International Journal of Medical Physics, Clinical Engineering and Radiation Oncology, 2013, 2, 6-14
Published Online February 2013 (http://www.scirp.org/journal/ijmpcero)
http://dx.doi.org/10.4236/ijmpcero.2013.21002
Verification and Dosimetric Impact of Acuros XB
Algorithm for Stereotactic Body Radiation Therapy (SBRT)
and RapidArc Planning for Non-Small-Cell Lung Cancer
(NSCLC) Patients
Suresh Rana1,2, Kevin Rogers2, Terry Lee2, Daniel Reed2, Christopher Biggs2
1Department of Medical Physics, ProCure Proton Therapy Center, Oklahoma City, USA
2Department of Radiation Oncology, Arizona Center for Cancer Care, Peoria, USA
Received October 11, 2012; revised November 14, 2012; accepted November 20, 2012
ABSTRACT
Purpose: The experimental verification of the Acuros XB (AXB) algorithm was conducted in a heterogeneous rectan-
gular slab phantom, and compared to the Anisotropic Analytical Algorithm (AAA). The dosimetric impact of the AXB
for stereotactic body radiation therapy (SBRT) and RapidArc planning for 16 non-small-cell lung cancer (NSCLC) pa-
tients was assessed due to the dose recalculation from the AAA to the AXB. Methods: The calculated central axis per-
centage depth doses (PDD) in a heterogeneous slab phantom for an open field size of 3 × 3 cm2 were compared against
the PDD measured by an ionization chamber. For 16 NSCLC patients, the dose-volume parameters from the treatment
plans calculated by the AXB and the AAA were compared using identical jaw settings, leaf positions, and monitor units
(MUs). Results: The results from the heterogeneous slab phantom study showed that the AXB was more accurate than
the AAA; however, the dose underestimation by the AXB (up to −3.9%) and AAA (up to −13.5%) was observed. For a
planning target volume (PTV) in the NSCLC patients, in comparison to the AAA, the AXB predicted lower mean and
minimum doses by average 0.3% and 4.3% respectively, but a higher maximum dose by average 2.3%. The averaged
maximum doses to the heart and spinal cord predicted by the AXB were lower by 1.3% and 2.6% respectively; whereas
the doses to the lungs predicted by the AXB were higher by up to 0.5% compared to the AAA. The percentage of ipsi-
lateral lung volume receiving at least 20 and 5 Gy (V20 and V5 respectively) were higher in the AXB plans than in the
AAA plans by average 1.1% and 2.8% respectively. The AXB plans produced higher target heterogeneity by average
4.5% and lower plan conformity by average 5.8% compared to the AAA plans. Using the AXB, the PTV coverage
(95% of the PTV covered by the 100% of the prescribed dose) was reduced by average 8.2% than using the AAA. The
AXB plans required about 2.3% increment in the number of MUs in order to achieve the same PTV coverage as in the
AAA plans. Conclusion: The AXB is more accurate to use for the dose calculations in SBRT lung plans created with a
RapidArc technique; however, one should also note the reduced PTV coverage due to the dose recalculation from the
AAA to the AXB.
Keywords: Acuros XB; AAA; Heterogeneity Correction; SBRT; RapidArc; Lung Cancer
1. Introduction
Lung cancer is the leading cancer killer in both men and
women in the United States, causing more deaths than
the next three most common cancers combined (colon,
breast and prostate) [1]. According to the American Can-
cer Society’s most recent statistics, an estimated 226,160
new cases of lung cancer were expected to be diagnosed
in 2012, representing almost 14 percent of all cancer dia-
gnoses [1]. Surgical resection is considered to be the
preferred treatment for the early stage non-small-cell
lung cancer (NSCLC). For NSCLC patients who are
not candidates for surgical therapy, stereotactic body
radiotherapy (SBRT) is an alternative method for the
treatment of small lung tumors. SBRT is a highly con-
formal technique that delivers high radiation dose with
few treatment fractions to the tumor while limiting the
doses received by the organs at risk (OARs).
