The prevailing histological subtype of tumors originating from the abdominal wall typically arises from desmoid sources. Differential diagnoses for abdominal wall tumors include desmoid tumors, endometriosis, fibromatosis, gastrointestinal stromal tumors, leiomyosarcomas, and rhabdomyosarcomas [1]. Synovial sarcoma is uncommon within the anterior abdominal wall, documented in only 44 cases over 55 years [2]. Globally, synovial sarcoma accounts for 8% of all primary soft-tissue malignancies, ranking fourth in frequency, succeeding undifferentiated pleomorphic sarcoma, liposarcoma, and rhabdomyosarcoma [3]. Approximately 70% of synovial sarcoma cases manifest around the knee joint [4]. Despite extensive investigations, the precise pathogenesis of synovial sarcoma remains unresolved. Some researchers propose that these tumors originate from multipotent stem cells that can differentiate into mesenchymal and/or epithelial structures, as suggested by ultrastructural and histochemical analyses. Despite their name, synovial sarcomas predominantly occur outside joint structures and lack morphological resemblance to synovial tissues. Nonetheless, owing to similarities with primitive synoviocytes, the term “Synovial Sarcoma” has been adopted [5, 6].

In our case, a 40-year-old female sought medical attention due to a complaint of a large swelling in the left lower back region. It was associated with mild pain, managed with painkillers, and has grown to approximately 10 × 10 cm over four months. There were no reports of swelling elsewhere in the body, skin changes, or discharge from the swelling. Additionally, there was no history of appetite loss, weight loss, altered bowel habits, blood in stools, any therapy, or drug intake. The patient underwent an MRI, which revealed a large well-defined lobulated heterogeneous mass lesion in the left lateral abdominal wall muscles showing varying signal patterns on T2 weighted sequence (Fig. 1A), giving a “triple signal pattern” or “bunch of grapes” sign; the lesion showed restriction of diffusion on diffusion-weighted imaging with corresponding low signal on apparent diffusion coefficient maps (Fig. 1B and C); axial T1 weighted fat suppressed pre-contrast (Fig. 1D), post-contrast (Fig. 1E) and subtraction image (Fig. 1F) showed heterogeneous enhancement of the lesion. The swelling was removed by wide local excision with meshplasty. Histopathological images shown in Figs. 2 and 3 reveal gross photo macrograph showing a multinodular solid cystic grey-white tumor with attached fibrofatty tissue (Fig. 2A) with Hematoxylin and eosin stained sections (Figs. 2B, C, and D) showing a biphasic tumor comprising of epithelial and spindle cell components. Immunohistochemistry (IHC) images in Fig. 3 showed the tumor cells having diffuse membranous positivity for CD99, nuclear TLE1 staining, EMA highlighting the epithelial cell component, Pan cytokeratin positive in epithelial component, CD34 negativity, and Ki67 proliferating activity. Diagnosis of Biphasic synovial sarcoma of the abdominal wall was made.

MRI shows a large well-defined lobulated heterogeneous mass lesion (arrow) in the left lateral abdominal wall muscles showing varying signal pattern on T2 weighted sequence (A); the lesion shows restriction of diffusion on diffusion-weighted imaging with corresponding low signal on apparent diffusion coefficient maps (B and C); axial T1 weighted fat suppressed pre-contrast (D), post-contrast (E) and subtraction image (F) showing heterogeneous enhancement of the lesion. Areas of high signal on pre-contrast T1 weighted sequence represent locules of hemorrhagic contents (arrow in D)

Gross photo macrograph shows a multinodular solid cystic grey white tumor with attached fibrofatty tissue (A); Hematoxylin and eosin stained (H&E) section (B) shows a biphasic tumor comprising of epithelial and spindle cell components (H&E × 40); higher magnification (C) showing spindle cell component (H&E × 400); and higher magnification (D) showing epithelial cell component (H&E × 200)

