[Split-course neutron and photon irradiation of experimental tumors: the importance of the sequence and interval...

In a split course mixed beam experiment we have investigated the influence of the interval and beam-quality on tumour growth delay. Fast growing sarcomas (SaF) in grey CBA-mice were irradiated with different sequences of neutrons (N) and photons (X) varying the length of time interval from 0 up to 300 minutes between two doses. The tumours were treated with isoeffective doses of X-rays and/or neutrons (XX, NN, NX, XN). In order to achieve a homogeneous radiosensitive cell population, tumours were reversibly made hypoxic by clamping ten minutes prior and during irradiation. Tumour growth delay is dependent on the sequence and on the interval. As expected the growth delay in tumours treated with neutrons only was less dependent on the interval than after pure photon irradiation. Beyond an interval of 30 minutes in the mixed beam schedules the one giving neutrons first was more effective than the one giving photons first.