Infantile Fibrosarcoma of the Upper Arm in a Neonate: A Rare Case Report.

Infantile fibrosarcoma (IFS) is a rare soft tissue malignancy typically presenting within the first year of life. It commonly affects the extremities but remains underreported in neonates. This case report aims to contribute to the limited literature on IFS involving the upper limb in neonates. A one-month-old female presented with a gradually enlarging, painless swelling in the right upper arm since birth. Magnetic resonance imaging (MRI) suggested a soft tissue mass without bone involvement. Fine needle aspiration cytology (FNAC) resulted in a probable mesenchymal neoplasm. The mass was surgically excised and sent for histopathological evaluation. The final diagnosis revealed low-grade infantile fibrosarcoma (spinal muscular atrophy [SMA] negative, Ki-67 <2%). At one-year follow-up, the child had no recurrence or functional impairment. Infantile fibrosarcoma, though histologically malignant, has a favorable prognosis with surgical management. Early recognition and complete excision are crucial for achieving excellent long-term outcomes. This case highlights the importance of considering IFS in the differential diagnosis of neonatal limb swellings.