Table 4.

Risk of breast cancer for combinations of lobular involution and parenchymal pattern of mammographic breast density^*

Combination of lobular involution† and parenchymal pattern‡	No. of women	No. of women with breast cancer	No. of person-years of follow-up	Incidence rate§ (95% CI)	Age-adjusted,‖ HR (95% CI)	P¶	Multivariable adjusted,# HR (95% CI)	P¶
Complete† and N1 or P1‡	300	9	3644.46	247 (86 to 408)	1.00**	<.001	1.00**	.006
Complete† and P2 or DY‡	341	19	4330.29	439 (242 to 636)	1.85 (0.84 to 4.10)		1.66 (0.75 to 3.70)
Partial† and N1 or P1‡	546	28	7241.11	387 (244 to 530)	2.03 (0.95 to 4.31)		1.57 (0.73 to 3.36)
Partial† and P2 or DY‡	1120	92	15 261.88	603 (480 to 726)	3.88 (1.92 to 7.83)		2.70 (1.32 to 5.53)
None† and N1 or P1‡	87	5	1149.02	435 (54 to 817)	3.77 (1.22 to 11.6)		3.24 (1.05 to 9.98)
None† and P2 or DY‡	272	19	3745.81	507 (279 to 736)	5.14 (2.18 to 12.1)		4.08 (1.72 to 9.68)

^*Nested cohort study of women with benign breast disease (BBD) diagnosed at the Mayo Clinic between January 1, 1985, and December 31, 1991. HR = hazard ratio; CI = confidence interval.

^†Extent of Lobular involution was classified as none (0% lobules involuted), partial (1 to 74% lobules involuted), or complete (≥75% lobules involuted).

^‡Wolfe parenchymal pattern: N1—nondense, no ducts visible; P1—ductal prominence occupying less than a fourth of the breast; P2—prominent ductal pattern occupying more than a fourth of the breast; DY—homogenous plaque-like areas of extreme density.

^§Crude incidence rates per 100 000 person-years. 95% confidence intervals calculated based on large sample normal approximation to the Poisson distribution.

^║Age-adjusted univariate analysis; analysis was done using the Cox proportional hazards model.

^¶P values were calculated using two-sided Wald test with 5 df.

^#Multivariable analysis adjusting for age, BBD histology, body mass index, parity, menopause status, and family history.

^**Referent category in the Cox proportional hazards model.