Limitations of a pencil beam approach to photon dose calculations in lung tissue

Abstract

A common limitation in treatment planning systems for photon dose calculation is to ignore the impact on electron transport and photon scatter from patient heterogeneities. The heterogeneity correlation is often based on scaling operations along beam rays as for the method according to Batho or the more novel approach of 1D convolutions along beam paths applied in pencil-beam-based systems. The effects of the limitation have been studied in a mediastinum geometry for a wide range of beam qualities by comparing the results from a pencil-beam-based treatment planning system with the results from Monte Carlo calculations. As expected, the deviations within unit-density volumes are small while deviations in low-density volumes increase with increasing beam energy from approximately 3% for 4 MV to 14% for 18 MV x-rays as a result of increased electron disequilibrium.

Verification of dose calculations with a clinical treatment planning system based on a point kernel dose engine

“…et al 7 . has been used for two open beams (6 and 18 MV) where the dose has been calculated using the EGS4 code, including the PRESTA algorithm.…”

“…The pencil‐kernel model has previously shown to have some limitations in heterogeneous media, see Knöös et al 7 . where it was pointed out that scatter from lateral heterogeneities, as well as disregarding the loss of electron equilibrium, may lead to significant errors.…”

“…The scaling of the point kernel with the average electron density between the energy release point and dose deposition point allows a more accurate prediction of dose for a number of clinical situations (the lung region being the most prominent one) where pencil kernels yield large deviations for high‐energy photon beams 7 . The recalculation of data for the geometry used by Knöös et al 7 .…”

“…However, there are still cases where the results from the pencil‐beam algorithm may be outside accepted accuracy levels (see Discussion). The inability of the model to correctly handle changes in scatter from lateral heterogeneities, together with the lack of scaling of the electron transport, can result in significant dosimetric errors in, e.g., the thoracic region 7 . The integration volume for phantom‐scatter calculation is dependent on phantom/patient contour and can, in some cases, give rise to deviations between measured and calculated doses, as has been shown in specific phantom geometries 6 …”

Practical methods for improving dose distributions in Monte Carlo‐based IMRT planning of lung wall‐seated tumors treated with SBRT

“…Instead, the recommendation is to use treatment planning systems which employ alternative dose calculation algorithms, among the most accurate of which are Monte Carlo (MC)‐based 10 , 12 , 13 . These algorithms use underlying physics principles to statistically assess the radiation beam/patient interaction and derive dose distributions which more accurately reflect reality compared to other algorithms, especially in situations with high‐density/low‐density interfaces, such as in the lung 10 , 12 , 15 .…”