Teaching Case

A 24-year-old male patient with no prior medical history consulted because of lower back pain. Clinical evaluation revealed left sacral swelling that was painful on palpation.

An ultrasound of the left para-spinal region was performed showing a hypoechogenic lesion with subtle peripheral calcifications visible as a hyperechoic linear structure of the superficial component of the lesion (Figure 1). There was no hyperemia at Doppler.
A CT examination was performed and confirmed the presence of peripheral calcifications. (Figure 2).
MRI showed a well-delineated, lobulated lesion with a hyperintense T1 and T2 signal and homogenous enhancement (Figure 3, 4, 5). Peripheral calcifications confirmed by the CT were hypointense on T2 (Figure 3). The lesion measured 2.5 x 1 x 4 cm and was situated within the para-spinal muscles with a subcutaneous extension at the level of S3, in a dumbbell form (Figure 3).
There was no significant hyper-metabolism of the lesion with a SUV-max 0f 1.0 on the PET-MRI (Figure 6).
Surgical removal of the lesion revealed a malignant tumour with the translocation t(X-18) of the SYT and SSX2 genes.

Synovial sarcoma is a malignant tumour that represents 8-10% of all sarcomas affecting adults aged from 15-50 [1, 2]. The majority of lesions occurs in the extremities and is most commonly situated in the lower extremities especially in the popliteal fossa [1, 2]. Histologically, synovial sarcoma does not originate from synovial cells despite its name and is usually not located within a joint. Synovial sarcoma poses a diagnostic challenge due to its indolent course and seemingly benign imaging characteristics with slow growth and small size (<5cm on initial presentation) leading to an average time to diagnosis of 2-4 years [2]. Patients usually present with a palpable soft-tissue mass that is slow growing. Pain and tenderness are frequent [2].
Radiography is often the initial examination but is normal in 50% of cases [1]. Imaging may show a lobulated soft-tissue mass with calcifications in 30% of cases, often eccentric or peripheral [1, 2]. Periosteal reaction is observed in 20% of patients. Aggressive bone invasion and destruction are uncommon [2].
CT is useful to depict calcifications and to determine bone invasion or periosteal reaction. Findings are non-specific and show a well-defined soft-tissue mass usually found near a joint.
MRI is the modality of choice because of excellent tissue contrast enabling to depict the extent of the lesion. 91% of patients present with a well-defined lesion with rounded or lobulated margins [1]. The tumour displays a heterogeneous intermediate signal on T1-weighted images and high signal on T2-weighted images. Lesions may bleed and lead to high T1-signal due to methaemoglobin. Cystic components are seen in 77% of patients [1]. Approximately one third of lesions demonstrate the “triple signal pattern” which translates different signal intensities due to cystic, fibrous and tissue components [1]. Lesions show enhancement that is more commonly heterogeneous but may be homogeneous in up to 17% of cases [2].
Nuclear imaging does not play a significant role in the workup of synovial sarcoma but may be used in the evaluation of metastases or recurrent disease and in targeting biopsies. PET imaging demonstrates increased tracer accumulation with high standard uptake values being predictive for overall survival in various studies [2, 3].
Treatment of the lesion is achieved with surgery. Chemotherapy may be used in cases of metastatic, most commonly to the lung, or residual disease. Radiation therapy plays a role in the treatment of marginally resected tumours [2]. 5-year survival rates range from 36%-76% [2]. A favourable prognosis is expected in patients with a tumour size <5cm [4].