Apocrine Adenoma of the Breast Showing Unique Image Findings

Apocrine adenoma of the breast is extremely rare and has been overwhelmingly found incidentally as a small mass on screening mammography [1‒3]. Pathological studies have reported that apocrine adenomas have compact proliferation of round and regular glands with a luminal apocrine cell layer and an outer attenuated basal myoepithelium cell layer [4]. Limited number of apocrine adenoma studies have exclusively focused only on the pathological characteristics of this disorder. No studies, however, have reported the image findings of apocrine adenoma of the breast to date. We herein report a case of breast apocrine adenoma showing unique image findings. The CARE Checklist has been completed by the authors for this case report, attached as online supplementary material (for all online suppl. material, see https://doi.org/10.1159/000539644).

A 66-year-old woman was referred to our hospital for the treatment of ascending colon cancer. Enhanced computed tomography for colon cancer staging work incidentally revealed a small mass in her left breast. Mammography showed a circumscribed oval mass, 10 mm in size (Fig. 1a). Ultrasonography showed an oval mass with indistinct borders, internal iso-echoes with high echo spots, and unchanged posterior echoes (Fig. 1b). Magnetic resonance imaging (MRI) of the mass showed a hypo-intense pattern both on T1- (Fig. 2a) and T2-weighted (Fig. 2b) images and a long-lasting rim enhancement pattern up to the late phase on time-signal intensity images (Fig. 2c, d). The small size and obscureness of the mass borders on ultrasound probably caused inappropriate tissue sampling of the lesion with core needle biopsy, leading to no definitive diagnosis of the mass. All image findings, however, strongly suggested the mass to be a benign disorder. After thorough discussion of the pathological results with the patient, an additional tissue sampling was scheduled at the time of colon cancer surgery. Therefore, without repeated core needle biopsy or vacuum-assisted biopsy to the lesion, diagnostic and therapeutic lumpectomy was concomitantly done to the mass just before the colon cancer surgery under general anesthesia. The resected breast mass was hard, clearly demarcated, and 8 mm in size. Cut surface of the mass was flesh-colored and had multiple white spots (Fig. 3a). Pathological study showed dense predominantly oval glandular ducts with luminal apocrine cell layers, outer myoepithelium cell layers, and slight fibrosis (Fig. 3b, c). Glandular cavities on pathology well matched to white spots observed on macroscopic view, seemed vacant in a cyst-like fashion, and had aggregated foamy histiocytes in some of them (Fig. 3d). These pathological findings led us to the diagnosis of apocrine adenoma of the breast. The patient was discharged on the 9th day after operation without any events and is scheduled both for follow-up for colon cancer recurrence and annual mammography screening on an outpatient basis.

On mammography, the density and margin of a mass are determined by the X-ray attenuation coefficient of it. It is well known that fat has a much lower X-ray attenuation coefficient than the mammary gland [5]. The target lesion, therefore, was clearly depicted as a mass with clear boundaries on mammography due to the presence of fat surrounding the mass. If other substances with similar X-ray attenuation coefficients to the mass had existed around the tumor, tumor borders would have been much unclear.

Reflection and back scattering of ultrasound waves determine the shape and internal echoes of a mass, respectively. They are both phenomena that occur when ultrasound waves hit an object and return toward the ultrasound probe. The former, however, is caused by the ultrasound return with an object overwhelmingly larger than the ultrasound wavelength, while the latter by that with a much smaller one. Brightness of both tumor boarders and internal echoes is determined by the difference of acoustic impedance between the pathological tissue components in contact. In short, the larger the difference of acoustic impedance between the adjacent two components is, the brighter both the tumor boarders and internal echoes are. Therefore, calcifications with very high acoustic impedance are visualized as point-like echoes, i.e., echogenic spots, with extremely high brightness. In addition, it is well known that some kind of papillary structures themselves make a lot of back scattering of ultrasound waves. Therefore, the presence of multiple cystic structures in this case well explained the intra-tumoral high echo spots.

In this case, small size and lobulated shape of the mass should have lessened the vertical return of the reflected ultrasound waves and had made the mass boarders unclear. Much less difference in acoustic impedance than that of X-ray attenuation coefficient between the mass and the surrounding fat made the tumor borders less bright and unclear on ultrasound. These image-formation mechanisms mentioned above made the tumor margins clear on mammography and unclear on ultrasound, respectively.

Vast majority of tumors, regardless of whether they are benign or malignant, are at least somewhat edematous and generally have a hyper-intense pattern on T2-weighted images [6]. In this case, T2-weighted images showed a hypo-intense pattern, highly indicating the less movability and/or the paucity of protons in the mass. Representative substances with poor proton movability or less protons include bone, cartilage, fibrous tissue, and solid proteins in human beings. The presence of a bone component can be easily ruled out from mammographic findings. In breast diseases, cartilage components are overwhelmingly present in a certain size, highly leading to at least focal low internal echoes in the lesion. A large amount of collagen fibers, if present in the mass, should have made the attenuated posterior echoes, being inconsistent with the findings observed in this case. The whitish substance, presumable solid protein, should have resided in the cystic parts and probably gave hardness to the tumor. Sahin et al. [4] reported apocrine adenoma of the breast had three proteins such as GCDFP15, 24 and 44 in the cystic glands, being consistent with the hypo-intense pattern on T2-weighted images in this case.

This disorder must be differentiated from fibroadenoma and tubular adenoma. The latter two disorders, however, typically have clear borders on ultrasound and tumor enhancement in either a plateau or a persistent pattern on enhanced MRI. It, therefore, is not very difficult to differentiate this disorder from other benign breast diseases.

A small well-demarcated mass on mammography which has indistinct borders, internal iso-echoes with high echo spots, and unchanged posterior echoes on ultrasound highly suggest apocrine adenoma of the breast when the lesion shows a hypo-intense pattern on T2-weighted images.

This article does not contain any studies involving human participants or animals performed by any of the authors. Ethical approval was not required for this case study in accordance with local or national guidelines. Written informed consent was obtained from the patient for the publication of this case report and accompanying images.

The authors have no conflicts of interest to declare.

The authors have no funding sources to declare.

S.K. contributed to the design of the report. O.S. drafted the manuscript. All authors have read and approved the final version of the manuscript.

All data generated or analyzed during this study are included in this article and its online supplementary material files. Further inquiries can be directed to the corresponding author.