Simulation modelling to validate the flow method for estimating completeness of case ascertainment by cancer registries.

BACKGROUND: To validate estimates of completeness of cancer ascertainment obtained by the flow method. METHODS: We generated a computer simulation of patient-level cancer registration processes, based loosely on the age distribution and survival of colorectal carcinoma patients, and utilizing a mixture of 'cured' and 'killed' subjects with an age-dependent fraction of 'cured' cases. The simulated data were then used in an analysis of completeness using the flow method. Validation of the simulation process was based on similarity of outputs to those obtained using real data, and validation of the flow method on its ability to correctly estimate the known proportion of cases in the simulated data which would never be registered. RESULTS: We successfully generated realistic data and have shown that completeness estimated by the flow method is close to the true value, whereas another method of estimating completeness (Ajiki's) was shown to be strongly biased. We also modelled what happens to completeness estimates when a new registry is set up. CONCLUSIONS: When its assumptions are met (steady state for incidence, survival and stable population structure), the flow method works well but is biased for cancers with good survival. Further research is required to assess the robustness of the method when these conditions are not met.

The effects of twins, parity and age at first birth on cancer risk in Swedish women.

The effect of reproductive history on the risk of cervical, colorectal and thyroid cancers and melanoma has been explored but the results to date are inconsistent. We aimed to examine in a record-linkage cohort study the risk of developing these cancers, as well as breast, ovarian and endometrial cancers, among mothers who had given birth to twins compared with those who had only singleton pregnancies. Women who delivered a baby in Sweden between 1961 and 1996 and who were 15 years or younger in 1961 were selected from the Swedish civil birth register and linked with the Swedish cancer registry. We used Poisson regression to assess associations between reproductive factors and cancer. Twinning was associated with reduced risks of breast, colorectal, ovarian and uterine cancers, although no relative risks were statistically significant. The delivery of twins did not increase the risk of any cancers studied. Increasing numbers of maternities were associated with significantly reduced risks of all tumors except thyroid cancer. We found positive associations between a later age at first birth and breast cancer and melanoma, while there were inverse associations with cervix, ovarian, uterine and colorectal cancers. These findings lend weight to the hypothesis that hormonal factors influence the etiology of colorectal cancer in women, but argue against any strong effect of hormones on the development of melanoma or tumors of the thyroid.

Completeness of ascertainment by cancer registries: putting bounds on the number of missing cases.

BACKGROUND: When comparing cancer incidence or mortality rates between different regions, it is important to know how complete the registration data are on which these figures are based. A number of ways of estimating completeness have been proposed, but it is often difficult to say how precise these estimates are. We describe a computer program developed to produce measures of precision for estimates of completeness obtained by one such method, the flow method. METHODS: The program works by resampling the required data sets, and repeatedly calculating completeness estimates until convergence of the standard errors occurs. It was tested on colorectal tumours from a single health district, and empirical confidence limits for 1 and 5 year completeness were compared with those obtained by applying various normalizing transformations and a beta distribution. The method was then applied to tumours of the head and neck, breast and lung and the results compared with those from a capture-mark-recapture exercise carried out 4 years previously. RESULTS: The sampling distribution was close to normal for 1 year completeness, but much less so for 5 year completeness, as assessed by quantile plots. Approximation by a beta distribution was better than by normalizing transformation. Although there were differences between the results produced by the flow method and capture-recapture, the flow method is more reproducible and easier to apply. CONCLUSION: It is now possible to estimate confidence limits for the results of the flow method, and thus determine whether comparative results between registries are likely to be affected by sampling error.