A convolution-superposition fluence model for the Siemens HD120 multi leaf collimator with application to a 3D VMAT dose engine

Abstract

Purpose: To construct a fast-calculating fluence model for the Siemens HD120 multi leaf collimator (MLC) using convolution-superposition techniques, and to develop a 3D VMAT dose engine using this fluence model. This work offers an alternative to time-consuming open-source Monte Carlo simulations for those developing in-house dose-calculating software for research or clinical needs.
Methods: EPID-acquired images of sweeping-window and sweeping-checker field profiles were used to commission transmission, 2D interleaf leakage, and tongue-and-groove maps specific to the HD120 MLC. These maps, along with a 2D head-scatter model were incorporated into a convolution-superposition algorithm to provide a fluence model for the HD120 MLC. This fluence model was used to develop a 3D VMAT dose engine, where 3D pre-computed 6MV dose kernels (EGSnrc) and a 3D fluence curvature-correction map were incorporated to calculate 3D VMAT doses in a 22 cm diameter cylindrical phantom. Four VMAT patient plans with a large range of PTV sizes (36 cc to 604 cc) were chosen to test the fluence model and dose engine. 
Results: Excellent agreement was observed between the simulated commissioning fields and measured EPID-responses. 2D 2%/2mm gamma analysis yielded a 98.9% pass rate for 1cm, 2cm, and 4cm sweeping-window fields. 2D 2%/2mm gamma analysis for outer/inner MLC leaves yielded 89.1%/77.0% and 95.2%/91.1% pass rates from1cm and 2cm sweeping-checker fields. Mean 3%/3mm gamma analysis showed excellent agreement between our dose engine and Eclipse (Acuros) regardless of PTV size: 98.7% pass rate, with 95.1% pass rate in the high-dose volume. Fluence calculation times were 13.6 seconds per dynamic MLC field and 1.4 minutes/arc for 3D VMAT dose on a standard PC.
Conclusions: A fast-calculating convolution-superposition fluence model has been commissioned for the Siemens HD120 MLC and incorporated into a 3D VMAT dose engine. This work can be used to facilitate the development of fast in-house dose-calculating software.