Previous studies have shown that high probability of
tumor control can be achieved with SBRT when com-
pared to the conventional radiation therapy [2,3], with
local control rates at 3 years up to 90% [4]. However,
advanced cancer treatment techniques such as SBRT also
demand for more accurate dose calculation algorithms

S. RANA ET AL. 7
[5]. The volumetric modulated arc therapy (VMAT) in
the form of RapidArc (Varian Medical Systems, Palo
Alto, CA) for SBRT lung case involves the use of small
fields with the presence of air, which causes the elec-
tronic disequilibrium effect near the air/tissue interfaces
as the lateral range of secondary electrons becomes
longer than the width of the small field segments [6,7].
Dutreix et al. [8] reported the strong dependence of the
calculated and delivered dose on the irradiated inhomo-
geneous media when electronic disequilibrium occurs
such as in the case of lung. Thus, when a lung tissue is to
be irradiated, dose calculation algorithms must have tis-
sue heterogeneity corrections that will account accurately
for the electron transport near air/tissue interface.
Some of the most common dose calculation algorithms
implemented in commercially available clinical treatment
planning system (TPS) such as collapsed cone (CC) and
anisotropic analytic algorithm (AAA) have several limi-
tations. The inadequacy of CC [9-11] and AAA [12-15]
to calculate the dose accurately inside heterogeneous
media is well documented. Ideally, clinical usage of
Monte Carlo (MC) simulation would be more accurate
method for dose calculations in heterogeneous media
[16-18]. A new photon dose calculation algorithm called
Acuros XB (AXB) has recently been implemented in the
Eclipse TPS (Varian Medical Systems, Palo Alto, CA).
The AXB utilizes the Linear Boltzmann Transport Equa-
tion (LBTE) and solves numerically that describes the
macroscopic behavior of radiation particles as they travel
through and interact with the matter [19].
Several validation studies on the AXB in inhomoge-
neous media have shown that the results of dose calcula-
tions from the AXB were better than that of AAA when
compared against the MC results [19-23]. Few experi-
ments involving phantom measurements have been done
by comparing the calculated doses of AAA and AXB
against the measured doses in a slab phantom containing
a single air gap [23] and in anthropomorphic phantoms
for head and neck [24] and thorax [23,25] regions. The
results from these studies [23-25] reported the better
agreement between the measurements and AXB than
between the measurements and AAA. Few other studies
have compared the AXB computations with the AAA
using real computed tomography (CT) dataset of patients
for nasopharyngeal [23], breast [26] and lung [27] cases.
These clinical studies concluded that, in comparison to
the AAA, the AXB predicted a lower target coverage and
a lower minimum target dose [23], and the AXB could
improve the dose estimation in the dose plans computed
for the treatment of patients [26,27].
Although the MC studies and the measurements have
already shown that the AXB calculations are clearly su-
perior to the AAA calculations [19-25], data on the rele-
vance of dose calculation differences between the AXB
and AAA in clinical cases are lacking. To our knowledge
no study has been conducted comparing the AXB and
AAA calculations on real patient data of SBRT lung
plans with focus on RapidArc. The main purposes of this
study were to 1) further validate the AXB by comparing
the calculated doses computed by the AXB and the
measured doses by an ionization chamber in a heteroge-
neous rectangular slab phantom containing two air gaps,
and 2) compare the AXB dose calculations with the AAA
calculations on CT data sets of 16 NSCLC patients trea-
ted with SBRT and RapidArc planning.
2. Methods and Materials
2.1. Dose Calculation Algorithms
The AAA and AXB are implemented in the Eclipse TPS
(version 10.0.26). The configuration of AXB was done
by importing the same set of beam data used by the AAA
through beam configuration feature in the Eclipse TPS.
For the AXB, there are two options available to calculate
the dose: 1) dose-to-medium (Dm), and 2) dose-to-water
(Dw). For the Dm calculations, the macroscopic energy
deposition cross-section and atomic density are based on
the material properties of local voxel [19,21] whereas;
energy deposition cross-sections for water are used for
the local media in the case of Dw calculations [19,21].
