Verification of beam delivery using fibrosis after proton beam irradiation to the lung tumor.

BACKGROUND AND PURPOSE To investigate the geometrical accuracy in proton beam therapy (PBT) to the lung by comparing the location of the fibrosis after PBT and the tumor using less invasive method. PATIENTS AND METHODS We examined 50 lung tumors that had been treated by respiratory-gated PBT (33-74 Gray equivalents). Image registration and re-slicing of computed tomography (CT) before and after PBT were performed using an image analysis software system. We investigated whether the location of fibrosis was accurate with the tumor site and which factor among the respiratory rhythm, tumor volume, location, fixation, beam direction and clinical outcome was affected with accuracy. RESULTS The area of fibrosis was accurate with the tumor in 45 tumors (Group A) and not accurate in 5 tumors (Group B). All lesions with a small irregularity of respiratory rhythm showed fibrosis in an area accurate with the tumor. This percentage (100%) was higher than that of the lesions with a large irregularity of respiratory rhythm (67%, p = 0.037). Radiation pneumonitis was found for 1 lesion in group A and 2 lesions in Group B (p = 0.0009). CONCLUSION The geometrical accuracy of beam delivery to the lung was higher in the patients with a regular respiratory rhythm in PBT. The patients of Group B had greater risk of pneumonitis.