Analysis of treatment in childhood leukemia. IV. The critical association between dose fractionation and immunosuppression induced by cranial irradiation.

The degree of long term lymphopenia induced by cranial irradiation is shown to be dependent upon the number of fractions into which the standard cranial dose of 2400 rads is divided. The mean lymphocyte count of patients three months after receiving this dose in five fractions was 1.84 X 10(9)/1; in twelve fractions it was 1.12 X 10(9)/1 and in twenty fractions it was 0.64 X 10(9)/1. An explanation for this finding is offered which is based upon marked radiosensitivity of certain lymphocytes and the pattern of recirculation and redistribution of these cells.