CRANIAL IRRADIATION IN ACUTE-LEUKEMIA - DOSE ESTIMATE IN THE LENS

Cranial irradiation for subclinical arachnoid infiltration is standard treatment in childhood acute lymphocytic leukemia. The incidental dose received by the ocular lens is of potential importance since these children evidence a significant long-term survival rate. Comparison of the lens dose using 6 MV and 4 MV photon beams and a cobalt unit is presented in terms of ion chamber measurements in a water phantom and thermo-luminescent dosimetry (TLD) measurements in a head phantom. TLD measurements on patients treated for acute lymphocytic leukemia (ALL) are examined and used to estimate the dose to the lens. It is demonstrated that the dose to the lens depends strongly on the choice of field margin and on the daily patient set-up. However, using parallel opposed beams in a clinically determined optimal set-up, the dose to the lens is approximately 20-30% of the midline central axis dose. By angling the treatment head to eliminate the geometrical divergence of the beam, it is possible to reduce the lens dose to approximately 15% of the midline dose. © 1979.