Comparative analysis of intensity modulation inverse planning modules of three commercial treatment planning systems applied to head and neck tumour model

Abstract

Background and purpose: Three commercial treatment planning modules for intensity modulated radiation therapy (IMRT) Inverse Planning, MDS-Nordion Helax-TMS, Varian Cadplan-Helios, and CMS Focus, were compared in an attempt to determine potential application limits or dosimetric differences among various optimisation algorithms.

Materials and methods: A comparative analysis of intensity modulated dose distributions was conducted at planning level on a group of four patients presenting advanced head and neck cancers. In the study, we analysed primarily the static 'step and shoot' multileaf implementation of modulation realisation with some investigation, on the Cadplan-Helios implementation of the 'sliding window', the Varian dynamic approach to IMRT delivery. The whole study was carried out using the inverse planning tools implemented by vendors fully optimising each plan to obtain the best dosimetry given some general plan objectives. To achieve adequate target coverage, optimisation was carried out on Helax-TMS and CMS Focus adding extra margins of 5 or 6mm to the planning target volume (PTV). Beam arrangements were set with five and nine equally spaced fields. The study was conducted with two complexity levels. At the first level, dose-volume constraints were applied only to the target volume and to the spinal cord, while parotid glands were added at the second level. The relative values of dose distributions and dose-volume histograms were compared, together with an estimate of the biological implications in terms of Equivalent Uniform Dose to the target. In the Cadplan-Helios system also the dosimetric implications of the number of intensity levels selected for the discretisation of the fluence matrix were investigated.

Results: With the application of common planning strategies and the proper consideration of treatment planning system (TPS) specific features (e.g. the PTV margin problem), no substantial differences among the three algorithms were demonstrated at the first level for PTV and spinal cord. At the second level of the study differences were outlined for Helax-TMS, where sub-optimal results were obtained with the 5-field geometry. Mainly due to the differences in optimisation volumes, Cadplan-Helios presented significant better sparing of healthy tissue around the PTV, in terms of mean dose to healthy tissue and Irradiated Volume at 50% dose level. Finally, to achieve dosimetrically acceptable and stable results on target, a minimum of eight intensity levels should be applied for the multileaf collimator (MLC) segmentation, giving an average of 1.5 segments per field and per intensity level.

Conclusions: Results obtained for the three IMRT TPS show in first instance that the optimisation algorithms analysed, as well as the conversion from computed fluences to multileaf sequences implemented in the planning systems can produce substantially equivalent dose plans (for target coverage and organs at risk sparing) if planning is performed with common strategies and once a strong understanding of each system feature is achieved. Secondly, a limited number of dose levels (about eight) is adequate at planning level.