Response of pig lung to irradiation with accelerated C-12-ions

The response of pig lungs to irradiation with C-12-ions was assessed in two experiments to validate the procedures for heavy ion therapy planning at t he Gesellschaft fur Schwerionenforschung (GSI) and to explore their range o f applicability. In both experiments, the target volume (spread-out Bragg p eak, SOBP) was planned to be a 4 cm long cylinder with a diameter of 4 cm. Doses in the SOBP were prescribed to be equivalent to 5x4 Gy, 5x5.5 Gy and 5x7 Gy of x-rays in the first experiment, and to 5 fractions of 7 Gy and 9 Gy in the second experiment. The lung response in the first experiment was less than expected on the basis of earlier experiments with photons. Pneumo nitis reaction and chronic fibrotic changes were observed outside the presc ribed high-dose region. In the second experiment, the effects were more pro nounced than had been expected on the basis of the first experiment. Change s were most intense in the high-dose region, but were also seen throughout the lung along the beam channel. Moreover, significant skin reactions were observed at the beam entrance site in all animals and - less pronounced - a t the beam exit site in 3 of the 6 animals. In conclusion, the complex irra diation geometry of the pig lung, the changes of body weight between the tw o experiments, and insufficient accounting for a change in the relative bio logical effectiveness (RBE) computation led to substantial deviations of th e observed reactions from expectations, the reasons for which could be iden tified in a subsequent analysis. The less pronounced lung reaction in the f irst experiment was due to an overestimation of RBE in a preliminary versio n of the algorithm for its determination. The extension of the fibrotic rea ction resulted from the smear-out of the high-dose region due to density va riations in tissue structures, respiratory movement, and limited positionin g accuracy. The skin reactions at the entrance port reflect the different t reatment geometry in the two experiments. The one unexplained observation i s the mild skin reaction that was observed in the second experiment at the beam exit site.