Radiotherapeutic management of medulloblastoma in a pediatric patient with ataxia telangiectasia.

Ataxia telangiectasia (AT) is a genetic disorder with a predisposition to malignancy. Cells from patients with AT demonstrate an increased sensitivity to ionizing radiation which creates a problem when these patients require treatment for their malignant disease. An eleven-year-old boy with a previous diagnosis of AT was seen in consultation following partial resection of medulloblastoma in the posterior fossa. To estimate how much the conventional radiation dose might have to be reduced, we compared the radiosensitivity of bone marrow myeloid progenitor cells from this patient to that of cells from the marrow of normal individuals, using colony formation in an agar culture assay system as the endpoint (CFU-Cs). Neither radiation dose-survival curve exhibited a shoulder--each displayed an extrapolation number of 0.99. The survival curve of normal cells displayed a steep slope with a D0 of 0.98 Gy (0.83-1.19 Gy, 95% confidence limits); the slope for the AT cells was considerably steeper with a value for D0 of 0.32 Gy (0.29-0.35 Gy). The ratio of D0's indicated that these AT cells were approximately 3X more radiosensitive than normal cells. Based on this, the daily dose was reduced from 1.8 to 0.6 Gy and the radiation was restricted to 25 treatments to the posterior fossa rather than the conventional cranio-spinal treatment. An additional 5 treatments at 1.0 Gy per day were given to the whole brain. The patient's skin responded to these reduced fraction sizes and doses to a similar degree as normal patients' skin following a standard schedule and the patient is doing well nine months after initiation of treatment.