Intensity modulation in radiotherapy: photons versus protons in the paranasal sinus.

PURPOSE: The purpose of this study is to investigate whether successive tightening of normal tissue constraints on an intensity modulated X-ray therapy plan might be able to improve it to the point of clinical comparability with the corresponding intensity modulated proton therapy plan.
MATERIALS AND METHODS: Photon and proton intensity modulated plans were calculated for a paranasal sinus case using nominal dose constraints. Additional photon plans were then calculated in an effort to match the dose-volume histograms of the critical structures to those of the proton plan.
RESULTS: On reducing the low dose contribution to both orbits in the photon plan by tightening the constraints on these structures, an increased dose heterogeneity across the target resulted. When all critical structures were more strictly constrained, target dose homogeneity and conformity was further compromised. An increased integral dose to the non-critical normal tissues was observed for the photon plans as dose was progressively removed from the critical structures.
CONCLUSIONS: Both modalities were found to provide comparable target volume conformation and sparing of critical structures, when the nominal dose constraints were applied. However, the use of intensity modulated protons provided the only method by which critical structures could be spared at all dose levels, whilst simultaneously providing acceptable dose homogeneity within the target volume.