Risk of breast cancer subsequent to histological or clinical diagnosis of fibroadenoma - Retrospective longitudinal study of 3938 cases

Background: The widespread belief that fibroadenoma is associated with an increased risk of subsequent breast cancer is based on studies of histologically-confirmed fibroadenomas. In current practice only a minority of fibroadenomas are excised: most of them are diagnosed on the basis of palpation and imaging, and are not surgically removed. The decision for surgical excision may be influenced by the presence of individual risk factors, and this can act as a confounder and bias studies that are based only on surgically excised fibrodenomas. Patients and methods: To investigate this hypothesis we linked data from a consecutive series of 3938 fibroadenomas diagnosed histologically (n = 1335) or clinically (n = 2603) in women aged 30 to 69 years to the Tuscany Cancer Registry database. After exclusion of concurrent breast cancers or cancers occurring within six months after the diagnosis of fibroadenoma, the observed and expected incidence of subsequent breast cancer were compared. Results. The overall Standardized Incidence Ratio (SIR) for excised and non-excised fibroadenomas was 1.38 (95% CI = 1.1-1.7). The SIR for histologically-confirmed fibroadenomas was 2.0 (95% CI 1.4-2.7) whereas there was no apparent risk for non-excised fibroadenomas (SIR = 0.97, 95% CI = 0.7-1.4). Conclusion. This study suggests that assessment of breast cancer risk subsequent to a diagnosis of fibroadenoma may be biased if the analysis is limited to surgically-excised fibroadenomas.