The effect of changes in tumor size on breast carcinoma survival in the US: 1975-1999

BACKGROUND. Temporal comparisons of case survival are commonly used to assess improvement in cancer treatment at the population level. However, such comparisons may be confounded by secular trends in disease prognosis, even within conventional stage categories. The objective of the current study was to characterize within-stage migration of tumor size in breast carcinoma, and to estimate the effect of this shift on reported breast carcinoma survival. METHODS. Population-based Surveillance, Epidemiology, and End Results (SEER) cancer registry data were used to evaluate secular trends in tumor size at the time of diagnosis and relative survival among localized and regional invasive breast carcinoma patients diagnosed between 1975-1999. Outcomes were stage-specific tumor size distribution, 5-year relative survival, relative survival standardized to the tumor size distribution of the cohort diagnosed between 1975-1979, and the percentage of improvement in relative survival attributable to shifts in tumor size distribution. RESULTS. Within each stage category, the proportion of smaller tumors increased significantly over time. Comparing patients diagnosed between 1995-1999 with those diagnosed between 1975-1979, within-stage migration of tumor size accounted for 61% and 28%, respectively, of the relative survival increases noted in localized and regional breast carcinoma. CONCLUSIONS. The tumor size distribution of incident breast carcinomas in SEER has shifted toward smaller tumors. A substantial fraction of the improvement in breast carcinoma survival noted since 1975 may be attributable to within-stage migration of tumor size.