Synovial sarcoma involving suprapatellar location of the right knee

A 19-year-old male patient presented with chief complaints of swelling in the right knee for 10 years, gradually increasing in size, associated with pain, stiffness and gradually increasing difficulty in walking. There was no significant past medical history.

Plain radiograph of the knee AP and lateral view shows soft tissue density swelling with few cacified foci in suprapatellar location on medial and anterior aspect of the right knee joint. (Fig. 1)

On Magnetic Resonance Imaging, altered signal intensity lesion (iso to hypointense on T1, heterogeneously hyperintense on T2) is noted in supra patellar location of the right knee more on the medial aspect, infiltrating the surrounding vastus muscles with possible intraarticular extension. (Fig. 2)

On post contrast study, the lesion shows heterogeneous enhancement. (Fig. 4)

Synovial sarcoma is the third most common soft-tissue sarcoma (10% of all soft-tissue sarcomas), affecting both men and women equally usually seen in third decade. Synovial sarcoma mostly affects extremities, more commonly lower extremities, but rarely may it may be seen in head, neck, thorax, and abdomen. [1,2]

Though the name is synovial sarcoma it does not arise from the synovium, but it originates from primitive mesenchymal cells which undergo differentiation to resemble synovial cells. [2]

Histologically there are three main variants
1) Biphasic type (spindle cells and epithelial cells).
2) Monophasic type (only spindle cells).
3) Poorly differentiated type which mimics small round blue cell tumours. [2]

Presentation: Slowly growing soft tissue mass for many years which may be sometimes painful. No other significant complaints are noted. [2]

X-ray are usually normal except for soft tissue density mass. But if the mass contains calcifies foci, which may be seen in 30 % cases, then it could be seen in X rays. [3]

CT are also non-conclusive except for punctate, peripheral calcifications seen in some cases. [4]

MRI is the modality of choice, but final diagnosis is confirmed only by biopsy and histopathology. On MRI it appears as mass of variable size which is iso to hypointense on T1 and heterogeneously hyperintense on T2. Heterogenicity is due to areas of necrosis, cystic degeneration, calcifications and fibrotic bands, which gives so-called "triple sign" on fluid sensitive sequences. 10-25% of synovial sarcomas have the tendency to bleed with multiple haemorrhagic foci which shows blooming on GRE. On contrast MRI 80% of cases show diffuse or heterogenous enhancement on T1 while in the remaining 20% peripheral enhancement is seen. [2,3]

Synovial sarcoma may metastasise mainly to the lung, however, lymphatic spread is unlikely. [2]

Treatment
Localised tumours are surgically excised followed by radiotherapy, while for tumours with metastases radiotherapy is performed. [2]

Take Home Message: As synovial sarcoma is a slowly progressing tumour, in an early stage when clinical findings are insufficient, imaging modalities are the only way for early diagnosis.

Written informed patient consent for publication has been obtained.