Intensity modulated radiotherapy treatment planning for dynamic multileaf collimator delivery: influence of different parameters on dose distributions

Background and purpose: This is an investigation of a dose-based conjugated gradient optimization method (implemented in the CadPlan/Helios system) applied for head and neck tumours. Optimized field fluence distributions are created and transformed into dynamic multileaf collimator (MLC) movements. The aim was to gain knowledge about the influence of different parameters on the dose distribution and how to use the optimization algorithm in an optimum way. Material and methods: Parameters such as the number of beams, collimator angle and constraints and weight factors have been investigated. Dose escalation to the target volume, the target volume in the build-up region and the way of prescribing the target dose were also investigated. The dose distributions were mainly analysed with physical parameters. Results and conclusions: The optimization algorithm is well suited to create clinical Intensity modulated radiation therapy (IMRT) treatment plans for head and neck tumours even when the target volume is situated in the build-up region. The number of beams is a critical parameter and has a great influence on the dose distribution. The choice of collimator angles is not a critical parameter. The constraints and weight factors have a great influence on the dose distribution and varying these could easily control priorities regarding dose to the target volume or to the surrounding critical organs. Because of dose variations inside the target volume, prescribing to, normalizing to and reporting the mean dose in the target volume for IMRT treatment plans is preferable to the absorbed dose at a point, for example the isocentre point