The cribriform plate: a sanctuary site for meningeal leukaemia.

Cranial irradiation is an effective prophylactic treatment for subclinical meningeal infiltration in lymphoblastic leukaemia. However, central nervous system (CNS) relapse still occurs in 6-10% of cases overall and in as many as 30% of cases with a poor prognosis in some series. These recurrences may be due in part to inadvertent underdosage of the cribriform plate, which is centrally situated between the orbits and is projected over their upper third in a lateral view. The dose to the adjacent meninges may thus be reduced by shielding of the radiosensitive lenses. This problem is exacerbated if conventional lateral fields centred on the mid-skull are used, because the eyes will not then project over one another. If the field centre is moved to the edge of the orbit, this problem of beam divergence can be overcome. Central axis beam blocking of both lenses is then possible, and in some patients the cribriform plate can be adequately irradiated. However, in most children the geometry of the orbit is such that it is necessary to add an anterior electron beam to ensure homogeneous dosage. These refinements in irradiation technique might prevent meningeal relapse with a lower whole-brain dose and, hence, fewer neuropsychological sequelae. This hypothesis could be tested in a clinical trial.