Table 1. Demographic and clinical characteristics of the study population.

Characteristics	N0 (n=7481)		N1 (n=2355)		N2 (n=562)		N3 (n=373)		Pc
	No.	%	No.	%	No.	%	No.	%
Median follow-up duration (months) (IQR)	16 (8-26)		16 (8-25)		17 (10-25)		14 (7-22.5)
Age (years)									<0.001
Median	57		53		54		53
<50	2165	28.9	941	40.0	205	36.5	138	37.0
≥50	5316	71.1	1414	60.0	357	63.5	235	63.0
Race									0.003
White	5529	73.9	1649	70.0	409	72.8	258	69.2
Black	1422	19.0	525	22.3	121	21.5	87	23.3
Othera	530	7.1	181	7.7	32	5.7	28	7.5
Marital status									0.075
Married	4630	61.9	1443	61.3	327	58.2	211	56.6
Not marriedb	2851	38.1	912	38.7	235	41.8	162	43.4
Laterality									0.143
Left	3882	51.9	1211	51.4	313	55.7	180	48.3
Right	3599	48.1	1144	48.6	249	44.3	193	51.7
Histological type									<0.001
Infiltrating duct carcinoma	7397	98.9	2334	99.1	555	98.8	360	96.5
Lobular carcinoma	84	1.1	21	0.9	7	1.2	13	3.5
Histological grade									<0.001
I/II	1532	20.5	306	13.0	71	12.6	54	14.5
III	5949	79.5	2049	87.0	491	87.4	319	85.5
Tumor size (cm)									<0.001
≤2	4343	58.1	747	31.7	130	23.1	72	19.3
>2 and ≤5	2825	37.8	1294	54.9	327	58.2	194	52.0
>5	313	4.2	314	13.3	105	18.7	107	28.7
Type of surgery									<0.001
None	196	2.6	135	5.7	15	2.7	15	4.0
Breast-conserving surgery	4102	54.8	946	40.2	152	27.0	93	24.9
Mastectomy	3183	42.5	1274	54.1	395	70.3	265	71.0
Radiation therapy									<0.001
No	4039	54.0	1090	46.3	196	34.9	235	63.0
Yes	3442	46.0	1265	53.7	366	65.1	138	37.0

Abbreviation: IQR, interquartile range.

^aOther includes American Indian/native Alaskan and Asian/Pacific Islander.

^bNot married includes divorced, separated, single (never married), unmarried or domestic partner and widowed.

^cP values were calculated among all groups using a Chi-squared test, and bold type indicates significance.