a 12-year - old male patient visited our polyclinic with complaints of pain and swelling in the left knee . his medical record showed the pain had started after an impact to the lateral side of the left knee while playing football 2 months ago . radiological and clinical examinations of the traumatic area were performed . while following the injury of the left knee suspected of ligamentous strain , swelling of the knee regressed . the pain was localized to a 2 cm area on the lateral side of the distal thigh . on the anteroposterior and lateral radiographs of the left femur , a 1-cm calcified mass was detected in the distal region of the vastus lateralis muscle , where the pain was localized . before anesthesia , the painful region was marked with a marker . a ring - shaped wire was placed laterally on the marked painful region . under fluoroscopic vision , the ring - shaped guide wire was placed on the calcified mass ( fig . a bright - surfaced and coin - shaped cartilage - like mass with a size of 210 mm was observed through the incision line . the result of intraoperative consultation ( frozen section ) was the neoplasm with fusiform cell . in macroscopic examination of the excised mass , a grey - white sectional view of an irregular - shaped solid tumoral mass of 1 cm in diameter was observed besides firm regions at certain locations . in microscopic examination of the tumor , short and irregular bundle structures , which involve wide dystrophic calcification regions and consist of large fusiform cells with narrow cytoplasm and oval nucleus , were observed . in the tumor , under 100 magnification ( high magnification field ) , there were 12 mitoses , nuclear pleomorphic structures at mild - medium level of severity . according to the classification of soft tissue tumors in the grading scale of fedaration nationale de lutte contre le cancer3 ) , tumor differentiation of the mass was 3 , mitosis was 2 ( 12/10 high magnification field ) , tumor necrosis was 0 , and total score was 5 . furthermore , in immune - histochemical analyses of our tissue samples , ki-67 proliferation index was found to be slightly high ( range , 5% to 6% ) . late recurrence and metastasis to a distant organism were not found during the long - term follow - up ( 3 years ) . during active and passive flexion and extension movements of the left knee , the majority of sss are located around the extremity45 ) , and they are less frequently seen in the neck , heart , and lung2 ) . in extremities , they are located around large joints such as the knee joint . they are in close relation with the joint capsule , tendon sheath , bursa and facial structures . it is commonly believed that ss does not originate from synovial cells but from mesenchymal cells1 ) . if it is calcified , it can be detected within the soft tissue in radiography2 ) . prognostic indicators of ss include inefficient excision of the mass , a mass larger than > 5 cm , male gender , > 20 years of age , high - degree tumor , presence of necrosis , neurovascular invasion , high mitosis rate , and syt - ssx1 variant6 ) . the metastatic spread of ss is to regional lymph nodes and lung in general7 ) . ss appears as a heterogeneous multi - lobular soft tissue mass having similar or higher signal density than muscle on t1-weighed magnetic resonance imaging ( mri ) . on t2-weighed mri the image obtained in mri evaluation of our case was in accord with that presented in the literature . even though the radiological characteristics of ss are not pathognomonic , it is seen as a calcified mass in soft tissues in up to 30% of cases8 ) . considering that formation of calcified mass also occurs in case of mo , it should also be added to the differential diagnosis . it generally occurs after a soft tissue trauma9 ) . in the x - ray assessment of our case , in addition , the patient had a history of trauma in the mass region , and the clinical picture indicated mo . ss has 3 subtypes from the histological aspect ; biphasic , monophasic , and poorly differentiated types10 ) . in the histological analysis of our case samples , there was no symptom in our patient with ss . during the evaluation of the injury caused by a trauma , a < 1 thus , the diagnosis of ss was made in the early - stage before the symptoms emerged and appropriate surgical treatment was performed . therefore , it is advised to consider a diagnosis of ss in patients with a history similar to our patient 's and radiographic evidence of < 1 cm lesions suspicious of mo .
a calcification mass was incidentally found in the soft tissue of a patient who had a history of trauma to the extremity during examination . the patient had no symptom . 
 the pathological analysis of the mass revealed it was an early - phase synovial sarcoma ( ss ) . 
 the diagnosis was made before the onset of symptoms and proper surgical intervention was performed . 
 therefore , in case of a < 1 cm lesion clinically suspicious of myositis ossificans , ss should be taken into consideration as a possible diagnosis .