Table 4 Relative risk (95%CI) for contralateral breast cancer according to treatment of first breast cancer among 5-year breast cancer survivors in the SEER cohort.

		Outcome
Population	Treatment	All Contralateral Breast Cases
Population	Treatment	Observed	RR^a	95%CI	P-value^b
All first breast cancer	Radiotherapy
	No/Unknown^c	4599	1
	Yes	5871	1.07	1.03–1.12	<0.001
	Hormone therapy
	No/Unknown^c	6596	1
	Yes	3874	0.92	0.88–0.96	<0.001
ER+ first breast cancer	Hormone therapy
	No/Unknown^c	3511	1
	Yes	3421	0.95	0.90–0.99	0.039
	Radiotherapy with HT
	No/Unknown^c	1021	1
	Yes	2400	1.03^d	0.95–1.11	0.489
	Radiotherapy without HT
	No/Unknown^c	1784	1
	Yes	1727	1.09^d	1.02–1.17	0.010
		ER+ Contralateral Breast Cases
All first breast cancer	Radiotherapy
	No/Unknown^c	3245	1
	Yes	4345	1.07	1.02–1.12	0.006
	Hormone therapy
	No/Unknown^c	4618	1
	Yes	2972	0.96	0.91–1.01	0.090
ER+ first breast cancer	Hormone therapy
	No/Unknown^c	2714	1
	Yes	2679	0.94	0.89-0.99	0.035
	Radiotherapy with HT
	No/Unknown^c	785	1
	Yes	1894	1.03^e	0.94–1.12	0.414
	Radiotherapy without HT
	No/Unknown^c	1348
	Yes	1366	1.12^e	1.04–1.21	0.004
		ER- Contralateral Breast Cases
All first breast cancer	Radiotherapy
	No/Unknown^c	799	1
	Yes	1039	1.12	1.01–1.23	0.028
	Hormone therapy
	No/Unknown^c	1248	1
	Yes	590	0.84	0.76–0.93	<0.001
ER+ first breast cancer	Hormone therapy
	No/Unknown^c	401	1
	Yes	481	1.15	0.99–1.32	0.054
	Radiotherapy with HT
	No/Unknown^c	141	1
	Yes	340	1.01 ^f	0.83–1.24	0.69
	Radiotherapy without HT
	No/Unknown^c	203	1
	Yes	198	1.10 ^f	0.89–1.34	0.379

BC breast cancer, ER oestrogen receptor.
^aRelative risk estimated using Poisson regression, adjusted for year of diagnosis and age at diagnosis, grade, tumour size, radiotherapy, chemotherapy, hormone therapy and grade and tumour size as appropriate.
^bLikelihood ratio test for heterogeneity of relative risks.
^cTreatment variables in SEER are classified as “yes” or “no/unknown” since SEER cannot accurately distinguish between “no treatment” and “unknown if ^patients received treatment”.
^dP-difference=0.28 between the RRs for radiotherapy with and without hormone therapy.
^eP-difference=0.16 between the RRs for radiotherapy with and without hormone therapy.
^fP-difference=0.34 between the RRs for radiotherapy with and without hormone therapy.

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