Border irregularities in fibroadenomas: an under-recognised phenomenon of diagnostic impact

To objectively evaluate the prevalence of histological border irregularities in fibroadenomas (FAs) and characterise their association with specimen type, histological subtypes, prior procedures and associated pathologies.

A retrospective review of 912 FA specimens (549 core needle biopsies (CNBs), 45 vacuum-assisted biopsies (VABs), 242 excisions and 76 mastectomies) from 816 lesions was conducted. An irregular border was defined as an interface between the FA and adjacent parenchyma that was not smooth or circumscribed. Statistical analyses were performed to identify factors associated with irregularities in the FA contours.

Border irregularity was present in 20.6% (188/912) of specimens. Prevalence was significantly lower in CNB (8.4%) compared with VAB (31.1%), mastectomy (34.2%) and excision (42.1%). By subtype, irregularity was most frequent in myxoid (41.7%), cellular (35.8%) and hyalinised (28.9%) FAs, compared with complex (20.4%), usual (16.2%) and juvenile (13.3%) forms. Calcifications, carcinoma in situ and invasive carcinoma, occurring within FAs, were associated with increased frequency of irregular borders. In multivariable analysis, a history of prior procedure was the strongest independent predictor of irregularity (OR 3.69, p<0.001). We found 1.6%, 2.5% and 0.2% of FAs to be accompanied by atypical hyperplasia, carcinoma in situ and invasive carcinoma, respectively.

While FAs are conventionally described as possessing circumscribed contours, our study found irregular borders in approximately one in five specimens. This feature is significantly associated with prior clinical procedures, specific histological subtypes and certain concurrent pathologies. It is essential to recognise that an irregular border in fibroepithelial lesions can be encountered in FAs and is not exclusive to phyllodes tumours.