The incidence of bilateral breast cancer: II. A proposed model for the analysis of coincidental tumours.

A statistical model has been proposed in an attempt to integrate coincidental (or synchronous) diagnoses of multiple primary cancers into a general method of analysis. In the context of population-based surveys, such diagnoses form an integral part of the pattern of incidence within the population. Because of clinical surveillance, the diagnosis of subsequent tumours may be advanced in time in comparison with a first primary diagnosis. The model has been used to predict the altered pattern of diagnosis in order to adjust the value of expected numbers. Data from a previously reported survey of bilateral breast cancer have been used to illustrate the model. Analysis in terms of the model showed a 2.6-fold increase in risk for a second primary tumour in the contralateral breast in a series of nearly 22,000 breast-cancer patients. The corresponding risks for 3 main age-ranges (at the time of diagnosis of the first primary) were 5.3 (age 15-44), 3.3 (45-49) and 1.5 (60+). In addition, a maximal risk of 5.0-fold was observed in the series as a whole during the third year after the diagnosis of the first primary.