Background: The recently revised American Joint Committee on Cancer (AJCC) sixth edition cancer staging system increased the stratification within colon cancer stages II and III defined by the AJCC fifth edition system. Using nationally representative Surveillance, Epidemiology, and End Results (SEER) data, we compared survival rates associated with colon cancer stages defined according to both AJCC systems.

Methods: Using SEER data (from January 1, 1991, through December 31, 2000), we identified 119,363 patients with colon adenocarcinoma and included all patients in two analyses by stages defined by AJCC fifth and sixth edition systems. Tumors were stratified by SEER's "extent of disease" and "number of positive [lymph] nodes" coding schemes. Kaplan-Meier analyses were used to compare overall and stage-specific 5-year survival. All statistical tests were two-sided.

Results: Overall 5-year survival was 65.2%. According to stages defined by the AJCC fifth edition system, 5-year stage-specific survivals were 93.2% for stage I, 82.5% for stage II, 59.5% for stage III, and 8.1% for stage IV. According to stages defined by the AJCC sixth edition system, 5-year stage-specific survivals were 93.2% for stage I, 84.7% for stage IIa, 72.2% for stage IIb, 83.4% for stage IIIa, 64.1% for stage IIIb, 44.3% for stage IIIc, and 8.1% for stage IV. Under the sixth edition system, 5-year survival was statistically significantly better for patients with stage IIIa colon cancer (83.4%) than for patients with stage IIb disease (72.2%) (P<.001).

Conclusions: The AJCC sixth edition system for colon cancer stratifies survival more distinctly than the fifth edition system by providing more substages. The association of stage IIIa colon cancer with statistically significantly better survival than stage IIb in the new system may reflect current clinical practice, in which stage III patients receive chemotherapy but stage II patients generally do not.