Synovial sarcoma is a rare malignant mesenchymal tumor, accounting for 5–10% of soft tissue sarcomas. Primary breast involvement is extremely uncommon, with very few cases reported.
Case presentationA 37-year-old female presented with a progressively enlarging mass in the left breast over two years, with accelerated growth during pregnancy. Imaging revealed a BI-RADS 4C lesion. Core needle biopsy suggested a low-grade mesenchymal tumor. The patient underwent a total mastectomy, and final histopathology confirmed biphasic synovial sarcoma with negative margins. Seven months later, she developed local recurrence involving the chest wall and axillary region. She received chemotherapy with ifosfamide and docetaxel, followed by radiotherapy using IMRT/VMAT techniques. After two years of follow-up, she remains free of disease.
DiscussionDiagnosis is based on histopathological and immunohistochemical findings. Surgical resection is the mainstay of treatment, although recurrence is frequent and the role of adjuvant therapy remains controversial.
ConclusionThis case highlights the importance of multidisciplinary management in rare breast sarcomas.
Synovial sarcoma is a malignant neoplasm of mesenchymal origin that accounts for approximately 5–10% of all soft tissue sarcomas [1]. Despite its name, it does not arise from synovial tissue, but rather from pluripotent mesenchymal cells capable of both epithelial and spindle cell differentiation [2].
It predominantly affects adolescents and young adults and is most commonly located in the extremities, particularly in periarticular regions. Breast involvement is exceedingly rare, with only a limited number of cases reported in the literature [3,4]. Additional isolated case reports further emphasize the rarity of primary breast synovial sarcoma [5].
Histologically, synovial sarcoma is classified into monophasic, biphasic, and poorly differentiated subtypes [6-8]. The biphasic subtype is characterized by the simultaneous presence of epithelial and spindle cell components, which is highly suggestive of the diagnosis.
At the molecular level, more than 90% of cases harbor the t(X;18)(p11.2;q11.2) translocation, resulting in the SS18-SSX fusion gene, which is considered a key diagnostic marker [9].
Due to its rarity in the breast, there are no specific management guidelines, and therapeutic decisions are generally based on experience with soft tissue sarcomas. In this context, the reporting of new clinical cases is essential.
Case reportA 37-year-old woman with no personal or family history of cancer presented with a progressively enlarging mass in the left breast, with a disease duration of over two years and noticeable growth during pregnancy.
Physical examination revealed a 6 × 7 cm firm mass with limited mobility, located in the upper inner quadrant of the left breast, with no palpable axillary lymphadenopathy.
Mammography reported a BI-RADS 4C lesion corresponding to an irregular, multilobulated mass with poorly defined margins measuring approximately 60 × 48 mm, in contact with deep planes (Fig. 1). Microcalcifications were also identified in the right breast. Ultrasound and computed tomography demonstrated two heterogeneous hypoechoic solid lesions in the left breast.
Core needle biopsy revealed a hypercellular atypical spindle cell proliferation. Immunohistochemistry was positive for CD34 (focal), BCL2, CD99, and CD68, consistent with a low-grade mesenchymal tumor. The immunohistochemical profile of the surgical specimen was largely consistent with that of the core needle biopsy; however, TLE1 and AE1/AE3 were not assessed in the core biopsy specimen.
Staging workup showed no evidence of metastatic disease. A total left mastectomy with reconstruction using an autologous graft was performed. No formal nodal staging was performed, as neither the diagnosis nor the oncologic management required it. Final histopathological examination confirmed a biphasic synovial sarcoma measuring 9 × 6 × 5.5 cm, with negative surgical margins. Immunohistochemical analysis showed positivity for AE1/AE3 and nuclear expression of TLE1, together with CD99 and BCL2 positivity, further supporting the diagnosis [10] (Fig. 2).
Synovial sarcoma, histopathological findings. (A) Spindle cell proliferation with a fascicular pattern and “staghorn” vascular architecture (arrows). (B) At higher magnification, tumor cells show moderate nuclear pleomorphism with occasional mitotic figures (arrow). (C, D) Tumor cells were positive for TLE1 and CD99 (images), and negative for CD34, smooth muscle actin, desmin, and CD10; BCL2 was positive. Immunohistochemical staining demonstrated positivity for AE1/AE3 and nuclear expression of TLE1, supporting the diagnosis of biphasic synovial sarcoma.
Seven months later, the patient developed a local recurrence with multiple lesions in the chest wall and axillary involvement (Fig. 1). Biopsy confirmed sarcomatous recurrence. Chemotherapy with ifosfamide and docetaxel was initiated, with evidence of unresectable residual disease.
Subsequently, she received radiotherapy using IMRT/VMAT (40.05 Gy plus a 13.3 Gy boost). Surgery of the right breast was also performed due to microcalcifications, with no evidence of malignancy.
After two years of follow-up, the patient remains disease-free.
DiscussionPrimary synovial sarcoma of the breast is an exceptionally rare entity, with very few cases reported in the recent literature. Most correspond to isolated case reports, which limits the understanding of its clinical behavior. Wang et al. (2025) even describe the first case in a male patient, highlighting the rarity of this neoplasm [3]. Likewise, recent reviews report fewer than ten cases of metastatic involvement of the breast [4].
Clinically, it typically presents as a slow-growing mass, which may delay diagnosis. Synovial sarcoma is more frequently reported in adolescents and young adults, although breast presentation may occur later in adulthood [11]. In the present case, a disease course of more than two years is consistent with prior reports.
The differential diagnosis includes metaplastic carcinoma, malignant phyllodes tumor, and other primary breast sarcomas. Primary breast sarcomas themselves are rare entities with overlapping clinicopathologic features [12]. These entities should be distinguished according to breast tumor classification criteria [13]. Histopathological classification of soft tissue tumors remains essential for diagnosis [14]. Diagnostic confirmation relies on histological and immunohistochemical evaluation, with the biphasic pattern being highly suggestive. Clinicopathologic correlation remains fundamental in rare presentations such as breast synovial sarcoma [15].
Treatment is primarily based on surgical resection with negative margins, which remains the cornerstone of management in localized disease [16]. However, the high rate of local recurrence, particularly in large tumors, remains a major challenge. In this case, early recurrence at seven months reflects the aggressive behavior of the disease.
The role of chemotherapy and radiotherapy remains controversial, although ifosfamide-based regimens have shown benefit in advanced disease [17,18]. Synovial sarcoma is also characterized by late recurrence and distant metastasis, supporting prolonged surveillance [19]. The use of modern radiotherapy techniques in this case allowed for adequate locoregional control, in line with previous reports supporting adjuvant radiation in selected high-risk cases [20].
Finally, the favorable outcome at two years is a relevant finding, considering the generally guarded prognosis of these tumors.
ConclusionPrimary biphasic synovial sarcoma of the breast is an extremely rare neoplasm that poses significant diagnostic and therapeutic challenges. A multidisciplinary approach is essential to optimize outcomes. The reporting of additional cases is crucial to improve understanding and to establish more effective management strategies.
Informed consentThe authors confirm that they have obtained the patient's informed consent.
Ethical considerationsThis study was approved by the Ethics Committee of Almenara Hospital, and the patient provided written informed consent.
FundingThe authors received no financial support for this research.
The authors declare no conflicts of interest.
