Improving the therapeutic ratio of craniospinal irradiation in medulloblastoma.

Radiation therapy delivered to the entire cerebrospinal axis is indicated for a number of pediatric brain tumors, especially medulloblastoma. Improved radiotherapy techniques have changed the near fatal prognosis for children with medulloblastoma to a 50%, 5-year survival. Nevertheless, the treatment results in substantial acute toxicity, and many survivors have serious sequelae. Further improvement in survival with optimal surgery and radiotherapy is not expected unless chemotherapy is added. Refinements in radiotherapy technique, however, can improve the therapeutic ratio of the treatment by lowering its side effects. In the last year children who required craniospinal irradiation at M. D. Anderson Hospital were treated with 6 MV photons to the brain and primary tumor and with 15-17 MeV electrons to the spinal canal. The elective dose to the whole brain was 30 Gy in 17 fractions and 30 Gy in 20 fractions to the spine. The primary tumor received an additional 20-25 Gy. An electron-beam dose distribution was drawn on a computerized tomography (CT) reconstructed sagittal plane. The electron energy was selected so that the 90% isodose line was at least 3 mm anterior to the cord after correction for bone heterogeneity. The treatment was well tolerated in the first five patients. It is projected that the current technique will cause fewer late effects and improve the tolerance to chemotherapy.