Second primary cancer after female breast cancer: Familial risks and cause of death

BACKGROUND: With continuous increases in survival rates following breast cancer (BC) diagnosis, the challenge of multiple primary cancers has become an issue. The data on familial risk of SPCs after BC diagnosis and the related mortality in BC patients are scarce. METHODS: A total of 87 752 female BC patients were followed for SPC diagnoses and records of death. Relative risks (RRs) of SPC in BC patients who had first‐degree relatives (parents or siblings) affected by the same cancer were compared to the patients without family history. Causes of death were compared between patients with and without SPC. RESULTS: After a median follow‐up of 5 years, 14 952 BC patients developed SPCs, among which 10 280 (68.8%) had first‐degree relatives diagnosed with cancer. Familial risks were significant for 14 site‐specific SPCs, and the highest risk was for second ovarian cancer (RR = 6.28, 95%CI: 4.50‐8.75), compared to those without family history (1.49, 1.34‐1.65). In patients with SPC, SPC was the main cause of death, including diverse cancers and BC in approximately equal proportions. CONCLUSIONS: Family history contributed to the excess number of patients with SPCs, and SPC was the leading cause of death in patients with SPC. Taking family history at diagnosis of BC may provide warning signs with regard to possible subsequent SPCs and may offer possibilities for counseling, intervention and management.