Intensity modulated photon and proton therapy for the treatment of head and neck tumors.

PURPOSE

A comparative treatment planning study has been performed between intensity modulated photon and proton therapy to investigate the ability of both modalities to spare organs at risk in the head and neck region while keeping target dose homogeneous. Additional advantage of reducing the spot size for IMPT was also investigated. The treatment planning comparison was extended by varying the number of fields to study its effect on the performance of each modality. Risks of secondary cancer induction were also calculated for all modalities.

MATERIALS AND METHODS

Five planning CTs were selected for the study. Four different constraints were set to the organs at risk in order to measure the resulting dose homogeneity in the target volume. Five and nine field plans were made for IMXT and 3, 5 and 9 field plans were made for IMPT, for both spot sizes. Dose homogeneity as a function of the mean parotid dose was visualized using a 'pseudo' Pareto-optimal front approach. Risks of secondary cancer were estimated using the organ equivalent dose model.

RESULTS

Critical organs were best spared using 3-field IMPT and, at least for IMPT, little advantage was seen with increasing field numbers. Reducing the spot size does give an advantage. In contrast, there was a significant advantage in going from 5 to 9 fields for IMXT. Secondary cancer risk was lowest for the IMPT plans with reduced spot size, for which normal tissue received the lowest integral dose. Interestingly, although integral dose remained the same, increasing the number of IMPT fields increased the secondary cancer risk, due to the increased volume of tissue irradiated to low dose.

CONCLUSIONS

IMPT has a better ability to spare organs at risk than IMXT for the same dose homogeneity. It also significantly reduced the estimated risk of secondary cancer induction and the use of small numbers of fields further increased this advantage. Given that target homogeneity and normal tissue sparing were equally good with the 3 field IMPT, there appears to be a clear rationale to deliver small numbers of fields for IMPT.