Table 1.

Study characteristics.

Study	Country	Study design	No. of patients with breast cancer	No. of controls	No. of patients with breast cancer with tonsils removed	No. of controls with tonsils removed	Risk measurement	Risk of bias
Brasky et al. (16)	USA	Case control	736	801	389	380	Adj OR	High quality
Gross et al. (11)	USA	Case control	110	200	22	46	NR	Very high risk
Howie et al. (12)	Scotland	Case control	149	478	54	117	NR	High risk
Kessler et al. (13)	USA	Case control	89	85	29	20	NR	Very high risk
Lubin et al. (14)	Canada	Case control	558	824	286	384	Adj OR	High risk
Sun et al. (17)	Taiwan	R. Cohort	440a	1760b	7	14c	IRR	High quality
Yasui et al. (15)	USA	Case control	537	492	362	314	Adj OR	High quality
Cassimos et al. (10)	Greece	Case control	52	255	2	31	NR	Very high risk

a, number of patients with tonsillectomy; b, number of patients without tonsillectomy; c, number of patients with breast cancer without tonsils removed.Adj OR, Adjusted Odds Ratio; IRR, Incidence Rate Ratio; NR, Not Reported; R. Cohort, Retrospective Cohort.