Circumscribed Masses

Circumscribed masses are frequently detected on screening and diagnostic mammography and ultrasound. For a mass to be categorized as circumscribed, 75% of its margin must be sharply defined on mammography (25% of the margin may be obscured by surrounding tissue, but it may not be indistinct or spiculated), and the entire margin of the mass must be defined on ultrasound and MRI (Cohen et al. Am J Roentgenol 214, 2019; Berg et al. Radiology 268(3):673–83, 2013; Yoo et al Radiographics. 30(6):1689–702, 2010). While multiple, bilateral circumscribed masses seen on a baseline screening mammogram are categorized as benign, a solitary circumscribed mass seen on a baseline mammogram, or a new mass seen on an annual mammogram should be designated as BI-RADS 0 and additional evaluation should be recommended (Cohen et al. Am J Roentgenol. 214, 2019). Ultrasound evaluation is key in differentiating cystic from solid masses. Many such masses will be found to be simple cysts and can be characterized as benign, with the patient able to return to routine screening mammography. Circumscribed masses detected on baseline screening and found to be solid and otherwise bearing the characteristics of a benign fibroadenoma following baseline screening mammography can be designated as BI-RADS 3 (Feig. Radiol Clin North Am. 30:67–92, 1992). A total of 1.4% of circumscribed masses detected on screening mammography are malignant, while 9% of those that are recommended for biopsy will be found to be malignant (Geertse et al. Breast. 69:431–440, 2023). When evaluating a circumscribed mass on imaging, it is important to take into consideration patient characteristics, whether or not the mass is palpable or screening detected, as well as to carefully evaluate the features of the mass on both mammography and ultrasound. While many circumscribed masses are benign, malignant lesions, including invasive ductal carcinoma of no specific subtype, mucinous carcinoma, encapsulated papillary carcinoma and medullary carcinoma may be circumscribed. In addition, some particularly aggressive breast malignancies may mimic common benign lesions, including triple-negative and basal-like carcinomas (Kim et al. J Ultrasound Med. 34:1951-9, 2015). Benign fibroepithelial neoplasms commonly present as palpable or image-detected circumscribed masses. These lesions originate in the terminal duct lobular unit (TDLU) and are therefore predominantly seen in women but rarely can also be seen in male patients. Fibroadenomas are biphasic tumors with proliferation of epithelial and stromal components. Histologically fibroadenomas can show two patterns of growth. In an intracanalicular growth pattern the glandular elements are compressed into slit-like spaces by the surrounding stroma. In the pericanalicular pattern the glands maintain open lumen separated by stroma. In a cellular fibroadenoma the stroma may be cellular without an increase in stromal mitosis. Giant fibroadenomas are usually larger than 5 cm and are seen in adolescents and may be solitary or multiple. Tubular adenomas, which are a variant of fibroadenoma, are composed predominantly of round or oval glandular structures with scant intervening stroma. The stroma has a variable appearance from one fibroadenoma to another but is relatively homogeneous within a fibroadenoma with a balanced proliferation of epithelial and stromal components. The stroma can be myxoid or exhibit smooth muscle metaplasia which can make the stromal component appear cellular. Multinucleated giant cells may be seen in the stroma of fibroepithelial neoplasms. Coarse calcifications can be seen in the stroma in postmenopausal patients. Fibroepithelial neoplasms can be involved by sclerosing adenosis, cysts, lobular neoplasia (ALH and LCIS), ductal carcinoma in situ, and rarely invasive carcinoma (Schnitt and Collins. Biopsy interpretation of the breast. Wolters Kluwer Health, 2018). On a core biopsy fibroepithelial lesions are assessed for stromal cellularity, presence or absence of stromal atypia and increased mitoses to distinguish benign fibroepithelial lesions from low grade or malignant phyllodes tumors. Due to the overlapping histomorphologic features between cellular fibroadenomas and some benign and borderline phyllodes tumors these lesions need careful evaluation to determine if an excision is required. Benign cysts can present as circumscribed masses, arise in the TDLU, and can be simple with a lining or capsule composed of an inner lining of luminal epithelial cells and outer layer of myoepithelial cells. Complicated cysts contain cellular debris, calcifications or a solid component. While simple cysts are aspirated and sent for cytologic evaluation if bloody or purulent, complicated cysts may need tissue sampling to exclude malignancy. Rarely invasive carcinomas may present as circumscribed masses and include mucinous, medullary, high-grade invasive ductal carcinoma and encapsulated papillary carcinomas (Schnitt and Collins. Biopsy interpretation of the breast. Wolters Kluwer Health, 2018).