The option of Dm was selected for all the AXB calcula-
tions in this study. For the AAA, the dose is reported in
Dw mode only since the AAA’s dose results are based on
an electron density scaled water [28]. For more detailed
descriptions on the AAA and AXB, readers are advised
to refer to previous publications [19,28]. The data pre-
sented in this study were taken for a 6 MV photon beam
of Varian Clinac iX accelerator equipped with a Millen-
nium 120 multi-leaf collimator (MLC) (Varian Medical
Systems, Palo Alto, CA).
2.2. Dosimetric Validation of AXB in a
Heterogeneous Rectangular Slab Phantom
Containing Two Air Gaps
2.2.1. C entral Axis De pth Dose Calculatio n
First, a set of rectangular solid-water blocks (30 × 30 cm2)
were scanned using GE LightSpeed CT scanner to de-
termine their average Hounsfield number (HU) for the
dose calculation purpose. Second, a virtual heterogene-
ous rectangular slab phantom (30 × 30 cm2) containing
no cavity was created as a 3D CT structure set in the
Eclipse TPS in order to simulate the experimental set up
(Figure 1). The phantom was defined as the body
structure and assigned with average HU number that was
obtained by scanning the solid-water blocks. The phan-
tom layers consisting of free air were assigned with HU
number of −1000. The central axis depth doses were
computed at points X, A and B (Figure 1) with AAA and

S. RANA ET AL.
8
Figure 1. Schematic diagram of the experimental setup for
the central axis depth dose c o mputations and measureme nts
in a heterogeneous rectangular slab phantom containing
two air gaps. The central axis depth doses were compared
in solid-water medium (third phantom layer) for points A
and B which are located at the distance of 1 and 2 cm from
the air/solid-water interface respectively. The normalization
point is 1.5 cm proximal to the phantom surface and mark-
ed with “X”.
AXB (both include version 10.0.26) for the same number
of MUs using an identical beam set up. The dose com-
putations were done for an open field size of 3 × 3 cm2.
The dose calculation grid was set to 2.5 mm for all the
AAA and AXB calculations.
2.2.2. Central Axis D epth Dose Mea su rement
In order to mimic the virtual phantom created in the
Eclipse TPS, rectangular Styrofoam blocks (2 × 2 cm2, 4
and 6 cm thickness) were placed on both the lateral sides
of the central beam axis and used only as support to
create the air gaps between the solid-water materials (30
× 30 cm2). The solid-water block that was used to house
the Exradin A1 cylindrical ionization chamber (collec-
ting volume: 0.053 cm3) (Standard Imaging, Middletown,
WI) contained single cavity. This solid-water block
(referred as the chamber block to differentiate itself from
other solid-water blocks with no cavity) was designed
such that the center of the chamber will be located under
the center crosshair inscribed on the surface of the
chamber block. For measurements at points of interest (X,
A and B), the crosshair on the surface of the chamber
block was aligned with the light field crosshair. By
keeping an identical field size (3 × 3 cm2), beam para-
meters and geometries that were used for the dose com-
putation by the AAA and AXB in the Eclipse TPS, 100
MUs were delivered to the phantom. The central axis
depth dose measurements were acquired with Exradin A1
cylindrical ionization chamber and the measurement at
each points of interest (X, A and B) was repeated three
times.
2.2.3. Central Axis Depth Dose Comparison
The calculated (AAA and AXB) and measured doses
were converted to the percent depth dose (PDD) by nor-
malizing to their respective central axis dose obtained at
1.5 cm depth and this dose normalization point is marked
with “X” in Figure 1. The calculated (AAA and AXB)
PDDs were then compared against the measured PDDs
for points A and B to evaluate the accuracy of dose pre-
dictions by the AAA and AXB in the presence of air gaps.
The difference, Dm (%) was calculated using Equation
(1).


