The use of modified single pencil beam dose kernels to improve IMRT dose calculation accuracy

Intensity modulated radiation therapy (IMRT) is used to deliver highly conformal radiation doses to tumors while sparing nearby sensitive tissues. Discrepancies between calculated and measured dose distributions have been reported for regions of high dose gradients corresponding to complex radiation fluence patterns. For the single pencil beam convolution dose calculation algorithm, the ability to resolve areas of high dose structure is partly related to the shape of the pencil beam dose kernel (similar to how a photon detector's point spread function relates to imaging resolution). Improvements in dose calculation accuracy have been reported when the treatment planning system (TPS) is recommissioned using high-resolution measurement data as input. This study proposes to improve the dose calculation accuracy for IMRT planning by modifying clinical dose kernel shapes already present in the TPS, thus avoiding the need to reacquire higher resolution commissioning data. The in-house optimization program minimizes a cost-function based on a two-dimensional composite dose subtraction/distance-to-agreement (gamma) analysis. The final modified kernel shapes are reintroduced into the treatment planning system and improvements to the dose calcula tion accuracy for complex IMRT dose distributions evaluated. The central kernel value (radius =0 cm) has the largest effect on the dose calculation resolution and is the focus of this study.