Data from the Cancer Registry of Slovenia were used in a cohort study
to determine whether the incidence of second primary cancers in patien
ts with first primary breast cancer differs from the incidence expecte
d in the general population. Special interest was given to long-term s
urvivors. The expected numbers of second primary cancers were calculat
ed by multiplying the number of appropriate person-years at risk by th
e corresponding age- and calendar-period-specific cancer incidence rat
es for women in Slovenia. The risk of a second primary cancer was expr
essed as the standardized incidence ratio (SIR). Of the 8,917 patients
newly diagnosed in the period 1961-85 and followed-up to the end of 1
994, 547 (6.2 percent) developed second primary cancers, whereas 410 (
4.7 percent) were expected (SIR = 1.3, 95 percent confidence interval
[CI] = 1.2-1.4). The risk was higher among younger patients. In long-t
erm survivors, the risk was increased significantly for second primary
cancer of the breast (SIR = 1.4, CI = 1.1-1.7), lung cancer (SIR = 1.
6, CI = 1.1-2.3), melanoma (SIR = 2.7, CI = 1.5-4.4) and non-melanoma
skin cancers (SIR = 2.0, CI = 1.6-2.4), corpus uteri cancer(SIR = 1.6,
CI = 1.2-2.1), ovarian cancer(SIR = 2.3, CI = 1.7-3.0), and thyroid c
ancer (SIR = 2.5, CI = 1.2-4.6). Our results confirm the findings of s
everal cohort studies carried out in Europe, the United States, and Ja
pan, indicating that breast cancer patients should be monitored carefu
lly for the occurrence of second primary cancers.