m
DPDD
AXB or AAAMeasurement100
Measurement





(1)
2.3. Dosimetric Evaluation of AXB for Clinical
SBRT Lung Cases
2.3.1. Pa tients and Con t o uring
Sixteen NSCLC patients were selected for this retrospec-
tive study and these patients were treated at Arizona
Center for Cancer Care with SBRT. The CT scans of all
SBRT patients were acquired with 512 × 512 pixels at
0.25 cm slice spacing on a flat tabletop of GE Light-
Speed CT Scanner. The Digital Imaging and Communi-
cations in Medicine (DI-COM) CT data was electroni-
cally transferred to the Eclipse TPS for contouring and
planning. The following volumes of interest (VOI) were
created in each axial CT slice: 1) planning target volume
(PTV) from a 5 mm wide isotropic expansion of the cli-
nical target volume (CTV), and 2) organs at risk (OARs):
heart, spinal cord, contra- lateral lung (contra-lung), and
ipsilateral lung excluding PTV (ipsi-lung).
2.3.2. Pl anning, Op timization and Dose Calculation
The beam parameters of the original patient treatment
plans in this study were set up in the Eclipse TPS (ver-
sion 10.0.26) and the treatment plans were created using
a RapidArc technique consisting of 2 - 4 partial arcs in a
coplanar field arrangement (Figure 2).
The beam-eye-view graphics in the Eclipse TPS was
used to better arrange the arc angles and define the field
sizes according to the location of the PTV and OARs
with an objective of achieving maximal PTV coverage
and minimal OARs dose. The isocenter of the plans was
placed at the center of the PTV and all the plans were

S. RANA ET AL. 9
Figure 2. A transversal view of RapidArc plan setup in the
Eclipse treatment planning system for lung cancer using
double partial-arc technique .
inversely optimized. The volumetric dose optimization
method followed the same systematic strategy regarding
the objectives and priorities such that at least 95% of the
PTV received the prescription dose of 60 Gy in 5 frac-
tions while keeping the doses to the spinal cord and heart
limited to not more than 18 and 30 Gy respectively. For
the contra-lung, the percentage volume receiving 20 Gy
or more (V20Gy) was restricted to 10%. For the ipsi-lung,
the dose limitation was <32% of the volume receiving 20
Gy or more.
The patient plans were generated by performing final
dose calculations of the optimized plans with the AAA
(version 10.0.26) and these plans were normalized such
that the 100% of the prescribed dose covered the 95% of
the PTV. The resulting final original patient treatment
plans (i.e., plans obtained after dose normalization) were
referred as the AAA plans. Next, for each patient, the
AAA plan was copied and the dose re-calculation was
performed retrospectively with the AXB (version 10.0.26)
using identical jaw settings, MLC leaf positions and MUs
as in the corresponding AAA plan. A second set of treat-
ment plans for all 16 patients resulting from the AXB
dose computation were referred as the AXB plans. Fi-
nally, a third set of treatment plans were created by nor-
malizing the AXB plans such that 100% of the pre-
scribed dose covered the 95% of the PTV and these plans
are referred as the AXB_Norm plans. The dose cal-
culation grid was set to 2.5 mm for all the calculations.
2.3.3. Plan Evaluation
The dose-volume histograms (DVH) of all the calculated
SBRT lung treatment plans (AAA and AXB) were gene-
rated in the Eclipse TPS for the PTV, heart, spinal cord,
contra-lung, and ipsi-lung. For the PTV, the maximum
dose, mean dose, minimum dose, the percentage of PTV
covered by 100% and 90% of the prescribed dose (V100
and V90 respectively), Paddick conformity index, CIPaddick
(defined in Equation (2)) [29], and heterogeneity index
(HI) (defined in Equation (3)) were compared.


Padd
2
P
ick
I
TV
PT
CI IV = (2)
where PI is the volume of the prescription isodose vol-
ume, TV is the target volume, and TVPI is the target
volume within the prescribed isodose volume PI. A per-
fect treatment plan would have TV = PI = TVPI and give
CIPaddick = 1.
