Implementation of random set-up errors in Monte Carlo calculated dynamic IMRT treatment plans

Area of Science:

• Medical Physics
• Radiation Oncology
• Computational Dosimetry

Background:

• The fluence-convolution method was previously validated for open-field radiotherapy.
• Its applicability to dynamic intensity-modulated radiotherapy (IMRT) needed confirmation.
• Set-up uncertainties can significantly impact IMRT dose distributions.

Purpose of the Study:

• To confirm the fluence-convolution method's applicability for dynamic IMRT dose calculations.
• To evaluate the impact of random set-up errors on clinical IMRT dose distributions.
• To assess the necessity of incorporating set-up errors in treatment planning.

Main Methods:

• Monte Carlo dose calculations using BEAMnrc and DOSXYZnrc codes.
• Simulation of sliding window IMRT delivery with a dynamic multi-leaf collimator (DMLC) model.
• Benchmarking with extended dose range (EDR) film and simulation of a clinical head and neck IMRT case with 2 mm random set-up errors.

Main Results:

• Calculated and measured relative dose values showed good agreement within statistical uncertainty.
• Incorporating random set-up errors resulted in minor differences in PTV dose-volume histograms, maintaining adequate coverage.
• Slice-by-slice dose distribution comparisons revealed up to 5.6% differences, with altered hot spot position.

Conclusions:

• The fluence-convolution method is valid for dynamic IMRT Monte Carlo dose calculations.
• Accounting for random set-up errors is essential for accurate hot spot identification and positioning in IMRT.
• This method improves the precision of radiotherapy treatment planning by considering patient positioning uncertainties.