Estimating the probability of multiple incidences of the same cancer type in a single workplace.

Objectives: This study aimed to estimate the probability of observing two cases of the same cancer type in a workplace with 300 employees, to help investigation of occupational cancer.

Methods: We assumed a workplace where chemicals are handled, employing 300 males aged 15 to 64, with an age distribution standardized to Japan's population from 2016 to 2019. Using national cancer statistics for newly diagnosed cases among males in this age range and period, we calculated the expected number of cancer cases for the workplace over 1- and 10-year period. We computed standardized incidence ratios (SIRs) for two instances of the same cancer type within these time frames and estimated the time required for the SIR to reach 2.0 and its lower 95% confidence interval limit (LL) to reach 1.0.

Results: The SIR for any cancer type exceeded 10 and was significantly high when two cases occurred within 1 year. Over 10 years, the SIR remained significantly high in some cancer types. An observation period required for SIR to reach 2.0 and LL to reach 1.0 for any cancer type were at least 5.4 and at least 1.7 years, respectively.

Conclusions: Considering that over 99% of Japanese workplaces employ fewer than 300 people, the likelihood of observing the same cancer type within 1 year is low. This study enhances our understanding of occupational cancer incidence and supports the integration of such data into prevention strategies.