Measurements of dose-averaged linear energy transfer distributions in water using CR-39 plastic nuclear track detector for therapeutic carbon ion beams

A CR-39 plastic nuclear track detector was used as a linear energy transfer (LET) detector for carbon ion radiotherapy. We compared dose-averaged LET distributions in water obtained using the CR-39 detector for a monoenergetic beam and spreadout Bragg peak beam by calculations using the one-dimensional heavy-ion transport code used in the current heavy-ion treatment planning. We confirmed that the CR-39 detector could measure the high LET particles that are dominant contributors to dose-averaged LET. On the other hand, the CR-39 result was overestimated in the tail region of the distal edge in depth-dose distributions, due to its detection limit for lower LET particles. However, physical dose in the region is quite small. Namely, the effect of this difference on the biological dose distribution is also small. These results demonstrate that the CR-39 detector is a useful detector for measuring the LET distribution in carbon ion radiotherapy.