Multisource modeling of flattening filter free (FFF) beam and the optimization of model parameters.

PURPOSE With the introduction of flattening filter free (FFF) linear accelerators to radiation oncology, new analytical source models for a FFF beam applicable to current treatment planning systems is needed. In this work, a multisource model for the FFF beam and the optimization of involved model parameters were designed. METHODS The model is based on a previous three source model proposed by Yang et al. ["A three-source model for the calculation of head scatter factors," Med. Phys. 29, 2024-2033 (2002)]. An off axis ratio (OAR) of photon fluence was introduced to the primary source term to generate cone shaped profiles. The parameters of the source model were determined from measured head scatter factors using a line search optimization technique. The OAR of the photon fluence was determined from a measured dose profile of a 40 x 40 cm2 field size with the same optimization technique, but a new method to acquire gradient terms for OARs was developed to enhance the speed of the optimization process. The improved model was validated with measured dose profiles from 3 x 3 to 40 x 40 cm2 field sizes at 6 and 10 MV from a TrueBeam STx linear accelerator. Furthermore, planar dose distributions for clinically used radiation fields were also calculated and compared to measurements using a 2D array detector using the gamma index method. RESULTS All dose values for the calculated profiles agreed with the measured dose profiles within 0.5% at 6 and 10 MV beams, except for some low dose regions for larger field sizes. A slight overestimation was seen in the lower penumbra region near the field edge for the large field sizes by 1%-4%. The planar dose calculations showed comparable passing rates (> 98%) when the criterion of the gamma index method was selected to be 3%/3 mm. CONCLUSIONS The developed source model showed good agreements between measured and calculated dose distributions. The model is easily applicable to any other linear accelerator using FFF beams as the required data include only the measured PDD, dose profiles, and output factors for various field sizes, which are easily acquired during conventional beam commissioning process.

[1]  U Titt,et al.  A flattening filter free photon treatment concept evaluation with Monte Carlo. , 2006, Medical physics.

[2]  P. Bloch,et al.  Extraction of the photon spectra from measured beam parameters. , 1998, Medical physics.

[3]  Bruce A. Faddegon,et al.  Simulation of large x-ray fields using independently measured source and geometry details. , 2009 .

[4]  Bo Lu,et al.  Comparison of analytic source models for head scatter factor calculation and planar dose calculation for IMRT , 2008, Physics in medicine and biology.

[5]  Timothy C Zhu,et al.  Head scatter off-axis for megavoltage x rays. , 2003, Medical physics.

[6]  Benoît Ozell,et al.  Fast convolution-superposition dose calculation on graphics hardware. , 2009, Medical physics.

[7]  Lei Xing,et al.  Incorporating leaf transmission and head scatter corrections into step-and-shoot leaf sequences for IMRT. , 2003, International journal of radiation oncology, biology, physics.

[8]  D A Jaffray,et al.  Extrafocal radiation: a unified approach to the prediction of beam penumbra and output factors for megavoltage x-ray beams. , 1995, Medical physics.

[9]  Tae-Suk Suh,et al.  Inverse planning for IMRT with nonuniform beam profiles using total-variation regularization (TVR). , 2011, Medical physics.

[10]  Lei Xing,et al.  A three-source model for the calculation of head scatter factors. , 2002, Medical physics.

[11]  M. Alber,et al.  A virtual photon source model of an Elekta linear accelerator with integrated mini MLC for Monte Carlo based IMRT dose calculation , 2007, Physics in medicine and biology.

[12]  Radhe Mohan,et al.  Dosimetric properties of photon beams from a flattening filter free clinical accelerator , 2005, Physics in medicine and biology.

[13]  J. Chu,et al.  Determination of penumbral widths from ion chamber measurements. , 2005, Medical physics.

[14]  Jianrong Dai,et al.  Delivery time comparison for intensity-modulated radiation therapy with/without flattening filter: a planning study. , 2004, Physics in medicine and biology.

[15]  P A Jursinic,et al.  Clinical implementation of a two-component x-ray source model for calculation of head-scatter factors. , 1997, Medical physics.

[16]  Radhe Mohan,et al.  Monte Carlo study of photon fields from a flattening filter-free clinical accelerator. , 2006, Medical physics.

[17]  J J Battista,et al.  Generation of photon energy deposition kernels using the EGS Monte Carlo code. , 1988, Physics in medicine and biology.

[18]  J. Dempsey,et al.  Assessment of the setup dependence of detector response functions for mega-voltage linear accelerators. , 2010, Medical physics.

[19]  Robert Jeraj,et al.  Radiation characteristics of helical tomotherapy. , 2004, Medical physics.

[20]  Jason Cashmore,et al.  The characterization of unflattened photon beams from a 6 MV linear accelerator , 2008, Physics in medicine and biology.

[21]  Weiguo Lu,et al.  Accurate convolution/superposition for multi-resolution dose calculation using cumulative tabulated kernels , 2005, Physics in medicine and biology.

[22]  D. Low,et al.  A technique for the quantitative evaluation of dose distributions. , 1998, Medical physics.

[23]  Benoît Ozell,et al.  A convolution-superposition dose calculation engine for GPUs. , 2010, Medical physics.

[24]  Lisa C. Siskind,et al.  Deconvolution of detector size effect for small field measurement. , 1995, Medical physics.

[25]  Aldo Badano,et al.  Accelerating Monte Carlo simulations of photon transport in a voxelized geometry using a massively parallel graphics processing unit. , 2009, Medical physics.

[26]  J. Alakuijala,et al.  Determination of parameters for a multiple-source model of megavoltage photon beams using optimization methods , 2007, Physics in medicine and biology.

[27]  N. Sahoo,et al.  Semi-empirical procedures for correcting detector size effect on clinical MV x-ray beam profilesa). , 2008, Medical physics.

[28]  Radhe Mohan,et al.  Properties of unflattened photon beams shaped by a multileaf collimator. , 2006, Medical physics.