A virtual source model for Monte Carlo simulation of helical tomotherapy.

Jiankui Yuan,Quan Chen,Yi Rong

doi:10.1120/jacmp.v16i1.4992

Abstract

The purpose of this study was to present a Monte Carlo (MC) simulation method based on a virtual source, jaw, and MLC model to calculate dose in patient for helical tomotherapy without the need of calculating phase‐space files (PSFs). Current studies on the tomotherapy MC simulation adopt a full MC model, which includes extensive modeling of radiation source, primary and secondary jaws, and multileaf collimator (MLC). In the full MC model, PSFs need to be created at different scoring planes to facilitate the patient dose calculations. In the present work, the virtual source model (VSM) we established was based on the gold standard beam data of a tomotherapy unit, which can be exported from the treatment planning station (TPS). The TPS‐generated sinograms were extracted from the archived patient XML (eXtensible Markup Language) files. The fluence map for the MC sampling was created by incorporating the percentage leaf open time (LOT) with leaf filter, jaw penumbra, and leaf latency contained from sinogram files. The VSM was validated for various geometry setups and clinical situations involving heterogeneous media and delivery quality assurance (DQA) cases. An agreement of <1% was obtained between the measured and simulated results for percent depth doses (PDDs) and open beam profiles for all three jaw settings in the VSM commissioning. The accuracy of the VSM leaf filter model was verified in comparing the measured and simulated results for a Picket Fence pattern. An agreement of <2% was achieved between the presented VSM and a published full MC model for heterogeneous phantoms. For complex clinical head and neck (HN) cases, the VSM‐based MC simulation of DQA plans agreed with the film measurement with 98% of planar dose pixels passing on the 2%/2 mm gamma criteria. For patient treatment plans, results showed comparable dose‐volume histograms (DVHs) for planning target volumes (PTVs) and organs at risk (OARs). Deviations observed in this study were consistent with literature. The VSM‐based MC simulation approach can be feasibly built from the gold standard beam model of a tomotherapy unit. The accuracy of the VSM was validated against measurements in homogeneous media, as well as published full MC model in heterogeneous media.PACS numbers: 87.53.‐j, 87.55.K‐

Highlights

70 Yuan et al.: Virtual source model for tomotherapy involves complex delivery pattern and irregular beam apertures shown as a sinogram
The phase-space files (PSFs) created by full Monte Carlo (MC) simulation contain the most accurate physical description of the radiation beam exited from the linac, provided that the information characterizing every component in the linac head is precise in terms of detailed geometric and material specifications
Electron contamination was excluded from calculation due to its negligible effect shown in the full MC model of tomotherapy.[17]. To account for the difference in penumbra and fluence from different jaw settings (1 cm, 2.5 cm, and 5 cm jaw size), profiles for each individual jaw size were kept as part of the virtual source model (VSM)

Summary

Introduction

70 Yuan et al.: Virtual source model for tomotherapy involves complex delivery pattern and irregular beam apertures shown as a sinogram. The PSFs created by full MC simulation contain the most accurate physical description of the radiation beam exited from the linac, provided that the information characterizing every component in the linac head is precise in terms of detailed geometric and material specifications. Some of the input information required in the MC simulations, such as the incident electron energy spectrum and the focal spot size, may not be precisely modeled, even for those who have access to the proprietary information. This led to possible modeling uncertainties in photon spectrum and focal spot size, which explained the differences in the results reported by the Jeraj and Sterpin studies. The correction of focal spot size from their studies was estimated from 1.4 mm[17] to 1.1 mm,(25) which was still different from the direct measurement.[26]

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Conclusion