Validation of fast motion-including dose reconstruction for proton scanning therapy in the liver

This study validates a method of fast motion-including dose reconstruction for proton pencil beam scanning in the liver. The method utilizes a commercial treatment planning system (TPS) and calculates the delivered dose for any translational 3D target motion. Data from ten liver patients previously treated with photon radiotherapy with intrafraction tumour motion monitoring were used. The dose reconstruction method utilises an in-house developed program to incorporate beam's-eye-view tumour motion by shifting each spot in the opposite direction of the tumour and in-depth motion as beam energy changes for each spot. The doses are then calculated on a single CT phase in the TPS. Two aspects of the dose reconstruction were assessed: (1) The accuracy of reconstruction, by comparing dose reconstructions created using 4DCT motion with ground truth doses obtained by calculating phase specific doses in all 4DCT phases and summing up these partial doses. (2) The error caused by assuming 4DCT motion, by comparing reconstructions with 4DCT motion and actual tumour motion. The CTV homogeneity index (HI) and the root-mean-square (rms) dose error for all dose points receiving  >70%, >80% and  >90% of the prescribed dose were calculated. The dose reconstruction resulted in mean (range) absolute CTV HI errors of 1.0% (0.0-3.0)% and rms dose errors of 2.5% (1.0%-5.3%), 2.1% (0.9%-4.5%), and 1.8% (0.7%-3.7%) for  >70%, >80% and  >90% doses, respectively, when compared with the ground truth. The assumption of 4DCT motion resulted in mean (range) absolute CTV HI errors of 5.9% (0.0-15.0)% and rms dose errors of 6.3% (3.9%-12.6%), 5.9% (3.4%-12.5%), and 5.4% (2.6%-12.1%) for  >70%, >80% and  >90% doses, respectively. The investigated method allows tumour dose reconstruction with the actual tumour motion and results in significantly smaller dose errors than those caused by assuming that motion at treatment is identical to the 4DCT motion.