Improved radiotherapeutic technique for prophylaxis of the central nervous system in patients with acute lymphoblastic leukaemia

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Abstract
The importance of prophylaxis of the central nervous system (CNS) in the management of acute lymphoblastic leukaemia is well recognized. Radiotherapy reduces the incidence of CNS relapse from 40–60% to less than 10% (Dritschilo et al., 1976; Evans et al., 1970; Hustu et al., 1973; MRC, 1973; Price and Johnson, 1973). Adequate coverage of the whole meningeal surface of the brain is essential and, since the retro-orbital tissues are also a potential site for relapse (Ridgeway et al., 1976; Robb et al., 1979), they should be irradiated to the same dose as the other parts of the CNS. However, adequate irradiation of the retro-orbital tissues may produce a high dose to the lenses of the eyes, introducing a high risk of cataract formation. The ideal CNS prophylactic irradiation therefore includes the retro-orbital tissues but gives a low dose to the lenses of the eyes. This requirement demands a radiation beam with a clearly defined edge and a sharp dose gradient at the edge of the field—a requirement which is found in a number of situations in clinical radiotherapy. In these situations, a half-blocked beam may be the best means of achieving a sharp fall-off in dose. We have used the half-blocked technique in CNS prophylactic irradiation and in retinoblastoma, again to keep the dose to the lens as low as possible, and in the treatment of certain head and neck tumours.