Technical note: TERMA scaling as an effective heterogeneity correction model for convolution-based external-beam photon dose calculations.

Background: In highly heterogeneous medium, such as one with lung tissue or air cavities, the dose in the low-density region or after it, as calculated by the conventional methods based on convolution with an energy-spreading kernel, is usually overestimated in comparison with measurements or more accurate predictions.

Purpose: To correct the overestimation, we propose a method of scaling the total energy released per mass (TERMA).

Methods: The scaling depends on both the density distribution and the effective beam size in the lateral direction.

Results: The corrected convolution method achieved a significantly improved accuracy in both the lung-like tissue and the water-like region after air, compared to the uncorrected method. The TERMA correction only adds about 10% to the overall computational cost.

Conclusions: Due to the improvement in accuracy and the preservation of computational efficiency, the proposed dose calculation method will be valuable for inverse treatment planning.