Safe range of preserved renal unit surgery for stage T1b renal cancer OBJECTIVE: To study the safe margin range of preserved renal unit surgery for stage T1b renal cancer. MATERIALS AND METHODS: 87 specimens of stage T1b renal cancer after radical nephrectomy were retrospectively studied, and serial large sections of the whole kidney were made to microscopically observe the integrity of the pseudo-envelope of renal cancer, the distance of tumor invasion out of the pseudo-envelope and the multicenter cancer foci, with 92 specimens of stage T1a renal cancer in the same period as the control group. RESULTS: The incompleteness of the pseudo-envelope (34% [30/87] vs 18% [17/92], P = 0.015) and the rate of cancer foci invading the pseudo-envelope (39% [34/87] vs 25% [23/92], P = 0.043) were significantly higher in the T1b group than in the T1a group. The distance of cancer foci invading the pseudo-envelope was within 3 mm, and the incidence of multicentric cancer foci was 6% (5/87) in the T1b group and 5% (5/92) in the T1a group (P = 0.928). CONCLUSION: In stage T1b kidney cancer, all cancer foci invading out of the pseudo-envelope did not exceed 3 mm; therefore, the safe margin range for preserving the renal unit for surgery of stage T1b kidney cancer should be 4 mm outside the pseudo-envelope.