A new algorithm for dose calculation in heterogeneous lung phantoms under condition of small field size

Background: Current dose algorithms, such as the collapsed cone convolution algorithm and anisotropic analytical algorithm, are widely used in commercial treatment planning systems. Nevertheless, it is difficult to calculate the dose distribution of heterogeneities for small fields by using these algorithms, because of the electronic disequilibrium. However, contemporary treatment uses small beamlets more and more frequently, such as stereotactic body radiation therapy and intensity-modulated radiation therapy. In this study, a new inhomogeneity method in lung medium for small fields was presented. Materials and Methods: Inhomogeneous lung phantoms for different small fields were established, and different locations and thicknesses of lung media in inhomogeneous phantoms were also considered. The Monte Carlo code EGSnrc was used to calculate the density factor and the percentage depthdose (PDD) distribution of lung phantoms. The PDDs were also calculated with the new algorithm, and then differences in the PDDs were determined. Results: The comparison shows that there is a good agreement between the new algorithm and the Monte Carlo code in different energy. The discrepancies of the three field sizes were less than 3%. With an increase in field size, the discrepancies were less than 1%. Even with changes in the location and thickness of the lung media in inhomogeneous phantoms, the discrepancies were always less than 1%. Conclusion: The comparative results revealed the effectiveness of the new algorithm in calculating depth-dose distribution, under different conditions, and that it can meet the requirements for calculating percentage depth dose distribution.