Abstract
Leukocyte depression is frequently observed during a course of localized radiotherapy. This depression often appears inordinate for the amount of bone marrow within the irradiated field. A possible mechanism is suggested by studies demonstrating the presence of stem cells in the peripheral blood. Swift et al. (2) demonstrated that rats irradiated with partial-body shielding, then receiving radiation 10 minutes later to the shielded portion, survived better than animals receiving wholebody irradiation. This suggests that circulating precursor cells were fortuitously shielded during both radiation courses. Cells capable of incorporating tritiated thymidine and therefore presumably able to synthesize DNA are found in the peripheral blood (3). These cells decrease after whole-body irradiation (4). Stored blood treated with phytohemagglutinin demonstrates cells with mitotic figures (5). Goodman and Hodgson (6) have shown that animals receiving whole-body irradiation can be protected by the peripheral blood of unirradiated donors and have identified the new marrow cells as descendants of the donor cells. It has also been shown that the spleens of irradiated mice can be colonized by elements of the peripheral blood of unirradiated donors (7). It appears, therefore, that cells capable of repopulating the marrow do circulate in the peripheral blood. Localized radiation therapy is often administered in daily fractions for an extended time. If stem cells circulate in ample quantities, a sufficient number could enter the irradiated field and be destroyed, and leukopenia could result even when the major marrow sites are not in the field. In order to test this hypothesis, animals were subjected to partial-body irradiation. One group was given the dose of radiation over a short time period; the second received the same dose of radiation in a much longer exposure. It might be anticipated that the usual dose-rate effect would occur, the high-intensity radiation being
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