Multiple malignancies in a child with de novo TP53 mutation. The authors present the case of an 8.5-year-old boy diagnosed with embryonal rhabdomyosarcoma of the infratemporal fossa at 21 months old. Five years later he developed an osteosarcoma of the tibia, for which he was treated with standard chemotherapy and resection. Soon after completing this treatment, ptosis of the right eye developed. A mass in the cavernous sinus was found, interpreted as an osteogenic metastatsis, although a third primary tumor could not be ruled out. Molecular genetic investigation revealed a de novo germline TP53 mutation, and heralded an aggressive clinical progression of multiple malignancies in a child.