Clinical examination of proton pencil beam scanning on a moving anthropomorphic lung phantom

The objective of this study was to examine the use of proton pencil beam scanning for the treatment of moving lung tumors. A single-field uniform dose proton pencil beam scanning (PBS) plan was generated for the standard thorax phantom designed by the Imaging and Radiation Oncology Core (IROC) Houston QA Center. Robust optimization, including range and setup uncertainties as well as volumetric repainting, was used for the plan. Patient-specific quality assurance (QA) measurements were performed using both a water tank and a custom heterogeneous QA phantom. A custom moving phantom was used to find the optimal number of volumetric repainting. Both analytical and Monte Carlo (MC) algorithms were used for dose calculation and their accuracies were compared with actual measurements. A single ionization chamber, a 2-dimensional ionization chamber array, thermoluminescent dosimeters (TLDs), and films were used for dose measurements. The optimal number of volumetric repainting was found to be 4 times in our system. The mean dose overestimations on a moving target by analytical and MC algorithms based on a time-averaged computed tomography (CT) image of the phantom were found to be 4.8% and 2.4%, respectively. The mean gamma indexes for analytical and MC algorithms were 91% and 96%, respectively. The MC dose algorithm calculation was found to have a better agreement with measurements compared with the analytical algorithm. When treating moving lung tumors using proton PBS, the techniques of robust optimization, volumetric repainting, and MC dose calculation were found effective. Extra care needs to be taken when an analytical dose calculation algorithm is used.