Evaluation of pan-TRK immunohistochemistry in infantile fibrosarcoma, lipofibromatosis-like neural tumour and histological mimics

Aims: Infantile fibrosarcoma is characterised by intersecting fascicles of spindle cells and ETV6- NTRK3 gene fusion in most cases. Given histological overlap with other spindle-cell tumours, the diagnosis can be challenging and often requires molecular confirmation. A recently developed pan-TRK antibody shows promise for identifying tumours with NTRK fusions. The purpose of this study was to evaluate the potential diagnostic utility of pan-TRK immunohistochemistry for infantile fibrosarcoma. Methods and results: We evaluated whole-tissue sections from 210 cases, including 15 infantile fibrosarcomas; five each lipofibromatosis-like neural tumour and lipofibromatosis; 10 each primitive myxoid mesenchymal tumour of infancy (PMMTI) and low-grade myofibroblastic sarcoma; 15 each fibrous hamartoma of infancy (FHI), myofibroma/myofibromatosis and desmoid-type fibromatosis; and 20 each low-grade fibromyxoid sarcoma, synovial sarcoma, spindle-cell rhabdomyosarcoma, malignant peripheral nerve sheath tumour, librosarcomatous dermatofibrosarcoma protuberans (F-DFSP) and nodular fasciitis. immunohistochemistry was performed using a rabbit monoclonal pan-TRK antibody. Immunoreactivity for pan-TRK was observed in all 15 (100%) infantile fibrosarcomas, including diffuse immunoreactivity (>50% of cells) in 14 (93%) cases. Pan-TRK was positive in all five (100%) lipofibromatosis-like neural tumours. Of the 190 histological mimics, diffuse pan-TRK immunoreactivity was noted in 16 (8%) cases, including five PMMTI, five FHI (highlighting predominantly the primitive myxoid spindle-cell components), three F-DFSP, one low-grade myofibroblastic sarcoma, one myofibroma and one spindle-cell rhabdomyosarcoma. Conclusions: Diffuse pan-TRK immunoreactivity is a highly sensitive but not entirely specific diagnostic marker for infantile fibrosarcoma, and may be helpful in selecting patients for TRK-targeted therapy. As expected, lipofibromatosis-like neural tumours, which harbour NTRK1 fusions, also show diffuse pan-TRK immunoreactivity.