An indirect study of cancer survival in the context of developing countries.

ABSTRACT

With classical approaches, survival refers to the life of a person after diagnosis of disease, and survival studies deal with measurement of the same to evaluate overall performance of a group of patients in terms of quality and quantity of life after diagnosis/treatment. There are numerous difficulties in the conduct of a population-based survival study in the context of developing countries, including India. Loss to follow-up is a typical problem encountered, causing biased estimates. In view of this difficulty with the classical approach, the objective of this study was to propose an indirect methodology for the study of survival. The proposed methodology is based on life table techniques and uses current data on incidence and mortality from the disease. It involves the estimation of person years free of disease (PYFD), person years with disease (PYWD), person years of life lost (PYLL) and average duration of disease (ADD) and their comparison over a time period. Empirical application was carried out for mouth and lung cancers in males and cancers of breast and cervix in females as well as for all sites combined together in each sex. Cancer incidence and mortality data by age and sex for the years 1989, 1993, 1997 and 2001 were obtained from published reports of Mumbai Cancer Registry, India. All causes of deaths for these years were obtained from Mumbai Municipal Corporation. Three life tables were constructed by applying various attrition factors (a) risk of death from all causes; (b) risk of incidence and that of death from other causes; and (c) risk of death from other causes only. The expectation of life from the second life table gave PYFD. PYWD and PYLL were calculated by suitable subtractions among three expectations of life. ADD was calculated by dividing person years lived with disease by number developing the disease. It was noted that during 1993-2001, PYFD for all sites increased from 59.4 to 62.1 and from 63.8 to 66 years in males and females respectively. PYLL was about 0.8 year in males and 1 year in females. Similarly, PYWD was 0.6 and 1 year in males and females. ADD for all sites varied from 4 to 4.7 years in both sexes. It was about 6 years for mouth cancers and 2 years for lung cancers in males and 4-5 years for breast and cervical cancers in females. Validation of the ADD was carried out by comparison with published data for calculating median duration of disease. Given the difficulties in conduct of classical survival studies, the proposed method may provide a useful tool for having a regular audit of prognostic factors in the community.