K.K.D.RameshNagarjuna Hospital

Physics aspects of Acuros algorithm

Uncertainty α 1/√ (# Events)

Accuracy impacts quality of treatment Speed heavily effects clinical flow

It was introduced to address 2 strategic needs

• Accuracy• Speed

Here we will discuss about

• Physics aspects of Acuros-XB( AXB) algorithm

• How it is comparable accuracy to Montecarlo simulation for full range of X-ray beams from 4-25 MV especially Lung , bone, air and non biological implants.

• AXB uses sophisticated technique( Numerical methods) to solve LBTE explicitly and directly accounts for the effects of heterogeneities in patient dose calculation.

• MC don’t solve LBTE explicitly, indirectly obtain the solution.

• MC simulates a finite number of particles interacting with medium, errors are random and having statistical noise.

• AXB simulates infinite number of particles ,absence of statistical noise and errors are systematic and are results from discretization of the variables in space, angle and energy.

Larger steps in discretization process result in a faster solution but less accuracy.

AXB calculates 3D patient dose deterministically using 4 components

• Primary photon source model• Scattered photon fluence• Scattered electron fluence• Dose calculation :

1o Photon Source model Scattered photon & e-fluence

• AXB explicitly models the physical interaction of radiation in material and solves the radiation transport problem numerically( Chebyshev-Legendre quadrature).AXB cross section library includes 25 photons & 49 e groups.

• It uses adaptive angular quadrature that varies by both particle type and energy.

Computational grid

Computational grid in AXB is spatially variable, the local element size is adapted and provides a rigorously defined solution at every point in computational domain.

Cut off energy :

It employs a spatial transport cutoff for electron energies <500Kev and for photon energies <1Kev in

the dose grid.

Grid Voxel size: up to 3mm

The choice of grid size is based on dimension of the planning geometry. Smaller grid sizes are normally used for smaller planning target volumes ,for smaller grid size of the dose matrices

becomes larger and total computation times become larger.

The resolution of dose calculation corresponds to the defined grid size, along Z axis AXB automatically sets grid resolution or closest to user defined grid size

Dose Points Calculated on a Grid

• Material assignment:

AXB explicitly model the physical interaction of photon in the material ,requires chemical composition of each material in its computational grid to perform dose calculation.

From CT calibration curve ,Eclipse assigns chemical composition for each voxel from Varian system database with density up to 3g/cc (Bone)for biological & 16 non

biological materials of density up to 8g/cc (Steel).

If the densities are more than 3g/cc for biological & 8g/cc for non biological material requires user assignment of material.

Dose reporting:

Dose to medium in medium(Dm,m):

Material for each voxel in the patient image is assigned according to that voxel CT # ,AXB uses energy dependent response function is based on the material properties of that Voxel

Dose to water in medium(Dw,m):Water based response function

Dose to water in water (Dw,w):Heterogeneity is off

6MV,5X5 cm2water bone lung slab phantom 18MV,5X5cm2 on multi layered slab phantom

From the above graphs e- transport is same but deposition of elec energy is different. This differences highlight the material significance of using actual composition.

Calculation time:

• Plan dose calculation: Steps in AXB

1.Transport of source model fluence in to the patient

2.Calculation of scattered photon fluence in the patient

3.Calcultion of scattered e- fluence in the patient

4. Dose calculation

Calculation timePlan dose calculation: Steps in AXB

1.Transport of source model fluence in to the patient

2.Calculation of scattered photon fluence in the patient

3.Calcultion of scattered e- fluence in the patient

4. Dose calculation

• Calculation time has very weak dependence on the # fields ,since majority of calculation time is spent on calculating scattered photon & e- fluence which are performed only once for all beams in the plan.

• As a result relative calculation speed of AXB increases with increasing # fields in the plan, when a separate AXB calculation is performed for each field scattered calculation phase has to run for every field which significantly increases calculation time.

Validation

AXB Vs MC:

MCNPX (N particle extended) computations run with very large no of particles to create results that were very smooth and w/o statistical uncertainties that may have influence the validation of AXB ,in practice MC results are much less smooth & statistical uncertainties are clearly visible.

Depth dose curves on multilayered slab phantom:

6X,10X10 cm2 (Dm,m)

20X ,10X10cm2(Dm.m)

Depth dose curves & profiles of high density material of 2cm3 inserted on a phantom

Depth dose curves & profiles of high density material of 2cm3 inserted on a phantom

Both codes are in close agreement even in high gradient e- disequilibrium region surrounded the implant

18x,10X10cm2

Depth dose curves on a slab phantom containing air block

2X2 Cm2,6MV, 2x2x10cm3 air block containing water phantom

TLD measurements on Radiological Physics center H & N phantom

Comparing depth dose & dose profiles of photon beams in both homogeneous & heterogeneous multilayered phantom the dose

agreement of AXB to MC for almost voxels is within 2% indicating that AXB was comparable to MC for dose prediction

3D Gamma Indices for 6MV

Ojala et al, done Vmat optimization for pelvic case in which hip is replaced by implantGI is 99.01%

AXB MC

Conclusion

• AXB provides comparable accuracy in treatment planning conditions to MC for full range of X-ray photon beams.AXB balance of both accuracy and computation speed.

Thank you