Further Observations on Inhibition of Lymphoid Tumor Development by Shielding and Partialbody Irradiation of Mice2

Abstract

Three related expts. are reported that confirm and extend previous evidence that local irradiation is ineffective in the induction of mouse lymphoid tumors and that an indirect systemic mechanism must therefore be implicated. One group of mice was given 4 doses of 300 r each at 2-week intervals over the upper half of the body only; a 2d group was given the same dose alternately over the upper and lower halves of the body; a 3d group received 4 similarly timed treatments of 150 r each over the whole body; a 4th group was treated like the 3d except that the mediastinal area was shielded with lead; and a 5th group was kept as control. The incidence of lymphoid tumors after 16 months was, respectively, 8, 8, 52, 0, and 8%. In a 2d expt. fractionated alternate irradiation of the upper and lower halves of the body at intervals of 0, 1, 6, and 24 hrs. yielded lymphoid tumors in 69, 74, 70, and 53% of the respective groups. This response was almost identical with that of mice receiving equivalent whole-body irradiation. Only 1 of 56 mice receiving alternate half-body treatments at 4-day intervals developed a lymphoma. The 3d expt. was intended, in part, to study the effect of preserving ovarian function. The ovaries were transplanted to the muscle of the right hind leg in groups I and II, and left intact in group III. During irradiation, mice of group I were unshielded while those of groups II and III had a lead shield placed over the right hind leg. The incidence of lymphomata to date has been 44, 10, and 10%,respectively. The significance of these results is discussed and a tentative working hypothesis formulated.