Case Report: Primary Intraosseous Poorly Differentiated Synovial Sarcoma of the Femur

Abstract

Primary intraosseous poorly differentiated synovial sarcoma is exceedingly rare. Here, we present a case of primary intraosseous poorly differentiated synovial sarcoma from the proximal femur in a 16-year-old girl. The case was initially misdiagnosed, but the correct diagnosis of synovial sarcoma was eventually confirmed by fluorescence in situ hybridization and next-generation sequencing. We review the literature pertaining to synovial sarcoma and show that this case is the second molecularly proven intraosseous poorly differentiated synovial sarcoma in the literature. Recognition of intraosseous synovial sarcoma composed of small round cells is imperative in order to avoid misdiagnosis of the tumor as Ewing sarcoma and other small round-cell tumors, all of which have markedly different clinical management.

Keywords: SYT-SSX fusion gene; bone tumor; poorly differentiated; small round cell; synovial sarcoma.

Figure 1

(A, B) An anteroposterior and lateral plain radiograph of the left femur. There is a comparatively well-outlined osteolytic lesion at the proximal part of the left femur. (C, E) MRI of the lesion of the proximal femur. The mass showed isointensity on T1-WI and high intensity on T2-weighted images (T2-WI). Well-defined oval-shaped heterogeneous soft tissue mass in close proximity to the medial side of the lesser trochanter. (D) PET-CT showed slight abnormal fluorodeoxyglucose (FDG) uptake in the lesion. (F) CT showed an expansile lesion at the proximal femur in the intertrochanteric region.

Figure 2

(A) CT showed multiple nodules in both lungs. (B) CT showed part of nodules in both lungs was enlarged 5 months later. (C) Plain radiograph of the left femur showed an aggravated lytic lesion in the upper segment of the left femur. (D) MRI showed the intramedullary soft tissue mass was enlarged. Intramuscular soft tissue mass in close proximity to the medial side of the lesser trochanter had no obvious changes. (E) Grossly, the tumor was centrically located in the proximal femur. Cut section of the proximal femur showed a black cut surface involving the intramedullary space with areas of hemorrhage and necrosis. The adjacent extraosseous mass had a yellowish-brown fleshy cut surface measured 6.4 × 4.8 cm in axial diameter. (F) Postoperative anterior posterior radiographs of the left femur demonstrated the endoprosthetic reconstruction of the proximal femur and hip joint.

Figure 3

(A) Highly cellular, malignant tumor composed of compact sheets of poorly differentiated rounded cells with ovoid or round hyperchromatic nuclei (hematoxylin and eosin staining; magnification, ×200). (B) Poorly differentiated round cells with a high nuclear-to-cytoplasmic ratio and vesicular nuclei with nucleoli (hematoxylin and eosin staining; magnification, ×400). (C) Immunohistochemistry of tumor cells showing focal positivity for membranous CD99 (magnification, ×400). (D) Strong nuclear TLE1 reactivity of the tumor cells (magnification, ×400). (E) Immunostaining for FLI-1 was negative (magnification, ×400). (F) Ki-67 staining indicated a proliferative index of 15%, ×400.

Figure 4

(A) RNAseq reads revealed exon 10 of SS18 was fused in-frame to exon 6 of SSX1. (B) DNA-seq reads revealed exon 10 of SS18 was fused in-frame to exon 6 of SSX1. Breakpoint was intron 10 of SS18 and intron 5 of SSX1.

Figure 5

The whole clinical process of this patient. VCR, vincristine; ADM, adriamycin; CPM, cis-platinum; PTX, paclitaxel; IFO, ifosfamide; VP-16, etoposide.

Figure 6

Positive fluorescence in situ hybridization (FISH) analysis for SYT (SS18) gene rearrangement, demonstrated by an abnormal signal pattern seen as disruption of the SYT gene through the breaking apart of the red and green probe signals.

Figure 7

Reported cases of primary intraosseous synovial sarcoma with molecular confirmation of the diagnosis.