Immunohistochemistry (IHC) images showing the tumor cells showing diffuse membranous positivity for CD99 (× 100) (A); nuclear TLE1 staining (× 400) (B); EMA highlighting the epithelial cell component (× 200) (C); Pan cytokeratin positive in epithelial component (× 200) (D); IHC for CD34 is negative and highlights blood vessels in tumor (× 100) (E); and Ki67 proliferating activity (× 400) (F)

Typically diagnosed during adolescence and adulthood, with a peak presentation occurring between 15 and 25 years of age, synovial sarcoma affects both genders equally, displaying no significant preference for ethnicity or race. Pathologically and immunohistochemically, three main types of synovial sarcomas are identified:

Monophasic, characterized by spindle-shaped cells and representing the most prevalent subtype

Biphasic, consisting of both epithelial and spindle cells

Poorly differentiated, exhibiting a rhabdoid pattern

More than 90% of cases exhibit the characteristic translocation t(X;18) (p11.2;q11.2), involving the SYT gene on chromosome 18 and the SSX gene on the X chromosome (SSX1, SSX2, or SSX4), detectable through fluorescence in situ hybridization (FISH) or reverse transcription-polymerase chain reaction (RT-PCR) techniques [6,7,8].

Radiological examinations play a pivotal role in the diagnosis, preoperative assessment, and postoperative monitoring of synovial sarcomas. Ultrasound typically reveals a solid hypoechoic soft-tissue mass or a heterogeneous appearance with irregular margins. However, ultrasound alone may not suffice for preoperative planning. Computed tomography (CT) commonly depicts a heterogeneous, well-circumscribed mass with attenuation values akin to or slightly lower than muscle, often exhibiting punctate calcifications and areas suggestive of cystic changes or necrosis. Upon intravenous contrast administration, heterogeneous enhancement is typically observed. Thoracic CT is routinely conducted to rule out pulmonary metastases before surgery. Magnetic resonance imaging (MRI) is the preferred modality for diagnosing synovial sarcomas. Typically, on T1-weighted images, these sarcomas appear as heterogeneous multi-lobulated soft-tissue masses with signal intensity comparable to or slightly higher than muscle. On T2-weighted images, synovial sarcomas exhibit prominent heterogeneity and high signal intensity, often displaying a “triple signal pattern” or “bunch of grapes” sign. MRI excels in evaluating local invasion and assisting in preoperative surgical planning. Post-gadolinium administration, synovial sarcomas typically demonstrate heterogeneous enhancement [9]. Differential diagnoses for abdominal wall synovial sarcomas include desmoid tumors, endometriosis, fibromatosis, gastrointestinal stromal tumors, leiomyosarcomas, and rhabdomyosarcomas.

Achieving local control in synovial sarcomas primarily involves thorough surgical excision, encompassing en bloc resection of the tumor with its pseudo capsule and a surrounding margin of normal tissue, typically at least 1 cm wide. Inadequate removal of normal tissue margins increases the risk of local recurrence eightfold. The efficacy of adjuvant radiation therapy and chemotherapy remains uncertain, with their impact on local control and overall survival debated. These modalities are typically offered to patients with marginally resected tumors or residual disease to enhance local control rates. Chemotherapy, particularly ifosfamide-based regimens, is primarily utilized for metastatic disease, potentially improving disease-specific survival rates. Reported 5-year survival rates for synovial sarcoma range from 56 to 76%, with frequent occurrences of local recurrence and distant metastases, affecting 50% to 70% of cases, commonly involving the lungs and regional lymph nodes. Given the tendency for late recurrences and metastatic disease, patients with synovial sarcoma should undergo extended follow-up, often exceeding a decade. Various prognostic factors have been identified, including patient age, tumor size, surgical margins, histological subtype, tumor grade, and fusion type. Notably, tumor size exceeding 5 cm consistently correlates with poorer outcomes [10,11,12].

This case highlights the importance of considering synovial sarcoma as a radiological possibility in the case of an abdominal wall tumor.