The Dose Accumulation and the Impact of Deformable Image Registration on Dose Reporting Parameters in a Moving Patient undergoing Proton Radiotherapy

Abstract

Introduction: Deviations of the irradiated object from the state captured during delivery planning have a substantial impact on the delivered dose in proton radiotherapy. The present study evaluates the feasibility of Monte Carlo N-particle tracking code version 6 (MCNP6) based dose engine and methodology to study the impact of deformable image registration on accuracy of the dose reporting quantities.

Materials and methods: A virtual lung tumor was inserted into a pair of single participant computed tomography images depicting opposite breathing phases, with deep inspiration breath-hold the planning reference and exhale the off-reference geometry. A novel MCNP6 dose engine was developed, validated and used in treatment plan optimization. Three deformable image registrations were compared and used to transfer the exhale simulated dose to the reference geometry. Dose conformity and homogeneity measures were evaluated on simulated dose distributions and compared with International Committee on Radioactivity Units and Measurements report number 83 standards.

Results: The MCNP6 dose engine can accommodate patient-like geometries and provides reasonable dose calculation times. All registration methods were able to align image associated landmarks to distances, comparable to voxel sizes. Registration associated dose reporting parameters differed by several tens of percent. D_98% dropped from 0.67 to 0.36 of the target dose and the homogeneity index grew from 0.45 to 0.78 when off-reference irradiation was simulated, violating dose delivery standards.

Conclusions: A combination of the MCNP6 dose engine, the deformable image registration and dose reporting parameters provides quantitative treatment evaluation. Violations can be mitigated using adaptive therapy protocols.