Risk factors for uncommon histologic subtypes of breast cancer using centralized pathology review in the Breast Cancer Family Registry.

Epidemiologic studies of histologic types of breast cancer including mucinous, medullary, and tubular carcinomas have primarily relied on International Classification of Diseases-Oncology (ICD-O) codes assigned by local pathologists to define histology. Using data from the Breast Cancer Family Registry (BCFR), we compared histologic agreement between centralized BCFR pathology review and ICD-O codes available from local tumor registries among 3,260 breast cancer cases. Agreement was low to moderate for less common histologies; for example, only 55 and 26 % of cases classified as mucinous and medullary, respectively, by centralized review were similarly classified using ICD-O coding. We then evaluated risk factors for each histologic subtype by comparing each histologic case group defined by centralized review with a common set of 2,997 population-based controls using polytomous logistic regression. Parity [odds ratio (OR) = 0.4, 95 % confidence interval (95 % CI): 0.2-0.9, for parous vs. nulliparous], age at menarche (OR = 0.5, 95 % CI: 0.3-0.9, for age ≥13 vs. ≤11), and use of oral contraceptives (OCs) (OR = 0.5, 95 % CI: 0.2-0.8, OC use >5 years vs. never) were associated with mucinous carcinoma (N = 92 cases). Body mass index (BMI) (OR = 1.05, 95 % CI: 1.0-1.1, per unit of BMI) and high parity (OR = 2.6, 95 % CI: 1.1-6.0 for ≥3 live births vs. nulliparous) were associated with medullary carcinoma (N = 90 cases). We did not find any associations between breast cancer risk factors and tubular carcinoma (N = 86 cases). Relative risk estimates from analyses using ICD-O classifications of histology, rather than centralized review, resulted in attenuated, and/or more imprecise, associations. These findings suggest risk factor heterogeneity across breast cancer tumor histologies, and demonstrate the value of centralized pathology review for classifying rarer tumor types.