smaller and present at an earlier stage than those detected in symptomatic patients (7,11,13,14,16,19,20,25-32).

Tsui et al. reviewed the records of 633 consecutive patients who underwent surgical treatment for renal cell carcinoma at University of California, Los Angels between 1987 and 1998. Stage I lesions were discovered in 62.1% of patients with incidental renal cell carcinoma and 23% with symptomatic renal cell carcinoma (p = 0.001). Mean tumor size was 5.1 cm versus 7.3 cm in incidental versus symptomatic cases (p < 0.05) (14). Patard et al. evaluated a series of 400 renal tumors and observed significantly smaller neoplasms in the incidentally detected group (5.7cm vs. 8.7cm; p < 0.001) (27).

Small asymptomatic tumors are more frequently benign. If proven to be renal cell carcinomas, they are on average lower grade than symptomatic ones (11,14,19,33-35).

Frank et al. recently reviewed the pathology of 2935 renal tumors at the Mayo Clinic and observed that as tumor size decreases there is a significant increase in the likelihood of having a benign tumor, a papillary compared to a clear cell histology and a low-grade compared to a high-grade malignancy. In their experience, 30% of tumors less than 4 cm in maximum dimension were benign and over 87% of those that were clear cell renal cell carcinoma were low-grade tumors (34).

Finally, several authors reported that small incidentally detected tumors are characterized by better survival outcomes (11,14,16,21,23,27,29,33,36). The five-year disease-free survival rate is 95% to 100% for incidental, less than 4 cm tumors treated with radical or partial nephrectomy (25,26,37). The first evidence of an association between tumor size

■ NATURAL HISTORY OF SMALL RENAL TUMORS

■ ACTIVE SURVEILLANCE OF SMALL RENAL TUMORS

■ REFERENCES

■ COMMENTARY : Morton A. Bosniak

Small renal neoplasms are generally removed soon after diagnosis. Therefore, their natural history has been historically poorly understood.

and prognosis was reported by Bell, who noted an increased rate of metastasis in patients found at postmortem to have renal cell carcinoma greater than 3 cm (38,39). Tumor size has always been incorporated in the tumor node metastasis staging system. In the 1997 version, the T1 stage was expanded from less than 2.5 to less than 7cm, because the lower cutoff value was not associated with a significant difference in survival (40). The current version defines the cutoff point at 4 cm to subdivide stage T1 into T1a and T1b (41-43). Tumor size remains the most important prognostic factor for renal cell carcinoma.

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