[The mixed fractionated radiotherapy of R1H tumors in the rat using neutrons and photons].

A number of experimental investigations has demonstrated, that tumor growth delay due to a mixed fractionated treatment schedule with photons and neutrons can be prolonged compared to pure photon treatment. That question after the mechanisms remains undissolved. In the presented study of the R1H tumor of the rat 30 fractions using 2 Gy of photons in 39 days are compared to results of mixed fractionated treatment schedules with neutrons and photons. Neither a prolonged net growth delay (NGD) was found nor does the sequence in which neutrons and photons are given influence the outcome when replacing photons by neutrons in twelve of 30 fractions (RBE 2.5). In contrast, hypofractionated schedules (five fractions/total dose 40 Gy/four days) have a considerable longer NGD when replacing two fractions of photons by neutrons (RBE 2.5) (p < 0.01). This could be explained if 8 Gy photons/fraction destroyed all euoxic tumor cells, a 24 hour interval was too short for reoxygenation and thus make hypoxia to a major problem in 8 Gy/fraction photon treatment.