X-irradiation chronosensitivity and circadian rhythmic proliferation in healthy and sarcoma-carrying rats' bone marrow.

Mitotic activity of bone marrow and tumor of sarcoma-bearing rats and of bone marrow of healthy controls is circadian periodic. Whereas the circadian rhythm in mitotic activity of bone marrow is similar for intact and tumor-bearing rats, with a peak occurring shortly after the onset of the dark (activity) span, the circadian rhythm in mitotic activity of tumor has a much smaller amplitude and a different acrophase occurring late in the dark span. This difference in acrophase of mitotic activity of bone marrow and tumor has important implications for scheduling the administration of oncotherapy. For the tumor model examined herein, it suggests the possibility to achieve concomitantly near maximal efficacy and near minimal myelotoxicity, a result awaiting further investigation in humans with different kinds of malignancies.