Failure to account for shift in tumor size can lead to overestimation of the impact of treatment advances

A new study finds that the size of newly diagnosed breast cancers has shifted towards smaller tumors, even within conventional cancer stage categories, and that this shift accounts for a proportion of the improvement seen in breast cancer survival over the last 30 years.

The authors of the report say that failure to account for this shift in tumor size can lead to overestimation of the impact of treatment advances. The study is published in the September 15, 2005 issue of CANCER, a peer-reviewed journal of the American Cancer Society.

Great strides in breast cancer survival have been made over the last 30 years, overall and within cancer stages, coinciding with advances in treatment and with increased use of screening mammography. However, if important prognostic factors have also changed over time, then observed improvements in breast cancer survival may be a result of such changes and of improvements in treatment. The authors studied changes in tumor size because it is a strong predictor of breast cancer prognosis and it is a straightforward, reliably evaluated, consistently available measure.

Elena B. Elkin, Ph.D. of the Memorial Sloan-Kettering Cancer Center in New York and her colleagues reviewed data on early-stage breast cancers from the Surveillance, Epidemiology and End Results (SEER) program to look for trends in tumor size and explore how those trends might impact survival rates. SEER is a population-based cancer registry system that collects and monitors data on cancers diagnosed in certain areas of the U.S.

Among localized and regional breast cancers, the size of newly diagnosed tumors decreased significantly from 1975 through 1999. While breast cancer survival rates improved during this time, adjusting for the changes in tumor size diminished the magnitude of the survival increases within each stage category. When the researchers compared five-year cancer survival rates in women diagnosed 1995-1999 with survival rates in women diagnosed 1975-1979, they found that the shift toward smaller tumors explained 61 percent of the survival increase in localized breast cancer and 28 percent of the survival increase in regional breast cancer.

"Failure to adequately control for this [within-stage tumor size] shift leads to inflated estimates of the impact of secular changes in treatment on stage-specific survival," conclude the study's authors. They add, "size-standardization is a refinement...that greatly improves our ability to interpret…trends in breast cancer survival."