Effect of linear accelerator settings on evaluation of dosimetric verification of VMAT plans

Souhrn

Background and purpose:
Comparison of the measured and the calculated 2D dose distributions in the planning system is one of the standard methods of VMAT (volumetric-modulated arc therapy) plans verification in radiotherapy. The aim of this paper is to assess the ability of the QA (quality assurance) method used at the University Hospital Olomouc to detect significant errors in the linear accelerator settings and to estimate clinical impact of the undetected changes.

Materials and methods:
To study this issue we chose a method based on introducing simulated errors in gantry and MLC (multi-leaf collimator) settings into 10 clinical VMAT plans of the prostate and the head and neck patients. The ability of the verification system to recognize changes in prescribed setting is - for the purpose of this paper - characterized by a maximum error, in which the two dose distributions are evaluated as sufficiently identical within the parameters of gamma analysis. VMAT plans with the maximum undetected error were recalculated back into the real patient anatomy in order to assess the potential clinical impact.

Results:
The verification method including 3%/3mm gamma analysis criteria together with Octavius II system is able to detect errors larger than 2mm in MLC and 3° in gantry settings in the head and neck VMAT plans and 3mm and 5° for prostate VMATs. In comparison with the original plan, dose recalculations with these errors in the settings back into the real patients showed differences in dose distributions.

Conclusion:
Comparison of dose volume histograms for the original plan and the plans recalculated with implemented errors indicate that this system of verification of VMAT plans could cover up clinically relevant errors.

Keywords:
Volumetric modulated arc therapy (VMAT), Octavius QA system, gamma analysis