A routine physical examination performed on an asymptomatic 63-year-old female patient revealed a stiff lump on the upper outer quadrant of the left breast, at one o'clock position. The patient was referred to our imaging department for further evaluation of these finding. There was no personal or familial history of prior breast pathology.
Classical mammographic views revealed a dense and spiculated mass located at the upper outer quadrant of the left breast - one o’clock position - highly suspicious for malignancy. The mass was relatively superficial and it focally retracted the skin. Thickening of the overlaying skin and increased density between the skin and the lesion was noted. Ultrasound revealed a taller-than-wide solid and hypoechoic nodular lesion measuring approximately 11 mm, with accompanying spiculated contour, a hyperechoic rim and marked posterior acoustic attenuation. There were no concurrent pathological axillary lymph nodes.
The lesion was classified as BI-RADS 5 and a core-biopsy with 14G needle was performed.
Due to the very high suspicion for malignancy, concurrent breast MRI was performed.
Breast MRI revealed a solid lesion with ill-defined margins measuring approximately 13 mm showing marked early enhancement followed by a plateau, following a type 2 kinetic curve.
There was no evidence of additional lesions in neither left nor right breasts.
Granular cell tumour (GCT) is a soft tissue tumour first reported by Abrikossoff by the name of granular cell myoblastoma. It is thought to originate in Schwann cells of peripheral nerves and it can be found in many different body sites . Most are benign, with malignant cases accounting for 1 or 2% of the cases. [1,2]
Around 5-6% arise in the breast, generally in women between 30 to 50 years of age. In the breast, GCT is commonly superficial, and some postulate that its distribution most often follows the cutaneous branches of the supraclavicular nerve. It usually presents as a solitary nodule with less than 3 cm and - unlike breast cancer - most often in the upper inner quadrant. [1,2,3]
Clinically, it usually presents as a hard but non-tender palpable mass. Imaging findings often exhibit findings highly suggestive of malignancy.
In mammography, irregular morphology, spiculations and stellate configuration with a dense core are frequent, although well-defined lesions can also be observed. Microcalcifications are uncommon.
In ultrasound, it usually presents as a solid mass with spiculated margins and posterior acoustic shadowing, although it can also have a more benign appearance.
Radiological findings in GCT thus overlap with those of malignant entities, with breast invasive carcinoma being the main differential diagnosis. Its suspicious appearance is due to GCT’s fibrous consistency and infiltrative growth pattern. 
The definitive verdict is only possible by histological assessment. GCT most often exhibit cells with eosinophilic granules and abundant cytoplasm and a single nucleus without evidence of pleomorphism or mitosis. Nuclear and cytoplasmic positivity for S100, cytoplasmatic staining for CD68 and negative reaction for cytokeratins are indicative of this tumour. 
MRI findings of breast GCT have been described but are unspecific and still scarce. GCTs are said to be isointense or slightly hyperintense in T2-weighted sequences. In T1-weighted sequences, lesions are most often isointense. Contrast enhancement with type 1, type 2 and type 3 kinetic curve have all been observed, as well as ring enhancement. [3,4]. It’s important to underline that MRI cannot exclude malignancy. For example, features such as strong enhancement and low ADC levels can be present, and even in the absence of enhancement, a spiculated lesion can be malignant.
Surgical resection with wide margins is the mainstay treatment for GCT, as these lesions are most often infiltrative and can locally recur.
In our case, core biopsy was performed and histopathology showed a lesion with “large and well-defined cells, with prominent cytoplasm and eosinophilic granules, with a single nucleus, without mitosis or atypia”. S100 and C68 staining were positive. Cytokeratins AE1/AE3 were negative. It was considered a B3 lesion.
Post-surgery histopathology confirmed negative margins and found no evidence of malignancy.
The final diagnosis of a breast GCT was made.
Written informed patient consent for publication has been obtained.
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