Validation of a new grid Boltzmann equation

solver, Acuros, for dose calculation

Treatment Planning Systems (TPS)

•Dose Calculation Algorithms used in TPS

•Simple dose calculation- Percentage Depth Dose(PDD), Tissue Air Ratio (TAR)

•Pencil Beam Convolution (PBC)

•Anisotropic Analytic Algorithms (AAA), Collapsed Cone Convolution (CCC)

•Monte Carlo (MC), Acuros XB (AXB)

AAA

• Analytical photon dose calculation algorithm based on a pencil beam convolution technique

• PBC -It is point oriented and thus faster than a full 3-D convolution algorithm and uses the same physical data base to characterize a clinical beam as a full 3-D convolution algorithm. It is shown that photon therapy beams can be characterized with great accuracy from a combination of precalculated Monte Carlo energy deposition kernels and dose distributions measured in a water phantom

Acuros XB algorithms

•Linear Boltzmann Transport Equation Solvers, similar to the classic MC methods, for accurate modeling of dose deposition in heterogenous media.

Linear Blotzmann Transport Equation

Linear Boltzmann Transport Equation

•After many manipulation of the equation through solving boundary equations,using Scalar flux and busbridge theorem of source iteration, we can approximate I

RapidArc Stereotactic Treatments for small fields

•a volumetric arc therapy that delivers 3D dose distribution with a single 360 rotation of LINAC gantry

•Made possible by the rotation speed of the gantry, movement of the MLCs and the delivery dose rate

Impact on RapidArc

•RapidArc plan Calculations repeated for both algorithms, AAA and Acuros XB, for all spot size configurations

•The aim is to see if Acuros XB is comparable to AAA in RapidArc for small fields

Why small fields?• Modelling point of view, photon source is

not a point source, might not be fully visible from the point of measurement, partially occluded by the collimating system

• fields might be small compared to the maximum lateral range of secondary electrons, consequently having a strong dependence of computed and delivery dose on compositio and density of irridiated media

ResultsBest agreement with

measurement for Acuros XB was spot

size 1mm

Comparison between measurements of Acuros XB and AAA for various test fields of

spot size 1mmGreatest dose difference when AXB spot size is 0.0-1.0

mm

Impact on RapidArc

•fixed spot size to 1mm, varying only DLG (dosimetric leaf gap)

•blurring effect observed when lowering DLG

Discussions

•2 main challenges in small field management, measurement and accuracy;implementation of dose calculation in algorithms

•first challenge solved by using equipment with higher sensitivity

•second challenge- by proper modelling of spot size

Reports

• Acuros XB and AAA show acceptable characteristics for stereotactic small fields provided adequate tuning of configuration parameters is performed

• Both algorithms can be considered safely applicable for stereotactic treatments with field sizes greater than 1 to 2 cm per side

• For extremely small targets, jaw settings might be kept above threshold and shelding and modulation generated by MLC only

Further possible research routes

•Instead of using a homogenous slab phantom, use a heterogenous slab phantom ( lung, head & neck) and voxelize the domain for small fields ( Vassiliev et al)

•Comparison of accuracy of AAA and AcurosXB in hetereogenous phantom