Acuros white paper

Question 1

accuracy compared toMC

Answer 1

comparable

Question 2

speed of acuros

Answer 2

calculates multiple fields as fast as a single field, so faster than AAA for VMAT plans

the majority
of the calculation time is spent calculating the scattered
photon and electron fluence, which is performed once for all
fields in the plan

Question 3

LBTE vs BTE

Answer 3

r. The LBTE
is the linearized form of the BTE, which assumes that
radiation particles only interact with the matter they are
passing through, and not with each other, and is valid for
conditions without external magnetic ields

Question 4

4 discrete steps in acuros

Answer 4

1. Transport of source model fluence into the patient
2. Calculation of scattered photon fluence in the patient
3. Calculation of scattered electron fluence in the patient
4. Dose calculation

In Steps 2 and 3, Acuros XB discretizes in space, angle, and
energy, and iteratively solves the LBTE.
In Step 4, the dose in any voxel of the problem is obtained
through applying an energy dependent fluence-to-dose
response function to the local energy dependent electron
fluence in that voxel (this is why acuros needs mass density info)
Unlike convolution/superposition
algorithms, where heterogeneities are generally handled as
density-based corrections applied to dose kernels calculated
in water, Acuros XB explicitly models the physical interaction
of radiation with matter

Question 5

what does this highlight?

Answer 5

These diferences highlight the signiicance of using
the actual material composition as opposed to scaling the
density of water material.

Question 6

how does acuros convert from Dm to Dw, versus MC?

Answer 6

acuros does energy deposition ratios whereas MC does stopping power ratios

Question 7

electron cutoff energy in Acuros

Answer 7

500 keV

Electrons
passing below this energy are assumed to dump all of their
energy in the voxel in which they are located

Question 8

plan dose calculation in acuros

Answer 8

In Acuros XB, the calculation time has a very weak
dependence on the number of ields, since the majority
of the calculation time is spent calculating the scattered
photon and electron luence, which is performed once for all
ields in the plan. When a separate Acuros XB calculation is
performed for each ield, the scatter calculation phase has
to run for every ield, which increases the calculation time.
Since ield weights cannot be edited when plan dose calculation is selected, this option is well suited for rapidly calculating intensity-modulated radiation therapy (IMRT) and Varian
RapidArc® radiotherapy technology plans. However, in 3D
conformal planning where ield weights may be individually
changed during optimization, plan dose calculation would
generally be turned of.

Question 9

does acuros convert HU to mass or electron density?

Answer 9

mass density

Question 10

acuros calculation time vs AAA

Answer 10

Calculations of a single or few fields are longer with Acuros
XB than AAA. For a 10 x 10 cm2
6 MVf ield on a 30 x 30 x 30
cm3
water phantom, Acuros XB will calculate the dose on a
2.5 mm voxel grid in about 51 seconds (Dell T5600 with dual
six-core Xeon 2.00 GHz processors and 32 GB DDR3 RAM).
AAA will require approximately 5 seconds for a similar case.
For a 5 x 5 cm2
field on the same phantom, Acuros XB will
require about 24 seconds. Larger fields and higher energies
take longer to calculate, as do phantoms containing large
amounts of bone. Most of the Acuros XB calculation time is
in solving for the scattered photon and electron fluencies,
which are performed only once for all beams in the plan. As
a result, the relative calculation speed of Acuros XB increases
with increasing numbers of fields in the plan. For cases with
larger numbers of fields, i.e., RapidArc, Acuros XB exploits
spatial adaption to speed up calculations in low dose, low
gradient regions.

It should be noted that when increased processing resources
are available and utilized, AAA calculation times will decrease given that the calculation times scale linearly with the
number of fields. Therefore, the more resources available, as
with a FAS environment, the greater the decrease in calculation time. Ultimately, this may result in shorter calculation
times for AAA compared to Acuros XB, even for plans with
a large number of ields, i.e., the number of control points/
fields in a RapidArc plan. The Monte Carlo equivalent accuracy achieved with Acuros XB should be taken in consideration when encountering this scenario.