IMRT with the sliding window: comparison of the static and dynamic methods. Dosimetric and spectral analysis

Aim of the study is the critical appraisal of the two delivery techniques for intensity-modulated treatments commonly known as dynamic and static step and shoot, in the framework of a sliding window multileaf sequencing method.

The study was performed using the solution commercialised by Varian with the Eclipse treatment planning system (TPS) and the Clinac accelerator. For a set of intensity modulated fluences, the calculated dose maps and the dose delivered to films were compared for the static and dynamic modes to verify the capability of the TPS to accurately model both the delivery modes. For these investigations, the gamma concept of Low et al. [Low D, Harms W, Mutic S, Purdy J. A technique for the quantitative evaluation of dose distributions. Med Phys 1998;25: 656-60] was applied demonstrating, in general, optimal modeling for both static and dynamic tests. Optimal and actual fluences, were analysed to ascertain the degree of the TPS accuracy in converting 'ideal' maps into realistic leaf motions.

Among the methods used, the Webb's Modulation Index [Webb S. Use of a quantitative index of beam modulation to characterize dose conformality: illustration by a comparison of full beamlet IMRT, few-segment IMRT and conformal unmodulated radiotherapy. Phys Med Biol. 2003;48: 2051-2062] proved to be useful. Also, dose volume histogram analysis was applied on deliverable plans using more clinical tools.

The dynamic delivery method seems to offer a more reliable agreement with the optimal calculations and, clinically, a slightly superior performance in terms of target coverage.