Study on LET distribution and dose distribution in heterogeneity for therapeutic carbon ion beams

In order to develop and verify dose averaged linear energy transfer (LET) calculation algorithm for carbon ion radiotherapy, we used a CR-39 solid state track detector as an LET distribution detector. First, depth dose averaged LET distributions in water obtained using the CR-39 for mono-energetic beam and spread-out Bragg peak beam were compared with calculations by treatment planning. We confirmed that the CR-39 could measure the high LET particles which contribute dominantly to dose averaged LET. On the other hand, the CR-39 overestimated in tail region of distal fall-off in depth-dose distributions, since its detection limits for lower LET particles. However, as physical dose in the region is considerably small, effect of this discrepancy on the biological dose distribution turn out to be small. Therefore, we conclude that it is useful for LET distribution measurements in carbon ion radiotherapy to use the CR-39 detector. Then, we would like to modify into more precise carbon ion dose calculation algorithm to be able to predict lateral dose averaged LET distributions. Therefore, lateral dose averaged LET distributions for collimated beams were also measured using the CR-39 detectors and analyzed. (author)