Photon characteristic line hybrid pencil beam model for lung phantoms under condition of small fields

Abstract:

In order to accurately calculate the three-dimensional dose distribution in a lung phantom photon under a small fied, photon characteristic line hybrid pencil beam model has been proposed. The model is based on the photon characteristic line algorithm to obtain the depth dose for lung phantoms in the reference field. On this basis, the pencil beam kernel of lung phantom is processed by heterogeneous correction, including equivalent depth correction and weighted density correction. Finally, the pencil beam kernel for lung phantom is calculated for obtaining the dose at each point on the central axis for the 6 MV photon beam. The results show that the central axis percentage depth dose obtained by this algorithm is basically consistent with the Monte Carlo simulation results, and the relative error is less than 3% in most cases. Especially in the middle and posterior portion of lung, the average relative error is less than 1%. However, the dose difference between the two methods is obvious at media interfaces and lung anterior portion, and the relative errors decrease with the increase of field size and lung density, with the maximum relative error ranging from 7.8% to 36.9%. Nevertheless, the model can accurately describe the dose distribution in small fields for lung radiotherapy.