Comparison of pencil-beam and Monte Carlo calculated dose distributions for proton therapy of skull-base and para-spinal tumors

Abstract

Purpose: Pencil beam algorithms rely on kernels to model proton range in density-scaled water equivalent material. Monte Carlo dose calculation methods are more accurate by design. This study addresses the issue of clinical significance of potential differences between a commercial pencil-beam algorithm and Monte Carlo dose calculation. Skull-base or paraspinal tumors are challenging for dose calculations due to interfaces between high and low density areas in the irradiated volume. In addition, in particular for para-spinal cases, there are often metallic implants that not only distort the CT image but also affect the accuracy of dose calculations.

Methods: Proton treatment planning is done at our institution by using FOCUS/XiO (Computerized Medical Systems Inc.). To do full Monte Carlo based dose calculation the treatment head was modeled including a simulation of the modulator wheel rotation as well as aperture and compensator. The capability to import patient CT data was implemented in the Monte Carlo code. The patient CT data information was converted into materials with explicit element composition and density. All secondary particles were tracked. Treatment plans designed with the treatment planning program were recalculated with the Monte Carlo code and compared by analyzing dose distributions and dose volume histograms.

Results: The agreement between our Monte Carlo dose calculation and pencil-beam algorithm is generally quite good. For some tumors in the head and neck region and near the spine, Monte Carlo based dose calculation can show significant differences compared to a pencil-beam algorithm.