Adamantinoma-like Ewing Sarcoma in a Metatarsal Bone After Chemotherapy Treated with an Osteocutaneous Fibular Transfer: A Case Report.

Background: Adamantinoma-like Ewing sarcoma is a rare variant of Ewing sarcoma known to have cytokeratin expression and squamous differentiation. It occurs more commonly in the head and neck region with only a few cases reported in long bones of the limbs. It also harbours EWSR1-FLI1 gene fusion which is required for the diagnosis of Ewing sarcoma. We present a case of Ewing sarcoma that manifested Adamantinoma-like morphology only post chemotherapy. Chemotherapy has been reported to induce neuronal maturation and rhabdoid morphology in cases of Ewing sarcoma, but no reports of treatment induced squamous differentiation with P40/P63 expression have been demonstrated to date.Case presentation: An 11 year old boy presented with a one year history of an enlarging painless mass over the left first metatarsal. Initial biopsy showed Ewing sarcoma with typical morphological features and EWSR1 rearrangement confirmed on fluorescent in-situ hybridization. The patient underwent neo-adjuvant chemotherapy and a subsequent wide local excision with an ipsilateral pedicled osteocutaneous fibula transfer. Subsequent histological examination showed frank squamous differentiation in the soft tissue component with keratin pearl formation and P40/P63 expression which was not observed in the initial biopsy and is compatible with Adamantinoma-like Ewing sarcoma.Conclusion: This case describes Adamantinoma-like Ewing sarcoma with P40/P63 expression after neo-adjuvant chemotherapy treatment. This immunophenotype was not apparent on the initial biopsy. It remains uncertain as to the reason for this change. This variant carries a poorer clinical outcome compared to the more conventional variant and could pose a diagnostic challenge(s) particularly if it occurs in an older patient or as a metastatic lesion.