Table 3.

Cancer Incidence in Danish Same-Sex Twins With Known Zygosity Born 1900–1918 Stratified for Age at Death of Cotwin

Lifespan of Cotwin	Observed Number of Cancers	Expected Number of Cancers*	SIR† (obs/exp)	95% CI
MZ
0–49 y	59	61.1	0.97	0.76–1.22
50–59 y	39	40.0	0.97	0.69–1.38
60–69 y	115	91.6	1.26	1.04–1.52
70–79 y	195	216.8	0.90	0.78–1.04
80–89 y	269	309.1	0.87	0.77–0.98
90+ y	120	167.1	0.72	0.59–0.87
Total	797	885.7	0.90	0.84–0.97
DZ
0–49 y	158	142.9	1.11	0.95–1.29
50–59 y	119	96.8	1.23	1.02–1.48
60–69 y	223	208.3	1.07	0.93–1.23
70–79 y	396	418.8	0.95	0.86–1.04
80–89 y	468	520.7	0.90	0.82–0.99
90+ y	229	259.0	0.88	0.77–1.01
Total	1,593	1,646.6	0.97	0.92–1.02

Notes: CI = confidence interval; DZ = dizygotic; MZ = monozygotic; SIR = standardized incidence rate ratio. Test for trend in SIRs associated with age of cotwin at death resulted in a statistically significant decline in SIRs in both groups. The p values were .003 for MZ twins and .001 for DZ twins.

^*Based on age-, sex-, and calendar time–specific cancer incidence in the general population.

^†Standardized incidence rate ratio.