Primitive neuroectodermal tumour arising within low grade astrocytoma: transformation, de novo or radiation induced? Report of three cases and review of literature.

The transformation from low grade to aggressive astrocytoma is well known. However, the development of a completely different tumour such as a primitive neuroectodermal tumour (PNET) within a low grade astrocytoma (LGA) is rare. Only two cases have been reported to date. We present three cases and review the literature. One case was identified at presentation. A subsequent review of our histopathology database revealed two further cases. All three patients had histologically proven low-grade astrocytoma and received radiotherapy following biopsy. The tumour location was infratentorial in one and supratentorial in two. The mean age at presentation with initial tumour was 20 years. Two patients underwent partial resection for recurrence, one at five and the other ten years later with histological confirmation of low-grade astrocytoma. At subsequent recurrence eight and thirty years following original presentation and eleven years later for the third patient, further tumour debulking was performed. Histology now revealed high grade PNET. Cytogenetics showed a complex karyotype with multiple chromosomal abnormalities in all three patients. All patients died within 1 year of final surgery. Among the six reported cases of PNET arising following prophylactic radiation therapy to low grade astrocytomas, only two occurred within the original tumour. Whether these cases represent transformation of low-grade astrocytoma, de novo formation of new tumour or radiation induced neoplasm is uncertain.