1%
99%
D
D
HI=

(3)
where D1% and D99% are doses at 1% and 99% of the
PTV respectively.
For the lungs, the maximum dose, mean dose and the
percentage of lung volume receiving 20 Gy and 5 Gy
(V20 and V5 respectively) were compared. The maxi-
mum dose was evaluated for the heart and spinal cord.
Additionally, the difference in MUs between the AAA
and AXB_Norm plans was evaluated. For the purpose of
comparison, the AAA plans were used as the standard,
and the percent difference of corresponding computed
value (Dc) between the AXB and AAA plans of the same
patient was calculated using Equation (4).
c
AXB AAA
Dx 100
AAA





 (4)
where x is a computed dose-volume parameter or a do-
simetric index in the AXB and AAA plans.
In order to test the observed differences between the
calculated AAA and AXB plans, a statistical analysis
was done using paired two-sided student’s t-test in a Mi-
crosoft Excel spreadsheet. A P-value of less than 0.05
(i.e., P < 0.05) was considered to be statistically signifi-
cant.
3. Results and Discussion
3.1. Dosimetric Validation of AXB in a
Heterogeneous Rectangular Slab Phantom
Containing Two Air Gaps
Table 1 shows the calculated (AAA and AXB) and mea-
sured PDD data at points A and B in a heterogeneous
rectangular slab phantom (Figure 1) for a 6 MV photon
beam and a 3 × 3 cm2 field size. It is seen from Table 1
and Figure 3 that the AXB’s values had better agreement
than the AAA’s values at both the points A and B when
compared against the measured data. Specifically, at
point A, the dose differences for the AXB and AAA were
−3.9% and −9.0% respectively; whereas at point B, the

S. RANA ET AL.
10
Table 1. The measured and calculated (AXB and AAA)
central axis PDD in a heterogeneous rectangular slab phan-
tom (see Figure 1) for an open field size 3 × 3 cm2 (6 MV
photon beam, 100 cm SSD, 100 MUs).
Measured AXB AAA
Points of
Interest PDD (%) PDD (%) Dm (%) PDD (%)Dm (%)
A 71.9 69.1 −3.9 65.5 −9.0
B 71.3 71.0 −0.4 61.7 −13.5
Figure 3. The calculated PDD curves by AAA and AXB in a
heterogeneous rectangular slab phantom (see Figure 1) for
an open field size 3 × 3 cm2. The measured PDD at points X
(1.5 cm depth), A (9 cm depth) and B (10 cm depth) are
provided too. (6 MV photon beam, 100 cm SSD, 100 MUs).
Abbreviations: AAA = Anisotropic Analytical Algorithm,
AXB = Acuros XB Algorithm, SSD = Source to Surface
Distance, MUs = Monitor Units, PDD = Percent Depth
Dose.
dose differences for the AXB and AAA were −0.4% and
−13.5% respectively. As the distance from the air/solid-
water interface increased (i.e., from point A to point B),
the discrepancies between the AXB and measured data
decreased by the value of 3.4%, whereas the opposite
trend was identified for the AAA as the discrepancies
between the AAA and measured data increased by the
value of 4.5%.
The results from the heterogeneous rectangular slab
phantom study showed that the AXB is more accurate
than the AAA when an air gap is involved along the
photon beam path; however, the dose discrepancies were
still observed for the AXB in the region distal to the
air/solid-water interface. A number of researchers found
that the secondary build up region occurred beyond air
cavities in the interface region [13,14,21-23]. The air gap
between the two solid-water materials will cause the re-
duction in scattered radiation reaching the measure- ment
points (A and B) due to a lateral spread of scattered ra-
diation within the air gap. Furthermore, Martens et al.
