Page 81 - New epidemiological and PSMA-expression based paradigms in salivary gland tumors
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Risk of breast cancer after a salivary gland tumor in the Netherlands
Statistical analysis: Time at risk started at date of SGC or SGPA diagnosis. For SGC follow-up ended at the date of BC diagnosis, death, emigration, or last follow- up, whichever came first. In SGPA patients, the expected number of cases and cumulative risk of BC were based on the cumulative follow-up time. Since vital status is not registered in the PALGA registry, mortality among SGPA patients was imputed using gender-, age-, and calendar-year specific life-tables for the Dutch population, generating 50 imputed datasets, under the assumption that SGPA patients, as SGPA is a benign disease, had a similar life expectancy as the Dutch general population. Estimated cumulative risks and standard errors in each imputed dataset were subsequently pooled using Rubin’s rule.
Expected BC incidence in the SGC cohort was estimated using the Hakulinen method[31]. The Standardized Incidence Ratio (SIR) was calculated as the ratio between the observed and expected number of BC cases in both the SGC- cohort and the SGPA-cohort. In order to compare the observed BC incidence in our study population with the BC incidence among Dutch females from the general population, we used external reference rates. Using age-specific (5-year age groups) and calendar-year specific BC incidence rates for the Dutch female population, we calculated the number of BCs we could have expected if our cohort would have had the same age-specific BC incidence as the general population, based on the number of person-years of follow-up our women accrued in each 5-year age group during each year of follow-up. This method of using external data as reference data has been extensively used previously[32]. Thus, the number of BC cases in both the SGC-cohort and the SGPA-cohort are compared with BC cases in a contemporized follow-up period in contemporized age categories, in the Dutch population.
As a sensitivity analysis, the SIR was also determined for BCs occurring ≥ 3 months after the index salivary gland tumor, thereby excluding potential synchronous BC. In this analysis time at risk started at 93 days after SGC or SGPA diagnosis. Also, as a sensitivity analysis risk of invasive and in situ BC was evaluated separately. BC risk was also assessed and stratified for SGC/ SGPA age, follow-up time and (in SGC patients) histological subtype. For SGC patients the 5 and 10-year survival after BC was calculated, with a subgroup analysis for patients younger than 65 years. The 95% confidence intervals (95%CI) were calculated assuming a Poisson distribution for the observed number of events. Tests for homogeneity and trend of SIRs were performed using Poisson regression models based on collapsed
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