Table 2 Relative risk (95%CI) for contralateral breast cancer according to treatment of first breast cancer among 5-year breast cancer survivors in the Kaiser cohort.

		Kaiser cohort
Population	Treatment	Observed	RR^a	95%CI	P-value^b
Population	Treatment	All Contralateral Breast Cases
All first breast cancer	Radiotherapy
	No	105	1
	Yes	248	1.07	0.85–1.35	0.560
	Hormone therapy
	No	149	1
	Yes	204	0.63	0.50–0.78	<0.001
ER+ first breast cancer	Hormone therapy
	No	83	1
	Yes	188	0.53	0.40–0.70	<0.001
	Radiotherapy with HT
	No	58	1
	Yes	130	0.94^c	0.69–1.29	0.717
	Radiotherapy without HT
	No	21	1
	Yes	62	1.57^c	0.94–2.62	0.072
		ER+ Contralateral Breast Cases
All first breast cancer patients	Radiotherapy
	No	78	1
	Yes	181	1.07	0.81–1.40	0.636
	Hormone therapy
	No	108	1
	Yes	151	0.63	0.48–0.81	<0.001
ER+ first breast cancer	Hormone therapy
	No	66	1
	Yes	140	0.52	0.38–0.71	<0001
	Radiotherapy with HT
	No	46	1
	Yes	94	0.88^d	0.61–1.26	0.480
	Radiotherapy without HT
	No	13	1
	Yes	53	2.22^d	1.20–4.14	0.007
		ER- Contralateral Breast Cases
All first breast cancer	Radiotherapy
	No	11	1
	Yes	47	1.85	0.95–3.59	0.055
	Hormone therapy
	No	22	1
	Yes	36	0.72	0.42–1.23	0.235
ER+ first breast cancer	Hormone therapy
	No	7	1
	Yes	32	0.75	0.32–1.79	0.531

RR Relative Risk, CI confidence Interval, ER oestrogen receptor, HT hormone therapy.
^aRelative risk estimated using Poisson regression, adjusted for year of diagnosis and age at diagnosis, radiotherapy, chemotherapy, hormone therapy, and study centre, as appropriate.
^bLikelihood ratio test for heterogeneity of relative risks.
^cP-difference = 0.09 between the RRs for radiotherapy with and without hormone therapy.
^dP-difference = 0.01 between the RRs for radiotherapy with and without hormone therapy.

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