Robust treatment planning for stereotactic lung treatment

Abstract

In radiotherapy, there are several uncertainties caused by positioning reproducibility, treatment planning, dose calculation and internal and external organ motion. Therefore, safety margins, such as the planning target volume (PTV) or internal target volume (ITV) are conventionally used to ensure the target receives the prescribed dose. As an alternative to this margin concept, robust planning directly includes various uncertainties into the optimization problem.For a sample set of ten patients, robust optimization was explored with systematic errors in patient position of 4 mm and organ motion uncertainty (four-dimensional Computed Tomography) in volumetric modulated arc therapy (VMAT) plans for stereotactic lung treatments in high energy photon external beam therapy. These patients showed different tumour sizes, tumour locations and tumour motion amplitudes. For every patient, five different treatment plans were calculated on the average CT and compared with each other: the clinical plan (three-dimensional conformal radiotherapy), VMAT (PTV based) and three robust optimized VMAT plans. In the different treatment plans the constraints were defined in a way to achieve the same quality as with the clinical plans. In all robust optimized plans, the gross tumour volume (GTV) was set as robust. Results showed that the higher the tumour motion, calculated via the ratio of the GTV/ITV, the more the results fluctuate. Also, the robust optimized treatment plans were more robust against motion uncertainties compared to the clinical plans, as trend-lines deviated up to almost one percent.Robust planning needs further research to evaluate the organs at risk (OAR) inall phases and to consider the different robust optimization settings (inter-fractionand intra-fraction).