Quality control of low dose craniospinal irradiation for low risk medulloblastoma

Between July 1984 and May 1990, 25 children (median age 9 years) classified as low risk medulloblastoma were treated post-operatively with low dose craniospinal irradiation (25 Gy) followed by a boost to the posterior fossa (total dose 55 Gy), with or without chemotherapy. Simulator and portal films were reviewed for all patients. Doses delivered at the mid cerebral and spinal axis, anterior vertebral body, field junctions and posterior fossa were recalculated and correlated with outcome. Three children were excluded because of inadequate data leaving 22 cases for study. The 5-year actuarial survival is 55%. Nine recurrences and one second tumour (astrocytoma) were observed. Sites of recurrence were either in the posterior fossa (5), in the brain (3), in the spinal axis (1), or combined (2). The dose delivered was within +/- 5% of that prescribed to the brain in 21/22 cases, and to the posterior fossa and the anterior surface of the vertebral body in 17/22 cases. The only patient who received less than 95% of the prescribed dose to the brain (23 Gy) failed in the subfrontal area and olfactory plate. One of the five patients who received less than 95% of the prescribed dose to the posterior fossa (50.3 Gy) failed at the primary site. None of the five patients who received less than 95% of the dose to the spinal axis failed. Quality control of radiation treatment showed that failures could not be correlated with incorrect technique.