Origin of neoplastic cells following prophylaxis of radiation‐induced mouse leukaemia

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Abstract
The cytogenetic details of 57 primary radiation‐induced leukaemias, occurring after post‐radiation cell supplementation, are recorded. Over 90% showed the presence of abnormal classes and clones as defined. The incidence of leukaemia (about 70%) following the leukaemogenic schedule of irradiation was lowered by grafts of bone marrow or foetal liver but not abolished. The incidence of leukaemia was also lowered by intravenously supplemented foetal thymus but by no other thymic or lymphoid post‐radiation treatment. Even massive lymphoid grafting after each fraction of radiation and lymph‐node grafting beneath the renal capsule were ineffective. All the cases of leukaemia which did occur, with one possible exception, were in irradiated cells as shown by cytogenetic examination after grafting with cells suitably labelled chromosomally. The finding is in contrast to the frequent occurrence of neoplasia in cell lines from unirradiated thymic grafts after thymectomy and the same fractionated radiation schedule. Although the thymus is necessary for the supervention of this type of radiation‐induced leukaemia, it is not necessary for it to have received irradiation. Thymic cells in association with irradiated bone‐marrow cells colonizing this organ become neoplastic but non‐irradiated bone‐marrow cells colonizing an irradiated thymus are not rendered neoplastic and indeed have an inhibitory effect on the incidence of leukaemia. In four out of 28 (C57BL × CBA.T6T6)F1 leukaemias abnormal chromosomes not distinguishable from the marker were seen drawing attention, together with presumed loss of the marker in another leukaemia, to the frequency of non‐disjunction for this chromosome.