High-grade osteosarcoma arising from a clinically aggressive infantile fibrosarcoma
Infantile fibrosarcoma (IFS) is a soft tissue tumour of intermediate malignant potential that commonly affects the extremities of young children and is characterised by an ETV6::NTRK3 fusion.1 2 Recurrent copy number changes, including the gain of chromosomes 8, 17 and 20, have been reported in IFS,3–5 but pathogenic mutations in other pathways are rare in this tumour type.6 We previously reported on a 5-year-old male patient with IFS of the right gluteal region/pelvis treated with chemotherapy, radiotherapy and TRK inhibition who developed metastatic, progressive disease with multiple acquired mutations, including TP53, SUFU and an NTRK F617L gatekeeper mutation.7 Due to acquired larotrectinib resistance, he was started on repotrectinib (according to phase I/II study NCT04094610 protocol).8 In this follow-up report, we describe the transformation (dedifferentiation) of the patient’s IFS to a high-grade osteosarcoma. The patient initially presented at 5 months of age with a mass on the