Fibroadenoma is a common benign breast mass and is the most common solid mass in women under 30; 15–20% of women have multiple, including bilaterally [1, 14]. If palpable, it can be a discrete, firm, mobile mass and may wax and wane in size and tenderness with cyclical hormonal changes [10]. Fibroadenomas measuring ≥8 cm in greatest length are referred to as giant fibroadenomas and can be seen in adolescents [10, 12, 14].

Mammographically, a fibroadenoma most commonly has a round or oval shape, low density, and circumscribed margins (Fig. 6.1) [10]. As women get older, and hormone levels decrease, these masses may hyalinize, resulting in decreased size, increased radiographic density, and sometimes the development of large, coarse, chunky “popcorn”-like calcifications (Fig. 6.2). However, as mentioned elsewhere in this handbook, even large, dense calcifications may start out fine and faint and if imaged at that stage may pose a diagnostic dilemma for the radiologist, since not all masses with calcifications can be classified as fibroadenomas. Moreover, sometimes the soft tissue mass is not evident radiographically, and so a group of developing calcifications may be the only finding of a fibroadenoma (Fig. 6.3), warranting biopsy for confirmation.

The sonographic appearance of a fibroadenoma is typically analogous tothe mammographic. It is oval, has an orientation parallel to the skin and chest wall (horizontal, traveling along tissue planes rather than disrupting them), uniform hypoechogenicity, and circumscribed margins (Fig. 6.4) [1–3, 10]. In some cases, thin internal echogenic fibrous septa may be seen. If hyalinized but without the characteristic large calcifications to define it radiographically, it may appear more hypoechoic, or nearly anechoic, and demonstrate posterior acoustic shadowing at ultrasound. If even partially calcified, there may be intense shadowing. If not hyalinized, its appearance in both modalities may be indistinguishable from that of a phyllodes tumor; the latter might be higher in echogenicity with slit-like anechoic spaces (Fig. 6.5) [10–12]. Phyllodes tumors have malignant potential, so the distinction is important. Although the classic teaching is that phyllodes are “large” masses,size should not be the sole criteria for exclusion or inclusion in a differential diagnosis.

According to the original prospective works and reviews by Sickles [4, 5, 6], if a non-palpable mass with circumscribed margins and no calcifications is identified after mammographic work-up from a baseline screening, it may be managed with periodic radiographic follow-up (assigned a BI-RADS 3 category assessment, with one short interval follow-up at 6 months, and then yearly), for a total of 2–3 years. After this, in the absence of significant growth or suspicious change, it may be deemed benign with no further follow-up. With support from later work focused on ultrasound, it is now recommended to include sonographic findings to support this recommendation for “watching and waiting,” or to rely only on ultrasound features in young patients for whom mammography is not done [3, 7, 8]. The data shows that the likelihood of malignancy is <2% (and in some studies, <1%) for an oval, hypoechoic mass with circumscribed margins, parallel orientation, and no posterior acoustic shadowing. Furthermore, palpability no longer excludes such masses from short-term follow-up, which makes sense, since the size and location in the breast (closer to the skin), relative to breast size and composition, do not incur additional likelihood of malignancy. If a mass looks like a fibroadenoma and has no suspicious imaging features, our practice is to discuss this with the patient and recommend a 6-month follow-up but also offer her the options of a needle biopsy or excisional biopsy if depending on the patient’s comfort level [3–9].

Classically, at follow-up, if the lesion has increased in volume by >20%, biopsy should be undertaken. However one must take into consideration the context of the patient: for example, these may normally grow in concert with breast development in adolescents and young women, or in pregnancy. If at follow-up there have been changes in morphology or margins suspicious for malignancy, biopsy is also warranted. Needle biopsy (rather than excisional biopsy) is the method of choice as it is less invasive and can be done right then, and the diagnosis may change the operative plan if surgery is in fact needed.

For core needle biopsy-proven fibroadenomas, some advocate an ultrasound follow-up in 6 months to ensure that a phyllodes tumor was not missed due to sampling error; if the lesion is truly a phyllodes, it would be expected to grow significantly in 6 months and thereby “declare itself.” Given the rarity of this occurring in our practice, we do not routinely recommend nor perform ultrasound follow-up for biopsy-proven fibroadenomas. Unless symptomatic, resection of a biopsy proven fibroadenoma is not typically warranted.

In some instances, a core biopsy sample is insufficient in distinguishing between a fibroadenoma and a phyllodes tumor. It is paramount that the physician performing the core biopsy be familiar with the nuances in the language of pathologists. Such a core biopsy may be reported as a “fibroepithelial lesion, phyllodes not excluded,” in which case an excisional biopsy may be needed for definitive results. The surgeon, also recognizing that a phyllodes tumor is possible based on this wording, would allow for wider margins at excision to ensure its entire removal given its propensity for recurrence.

Complex fibroadenomas are also most often indistinguishable from fibroadenomas by imaging. Findings suggestive of the former may include associated round, punctate, or amorphous calcifications radiographically and cystic spaces ≤3 mm sonographically. Although these lesions can increase a patient’s risk for developing breast cancer later on, since they themselves are not premalignant, we do not routinely recommend excision when a core biopsy yields a complex fibroadenoma.

Although not indicated for the work-up of a fibroadenoma, MRI characteristics can help narrow the differential diagnosis when done for other reasons. In premenopausal women, fibroadenomas have increased T2 signal and enhancement that is either homogeneous or interrupted by nonenhancing septa (Fig. 6.6). In postmenopausal women, these may have decreased T2 signal and not enhance, highlighting the hormonal influences on these benign tumors (Fig. 6.7). Phyllodes tumors, given their increased cellularity, may have more heterogeneous enhancement and T2 signal [11, 12, 14].