Synovial sarcoma is a rare soft tissue sarcoma with unclear differentiation, accounting for approximately 5–10% of all soft tissue sarcomas. It can occur in patients of any age but is commonly seen mainly in adolescents and young adults, with a median age at onset of approximately 39 years [3, 4]. According to the statistics, the incidence of synovial sarcoma is 0.81/1,000,000 among children and 1.42/1,000,000 among adults [5, 6]. It can occur anywhere in the body, though most often in the joints of the extremities, and presents as a growing soft tissue mass. Other areas, such as the oropharynx, larynx, mediastinum, lungs, kidneys, prostate, and abdominal wall, can also show synovial sarcoma growth [7]. Primary pelvic synovial sarcoma is even rarer and has been scarcely reported.
The clinical manifestations of synovial sarcoma are not obvious. It is usually a slow-growing, painless mass, and when detected, the tumor has grown very large and shows symptoms of pressure on the surrounding organs [8, 9]. Given the young age of onset, insidious course, and atypical clinical manifestations, it is easily misdiagnosed as a benign tumor, leading to treatment delay. In this case, the first symptom was the pressure of synovial sarcoma on the bladder, rectum, and other organs, which manifested as difficulty in urination and painful swelling in the lower abdomen and perianal area.
The diagnosis of synovial sarcoma relies on imaging tests such as computed tomography and magnetic resonance. On computed tomography, the mass appears as a round or lobulated mass with a density similar to or slightly lower than that of muscle. It is heterogeneous and often accompanied by punctate peripheral calcifications [10, 11]. Magnetic resonance is an imperative test to determine the diagnosis and staging of synovial sarcoma, and the imaging characteristics are diverse. Masses smaller than 5 cm appear as homogeneous masses on all sequences, favoring benign imaging features. Among sarcomas larger than 5 cm, there is significant heterogeneity. The heterogeneous mixed signal of the high, medium, and low intensities on T2WI, the so-called "triple signal sign", is due to calcification, cystic changes, hemorrhage, and fibrosis in the process of tumor growth [11, 12]. The MRI of our patient showed a clear triple-signal sign, with bleeding and cystic changes visible inside the tumor, and the intraoperative views confirmed the presence of hemorrhage inside the cancer.
In general, imaging is inadequate to confirm the diagnosis, and the final diagnosis is based on pathological findings. Synovial sarcomas are gray or yellowish-brown in appearance, and most are 3–10 cm in size [13]. Histologically, synovial sarcoma originates from mesenchymal cells and manifests as spindle cell sarcoma, which can be classified into monophasic, biphasic, and hypo-differentiated types according to the ratio of spindle cells to epithelial cells. The monophasic type is the most common type, accounting for approximately 50–60% of all synovial sarcomas and consisting of only spindle cells. This is followed by biphasic synovial sarcoma, which makes up approximately 20–30% of all synovial sarcomas and comprises both epithelial and spindle cells. Poorly differentiated synovial sarcoma is the least common type, accounting for 10–15% of cases [14]. In our patient, preoperative mass puncture biopsy suggested soft tissue spindle cell sarcoma, and postoperative gross specimen pathology indicated a biphasic synovial sarcoma with the coexistence of epithelial cells and spindle cells. Immunohistochemistry was positive for EMA, CK, CK19, CD99, and Bcl-2 protein and negative for S100 protein.
Molecularly genetically expressed as t(X:18) translocations, including the common SS18:SSX1 and SS18:SSX2 and the less common SS18:SSX4 translocations, this translocation is present only in synovial sarcoma and is seen in approximately 95% of cases[15–17]. Fluorescence in situ hybridization (FISH) and reverse transcription polymerase chain reaction (RT‒PCR) assays can confirm this translocation. Of course, the diagnosis of synovial sarcoma cannot be excluded in patients with no detectable t(X:18) translocation and imaging showing synovial sarcoma features (approximately 5% of all patients), as this group of patients may be associated with t(X;20) and SS18L1-SSX1 fusion transcripts [18].
Because of the rarity of synovial sarcoma, no standard treatment regimens are currently available. Surgery is still the preferred treatment modality according to the age of onset, location, and tumor size. Complete resection of the tumor is the key to reducing postoperative recurrence and improving survival. Postoperative supplementation with systematic radiotherapy can improve the treatment effect based on the characteristics of the case [19, 20].
In conclusion, Synovial sarcoma, a highly malignant soft tissue sarcoma, and primary intrapelvic synovial sarcoma is even rarer, with a poor prognosis. Ultrasound, CT and MRI can help specialists to detect the tumor at an early stage and treat it aggressively, especially by active surgical treatment, which can effectively improve the survival rate. Combined with the course of diagnosis and treatment of this case, it is possible to deepen the understanding of primary pelvic synovial sarcoma and discuss appropriate treatment strategies for this rare disease.