A granular cell tumor (GCT) is a soft tissue neoplasm derived from the Schwann cells of the nerve sheath (1). GCTs may occur throughout the body, especially in the head and neck, and 5%–8% of GCTs occur in the breast (2). It is necessary to distinguish this tumor from breast cancer because they show similar clinical and radiological features. However, diagnostic imaging presentation of GCT of the breast is variable (2). In order to delineate the diagnostic challenges and the therapeutic options of GCT, we report a case of a patient who developed a GCT in the axillary accessory breast, which is a rare location.

A 50-year-old woman came to our hospital with a palpable mass in the left axillary area with no skin change in the overlying skin for 2 months. Physical examination showed that the lump was approximately 1 cm in diameter, had poor mobility, a smooth surface, and no tenderness. A provisional diagnosis of left axillary lymph node enlargement was made and additional examinations were performed.

A circumscribed, round, and isodense mass that had not been observed 2 years earlier on mammography was discovered in the left axillary region on a recent mammogram (Fig. 1A). US examination of the axilla revealed a 1.1 cm, non-parallel, round, hypoechoic mass with a circumscribed margin and an echogenic halo (Fig. 1B). The mass was located within a small mammary gland-like structure in the axillary region. A color doppler image additionally revealed increased blood flow within the mass. The mass was adjacent to the skin layer on mammography and US. It was impossible to rule out breast carcinoma developing in the accessory breast tissue of the axilla. An US-guided core-needle biopsy of the mass was subsequently performed. Histologic examination of the core biopsy showed a benign GCT. Surgical excision was recommended, and wide local excision was performed. Intraoperatively, an axillary specimen of 3.2 cm × 1.5 cm × 1.5 cm was resected. The size of the attached skin was 2.6 cm × 1.0 cm × 0.1 cm. The cross-section showed a well-defined mass itself with a clear resection margin, measuring 1.3 cm × 1.2 cm × 1.1 cm in size. The mass was less than 0.1 cm apart from the skin margin. Histopathologic examination confirmed the diagnosis of a GCT of the axillary accessory breast. A surgical specimen showed tumor cells with a distinctive granular eosinophilic cytoplasm associated with typical nuclei, without an increase in nuclear division or another sign of malignancy (Fig. 1C). A diagnosis of GCT was established based on the immunohistochemistry (IHC) results. IHC analysis of the tumor showed strong S-100 positivity, CD68 positivity, and pancytokeratin negativity (Fig. 1D). The patient remains disease-free 2 years after surgery.

Fig. 1
A granular cell tumor involving the left axillary accessory breast in a 50-year-old woman.
A. Mammography shows an isodense mass with a well-circumscribed margin adjacent to the skin layer in the left axillary region (arrow).

B. Ultrasonography shows a round hypoechoic mass with a well-circumscribed margin and non-parallel orientation and echogenic halo.

C. Photomicrograph of a histopathological specimen shows nests of large cells with granular eosinophilic cytoplasm containing typical small nuclei (hematoxylin & eosin stain, × 200).

D. Immunohistochemical analysis of the resected specimen shows cells with immunopositivity for the S-100 protein (× 100).

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This study was approved by the Institutional Review Board of our institution, and the requirement for informed consent was waived (IRB No. 05-2022-122).

GCTs are rare benign soft tissue tumors that can occur anywhere in the body, especially in the head and neck. GCT was considered a myogenic tumor, however, IHC confirmed that GCTs were soft tissue tumors derived from nerve tissue. Currently, GCTs are accepted to be derived from the Schwann cells of the nerve sheath.

GCTs of the breast account for 5%–8% of all cases (3). In contrast to other breast tumors that frequently occur in the upper outer quadrant, GCTs of the breast most commonly occur in the upper inner quadrant, consistent with the location of cutaneous sensory territory of the supraclavicular nerve (4). There have also been reports of GCTs in other parts of the breast in the literature. However, it is known that GCTs in the accessory breast tissue are extremely rare. A literature search found only two cases of accessory breast tissue GCT. Liu et al. (5) described GCT of the accessory breast, which showed a hypoechoic lesion with a blurred margin and posterior shadowing on US and increased blood flow inside the mass on a color doppler image. Olivier et al. (6) described another patient with a GCT in the accessory axillary breast tissue that was also observed as an ill-defined, hypoechoic mass on US. The US findings in our case, in contrast to the two previously reported cases, revealed a round mass with a circumscribed margin and an echogenic halo.

GCTs can mimic breast cancer clinically and radiologically (3, 7). Clinically, they present as painless round nodules that are often palpable (1). Mammography has shown various findings, including round and circumscribed masses to indistinct or spiculated lesions (3). Microcalcifications have not been reported in GCT. On US, GCTs can be seen as spiculated or indistinct marginated masses with posterior shadowing or seemingly benign well-circumscribed masses (3). Because GCTs are often located subcutaneously, finding the epicenter of the lesion may provide a diagnostic clue, although they can be found at several depths (8). In our case, the mass was located in the subcutaneous fat layer, adjacent to the skin. MRI findings are not specific enough to differentiate the GCT from breast cancer (7, 8). Therefore, GCTs cannot be diagnosed based on radiological findings alone. The histological assessment of a GCT is essential, and core biopsy specimens are sufficiently representative to provide the pathological diagnosis (8).

GCTs of the breast are macroscopically solid, firm, homogeneous masses with a white to tan color without necrotic areas (7). Their histogenesis remains unclear. However, GCTs stain positive for S-100, and this protein is also found in Schwann cells, neural cells, and melanocytes. Although GCTs of the breast are sensitive to S-100, 10% of breast malignancies also stain positive for S-100. Therefore, other markers such as CD68 and neurospecific enolase are required to diagnose GCTs. The CD68 protein demonstrates the presence of lysosomal activity and stains positively for perineural Schwann cells and 90% of GCTs (9). Neurospecific enolase is found in the cytoplasm of neurons and neuroendocrine cells (10). The hypothesis that GCTs originate from the Schwann cells of distal nerves ending in the breast tissue is supported by these findings.

The treatment of benign GCTs in the breast is a wide local excision with free margins. Sentinel lymph node biopsies, lymph node dissections, and mastectomy are not required due to the benignity of the majority of GCTs. If the surgical margin is negative, the recurrence rate is 2%–8%; it is more than 20% when the surgical margin is positive. Therefore, it is necessary to follow up annually with clinical examination for about 10 years (9).

In conclusion, we report a rare case of an axillary accessory breast GCT with suspicious features of breast cancer, observed clinically and radiologically. The final diagnosis is guided by pathological assessment. Radiologists and clinicians should be aware of these findings of GCT in the differential diagnosis of breast and axillary lesions to avoid unnecessary overtreatment.