Cancer survival trends in Osaka, Japan: the influence of age and stage at diagnosis.

Background: Five-year survival is an important index for evaluating the overall effectiveness of cancer diagnosis and treatment. The aim of this study was to monitor trends in 5-year cancer survival in Osaka, Japan, during 1975-94, with adjustment for trends in age and stage at diagnosis.
Methods: Five-year crude survival was estimated for cancers of the stomach (39,697 patients), lung (17,400 patients) and breast (11,988 women) in four consecutive 5-year periods, with adjustment for age and/or stage, using the age and stage distribution of cases diagnosed during 1975-79 as standard weights. We estimated the impact of adjusting for either age or stage alone by their proportionate impact on the trend in fully (age- and stage-) adjusted survival.
Results: The absolute increase in unadjusted 5-year survival over 20 years was about 20% for stomach cancer, 14% for breast cancer and 5% for lung cancer. Lack of age adjustment would have caused proportionate under-estimation of these trends by 13-14% (stomach), 7% (breast), 14% (lung, men) and 4% (lung, women). Lack of adjustment for the trend toward earlier stage would have caused proportionate over-estimation (152%, men: 133%, women) of stomach cancer survival trends, which seemed more influenced by earlier diagnosis than more effective treatment. For breast cancer, the 31% over-estimation of trend from lack of stage adjustment suggests the impact of earlier diagnosis, while the improvement of survival after additional adjustment for age may be due to more effective treatment. Failure to adjust for stage led to a proportionate 21% under-estimation of lung cancer survival trends for men, and 4% over-estimation for women.
Conclusion: This study confirms the importance of adjusting for trends in age and stage distribution when evaluating time trends in cancer survival.