Proliferative index (Ki-67) was about 6-7%. The ultimate pathologic results ruled out the possibility of sarcoma and other spindle cell carcinoma, and confirmed that the breast mass was nodular fasciitis (NF). Based on the immunohistochemical findings, the patient received no adjuvant chemo-radio therapy.

NF is a proliferative lesion of the reactive fibroblastic cells. Proliferation originates from the fascia of the subcutaneous tissue. This lesion can occur in every place in the body but it is rare in the breast [3]. NF, was first explained by Konwaler, et al. [4]. However, the etiology is still unclear. It is believed that local injury may stimulate the fibroblastic proliferation. Although, the history of trauma was recognized in about 10–15% of patients [5]. There was no history of trauma in our case. NF is described a reactive rather than a true neoplastic lesion. For the reason, its quick expansion, it is often misdiagnosed as a sarcoma. Also, it can clinically and radiologically mimic invasive cancer [6] which in our case, radiologic findings was highly suspicious for malignancy (invasive cancer). NF is able to be mistaken for malignant fibrous lesions such as fibrosarcoma or breast carcinoma when located in breast. This present a difficulty for appropriate treatment strategy as the natural history of nodular fasciitis is spontaneous regression [7]. The imaging manifestation of NF frequently exhibits the histological appearance. Thus, suspicious appearances have been described. This diversity in imaging appearance may expose that as the lesion becomes more mature it becomes more fibrotic [8]. NF is not regularly diagnosed by fine needle aspiration cytology or core needle biopsy, and it commonly needs excisional biopsy for histologic confirmation. As the articles that have reported the proliferation of neoplastic spindle cells were not conclusive, surgical excision and examination of the whole specimen are necessary to reach final diagnosis [9]. The diagnosis of bland looking spindle cell proliferations of the breast is often complicated because there are similar morphological and immunohistochemical overlap between the diverse entities [10]. Histologically, such proliferated spindle cells may show a specific level of mitosis and cellularity, which can enhance suspicion of malignancy, even though they are benign. Recent articles have described that the most general diagnostic difficulty arise with other benign and malignant spindle cell tumors (including spindle cell carcinomas and sarcomas) and fibromatosis [11]. In our case, NF was diagnosed based on the histomorphology and immunostaining consequences (positivity for SMA and desmin with negativity for CK, P63, BCL-2, and CD34). Some reports point out that the key structures for diagnosis are the existence of inflammatory cells (principally lymphocyte) and red blood cells extravasation with reactive fibroblasts clusters that are arranged in short bundles, as in our case [12]. The rarity and the unclear clinical and radiological findings of breast NF create the differential diagnosis and management challenging. Awareness of the clinical, pathological and prognostic aspects of this condition is necessary for breast specialists in order to get better their diagnostic and therapeutic interventions [13]. If the breast surgeon considers NF as a probability alongside with other differential diagnoses, close observation could be main management choice [14]. Most cases are diagnosed after subsequent surgical excision, the natural history is not well known. Also, local recurrence is infrequent and those managed conservatively regress spontaneously [15]. It is difficult to diagnose NF using imaging or preoperative biopsy. Therefore, to avoid overtreatment, it is mandatory to keep the possibility of nodular fasciitis in mind.

In conclusion, NF of the breast is an exceptional lesion that mimics breast cancer clinically and radiologically. Correct diagnosis needs histopathologic and immunohistochemical examinations. The pathologist should also be alert of the patient presentation in order not to misdiagnose this benign entity.