Peripheral type of primitive neuroectodermal tumour arising from the left orbital floor.

Primitive neuroectodermal tumours (PNETs) are rare tumours that originate from primitive neural crest cells. They are usually found in children below ten years of age. Peripheral PNETs (pPNETs) occur in soft tissues of the body, but have the same genetic changes as Ewing's sarcoma of the bone (now called soft tissue Ewing's sarcoma). They commonly present in the thoracopulmonary region, abdomen, pelvis and the extremities. The head and neck regions may also be involved. Our case demonstrates a PNET in the peripheral tissue arising from the left orbital floor and spreading locally to involve the left maxillofacial region, cheek and gum. The incidence of pPNETs is likely to be under-reported in the literature. Recent diagnostic advances, including cytogenetic and immunohistochemical analysis, have allowed these tumours to be distinguished from other small, poorly differentiated round cell tumours such as rhabdomyosarcoma, lymphoma and poorly differentiated synovial sarcoma.