INTRODUCTION

Synovial sarcoma (SS) is a blue monomorphic spindle cell sarcoma with variable epithelial differentiation,1 characterized by single chromosomal translocation t(x; 18) (p11.2; q11.2),2.3.4 observed in more than 90% of cases,2,4,5 which results in the formation of the specific fusion gene SS18-SSX1/2/4.1,2,4 It is characterized by local invasion and propensity to metastases, affecting pediatric, adolescent, and adult populations.4 There are no known predisposing factors. Exceptionally, SS may be associated with a history of radiotherapy.1

SS can be classified as biphasic or monophasic1.6-8 depending on the proportion of epithelial and spindle cell components – the monophasic type is the most frequent in adults.6,7 In some cases, especially in the elderly, poorly differentiated variants can be found, with greater cellular mobility, greater nuclear atypia, and high mitotic activity, composed of fascicular spindle cells, small round hyper- chromatic tumor cells or epithelioid cells.1

SS is rare and represents 5-8% of all soft tissue sarcomas,5.7 having an annual incidence rate of 0.5-0.7 per million.4 It can occur at any age,1.9 and is distributed equally between genders.1 More than half of the patients are adolescents or young adults1-3,6,7 – 30% of cases oc- cur before 20 years of age4 and 77% occur before 50 years,1,9 with peak incidence observed in the third de- cade of life.4 The relative frequency of SS compared to other soft tissue sarcomas is age-dependent, ranging from 15% in patients aged 10 to 18 years to 1.6% in pa- tients aged > 50 years.1

Despite the name, SS is rarely intrarticular.4-6 Most (70%) appear in the deep soft tissues of the extremi- ties,1,3,4,6,7-9 often in juxta-articular sites.1,4,6,10 About 15% appear in the trunk and 7% in the head and neck re- gion.1 Unusual sites include male and female external reproductive organs, kidneys, adrenal glands, retroperi- toneum, stomach, small intestine, lung, heart, medias- tinum, bones, central nervous system, and peripheral nerves.1

Plain radiographs are often the first-line examina- tion in the evaluation of SS. In 50% of patients, radiogra- phic findings are interpreted as normal.4 Radiographs may eventually reveal a well-defined, round or lobulate soft tissue tumor.4 Calcifications in the tumor matrix constitute known radiographic findings of the SS,8,9,11,12 occurring in 30-50% of the cases,4.8 and may be focal or disseminated throughout most of the tumor, presen- ting a tenuous, dotted, or opaque appearance.4 Other findings include the presence of homogeneous mass in soft tissues, adjacent bone erosion, and, less commonly, bone invasion.9

Computed tomography (CT) and magnetic resonan- ce imaging (MRI) are useful in the evaluation of bone, muscle, and vascular invasion, as well as in the evalua- tion of tumor margins.9.11,12

CT is less useful in the evaluation of soft tissue tu- mors. However, SS is generally well defined by this me- thodology, and may occasionally appear infiltrative or, in the case of minor lesions, present homogeneous impro- vement.4 Alternatively, in the presence of hemorrhage or necrosis, the lesion may be multiloculated, presenting heterogeneous increase.4

MRI is the modality of choice, due to the excellent tissue contrast and ability to portray the lesion in mul- tiple planes, allowing adequate evaluation of tumor extension.4 Approximately 91% of patients present wel- l-defined lesion, with rounded or smoothly lobulated margins.4 The tumor usually causes displacement of ad- jacent structures, rather than invasion or destruction.4 Most tumors have heterogeneous intermediate signal