TABLE 2—

Relationship of Hospital Volume to All-Cause and Breast Cancer–Specific Mortality: United States, 1994–1996

	No.	Deaths From All Causes, %	All-Cause Mortality HR (95% CI)	Breast Cancer Deaths, %	Breast Cancer–Specific Mortality HR (95% CI)
Hospital volumea
0–19	3596	28.8	1.00	7.3	1.00
20–39	3698	25.6	. . .b	5.5	0.80 (0.66, 0.97)
≥ 40	3931	23.4	0.83 (0.75, 0.92)	6.2	0.78 (0.64, 0.96)
Comorbidity index
0	8369	21.0	1.00	5.8	1.00
1	1983	33.4	1.60 (1.46, 1.75)	7.2	1.25 (1.03, 1.51)
≥ 2	873	55.2	3.40 (2.71, 3.40)	9.0	1.67 (1.28, 2.17)
Tumor size, mm
0–20	7790	20.2	1.00	3.5	1.00
21–50	3183	38.7	1.61 (1.48, 1.74)	12.6	2.41 (2.05, 2.84)
>50	150	38.7	1.66 (1.27, 2.16)	16.0	4.25 (2.77, 6.53)
Missing	102	33.3	1.22 (0.86, 1.73)	11.8	1.19 (0.66, 2.15)
Lymph nodes
0	7897	21.1	1.00	3.8	1.00
≥ 1	2293	33.7	1.59 (1.44, 1.74)	14.2	3.07 (2.59, 3.65)
Missing	1035	44.3	1.55 (1.37, 1.75)	11.4	1.77 (1.33, 2.37)
Estrogen receptor/progesterone receptor status
Negative	1344	33.6	1.00	14.7	1.00
Positive	7741	24.0	0.66 (0.59, 0.73)	4.9	0.42 (0.35, 0.50)
Not available	2140	27.8	0.72 (0.63, 0.84)	6.2	0.48 (0.37, 0.62)
Tumor grade
1 or 2	6077	21.9	1.00	3.7	1.00
3 or 4	3016	33.6	1.38 (1.27, 1.51)	12.3	2.26 (1.89, 2.70)
Not available	2132	26.0	1.08 (0.97, 1.19)	5.4	1.26 (1.00, 1.60)

Note. HR = hazard ratio; CI = confidence interval. Values were adjusted for all of the factors shown, as well as for age, race, per capita income, population density, propensity group, and Surveillance, Epidemiology, and End Results program site.

^aHospital volume was defined as number of breast cancer operations performed per year.

^b Treated as a separate stratum to retain proportionality of hazards in other volume groups and other variables in the model.