Macroscopically, the fibroadenoma is usually encapsulated and shows circumscribed borders with a whitish-grey, bulging cut surface in which slit-like spaces are often evident (Figs. 3.2, 3.3, 3.4, 3.5, and 3.6). It is often rubbery in consistency and may have lobulated outlines. They can achieve large sizes, with fibroadenomas exceeding 5 cm in dimension being referred to as giant fibroadenomas.

Histologically, the fibroadenoma shows two patterns: “intracanalicular”, with stroma compressing epithelial elements, and “pericanalicular”, with stroma growing around open ductules (Figs. 3.7, 3.8, and 3.9). These microscopic patterns do not have clinical significance and often occur together in the same tumour. The epithelial compartment may show any alterations that can be observed in the breast, from usual ductal hyperplasia (Fig. 3.10), columnar cell change, atypical hyperplasia (Fig. 3.11), and in situ (Fig. 3.12) and invasive carcinomas (Fig. 3.13). It is reported that atypical hyperplasia confined within the fibroadenoma does not incur an increased risk of subsequent breast cancer development [1].

The stroma can exhibit multinucleated giant cells (Fig. 3.14), which do not indicate stromal atypia. Fibroadenomas with cellular stroma are referred to as cellular fibroadenomas (Fig. 3.15), though the level of stromal cellularity needed for this diagnosis is subjective and may differ among pathologists. Fibroadenomas in children and young females under 20 years of age tend to have increased stromal cellularity and need to be distinguished from phyllodes tumours [2]. The presence of extensive myxoid stroma is seen in the myxoid fibroadenoma (Fig. 3.16). While myxoid fibroadenomas are described in the Carney’s complex (myxoid neoplasms and pigmented skin lesions), most observed in the breast are unassociated with this clinical condition. In older women, the stroma can become hyalinised and sclerotic. Calcifications within the stroma can lead to radiological detection (Fig. 3.17). Lipomatous, smooth muscle (Fig. 3.18), and osteochondroid metaplasia may be seen (Fig. 3.19). Pseudoangiomatous stromal hyperplasia can be present (Fig. 3.20). Stromal mitotic activity is usually scarce, though small numbers of stromal cell mitoses may be seen in fibroadenomas in young or pregnant patients. In paediatric patients, mitotic activity can range up to seven mitoses per ten high-power fields [3]. Infarction may occur during pregnancy (Figs. 3.21).

The juvenile fibroadenoma is encountered in younger patients, though it has been documented in older adults as well (Figs. 3.22 and 3.23). Often demonstrating a cellular stroma, the term juvenile fibroadenoma has sometimes been used synonymously with cellular fibroadenoma. We prefer to diagnose a juvenile fibroadenoma when there is a predominant pericanalicular growth pattern accompanied by usual ductal hyperplasia that may resemble epithelial changes of gynaecomastia (gynaecomastoid hyperplasia), in addition to cellular stroma featuring intersecting fibroblastic and myofibroblastic fascicles. Sometimes the epithelial changes mimic atypical ductal hyperplasia.

The complex fibroadenoma is diagnosed when there is sclerosing adenosis, epithelial calcifications, papillary apocrine metaplasia, and cysts measuring 3 mm or more in size [4]. The presence of any of these features can designate a fibroadenoma as “complex”, though in our practice, we often reserve this diagnosis for fibroadenomas that either display a marked degree of these alterations or combination of alterations (Figs. 3.24 and 3.25). According to one study, complex fibroadenomas are associated with an increased relative risk of subsequent breast cancer (threefold), but the available data are insufficient to justify a different management approach for these lesions.