Society for Pediatric Pathology (SPP) Fall Meeting – October 11-October 14, Houston, TX

s are listed in presentation order, beginning with Platform Presentations. 1 TLE1 Immunohistochemistry for Diagnosis of Pediatric Synovial Sarcomas AF Lee, CH Lee, LM Sullivan, O Popescu. University of British Columbia, Vancouver General Hospital, and Children's and Women's Hospital of British Columbia, Vancouver, BC; The Children's Hospital of Philadelphia, Philadelphia, PA Background: Synovial sarcoma (SS), a high-grade sarcoma seen in young adults and adolescents, is characterized by t(X;18)(p11.2;q11.2). SS can be monophasic or biphasic in appearance. The monophasic variant has a histologic differential including a wide range of spindle and small round blue cell neoplasms. TLE1 was found in gene-expression profiling studies to be highly upregulated in SS and initial reports showed it was a sensitive and specific marker for SS. However, a subset of rhabdomyosarcoma (RMS) and peripheral nerve sheath tumors are also immunoreactive. Its expression in pediatric tumors is not fully characterized. We therefore performed TLE1 immunohistochemistry on various pediatric spindle/round cell tumors to address TLE1 immunoreactivity in this patient population. Design: Pathology files at Children's and Women's Hospital of BC, Vancouver General Hospital, and The Children's Hospital of Philadelphia were searched for formalin-fixed, paraffin embedded pediatric tumors to create 3 tissue microarrays containing 24 SS (19 molecularly confirmed), 83 RMS, 21 Ewing sarcoma, 4 infantile fibrosarcoma, 17 desmoid fibromatosis, 6 digital fibromatosis, 9 fibrous hamartoma of infancy, 3 fibromatosis colli, 13 hepatoblastoma, 7 myofibroma/myofibromatosis, 11 neuroblastoma, 5 superficial fibromatosis, and 25 Wilms tumor. Whole mount sections from 5 cases of molecularly confirmed SS were also obtained. All cases and an external positive control (molecularly confirmed adult SS), were stained oncurrently with anti-TLE1 antibody. Immunostaining was scored as 0 (no nuclear staining); 1+ (weak-moderate staining in < 50% of tumor nuclei); 2+ (weak-moderate staining in 50% or greater of tumor nuclei) and 3+ (strong staining in > 10% of tumor nuclei). Cases with 2+ or 3+ TLE1 staining were considered positive. Results: Table 1. Pediatric mesenchymal tumors with positive TLE1 immunoreactivity. Diagnosis 2+ or 3+ Staining 3+ Staining Only