[31] reported that if the lower attenuation of photon
beams within the low-density medium such as lung (air-
gap in this study) is not considered accurately, the dose
to tissues (solid-water in this study) downstream will be
underestimated. Thus, the dose discrepancies at point A
for the AXB (−3.9%) and AAA (−9.0%) may have been
due to their incorrect estimation of photon attenuation in
the air gap or improper modeling of scattered radiation
contribution to the measurement points in the second
build up and build down regions, especially for a smaller
field size.
Additionally, the media of low and high density cause
electronic disequilibrium near their interface due to im-
balance between the numbers of produced electrons and
absorbed electrons [5,28,30]. The dose underestimation
near the interfaces in the presence air can be attributed to
the incorrect modeling of electronic disequilibrium by
the AXB and AAA. Also, a higher dose discrepancy for
the AAA at point B may be due to AAA’s inability to
model the backscatter in the presence of air gap below
the measurement point. Bush et al. [21] showed that, in
the secondary build up region, the AXB differences with
MC results up to 4.5% and the difference between the
AAA and MC results was up to 13% for a 6 MV photon
beam of field size 10 × 10 cm2. In the same study [21],
for a 5 cm thick water medium placed before the 10 cm
air gap (density = 0.001 g·cm−3), a figure was presented
showing both the AAA and AXB underestimating the
dose beyond the secondary depth of maximum dose
(dmax) when a 6 MV photon beam of field size 4 × 4
cm2 was used.
Furthermore, our findings have shown an agreement
with the previous studies done in the anthropomorphic
phantoms [23,24]. Kan et al. [23] showed that the AAA
differed from the measurements by up to 10%, while the
measured doses matched those of the AXB to within 3%
near air/tissue interfaces in the anthropomorphic thorax
phantom. Han et al. [24] reported that both the AAA and
AXB calculated doses within 5% of the thermolumine-
scent dosimeter (TLD) measurements in the Radiological
Physics Center (RPC) head and neck phantom for both
the intensity modulated radiation therapy (IMRT) and
VMAT plans. That study [24] also showed the better
agreement of the AXB results (0.1% to 3.6%) than that
of the AAA results (0.2% to 4.6%) when compared to the
measurements.
Although the AXB provided better the agreement with
the measurements, the results from Bush et al. [21] and
our heterogeneous rectangular slab phantom study show-
ed that an error in the dose underestimation by the AXB
and AAA could still occur at the region distal from the
air/tissue interface, especially when a photon beam of
smaller field size passes through a large air gap or cavity.
Further experimental verification of AXB must be per-
formed in different clinical situations in order to deter-
mine the limitation of the AXB. For instance, the dose
prediction error may also occur when the photon beam

S. RANA ET AL. 11
passes through a high-density immobilization device pri-
or to entering the patient and then finally reaching the
centrally located tumor in the lung. Future work involves
the clinically relevant measurements to investigate the
dose predictions by the AXB in multi-layer phantoms
containing low and high density media.
3.2. Dosimetric Evaluation of AXB for Clinical
SBRT Lung Cases
Table 2 summarizes the results of the dose-volume pa-
rameters for the PTV (volume range: 10.2 - 21.0 cc),
ipsi-lung, contra-lung, heart and spinal cord, and the va-
lues are averaged over the sixteen analyzed patients.
3.2.1. Dose to PTV
The AXB produced a higher maximum PTV dose by
average 2.3% with a statistical significance (P = 0.00004)
but slightly lower mean PTV dose by average 0.3%
without a statistical significance (P = 0.21053) compared
to the AAA. Similar finding for the mean PTV dose was
reported by Fogliata et al. [27] for large NSCLC cases.
In that study [27], the mean PTV dose was found to be
lower for the AXB (IMRT: 0.4% ± 0.6% and RapidArc:
1.3% ± 0.2%) when the target was in the soft tissue;
however, the mean PTV dose was higher for the AXB
when the target was in the lung tissue (IMRT: 1.2 % ±
0.5% and RapidArc: 0.3% ± 0.2%). Kan et al. [23]
showed that the averaged minimum dose to the PTV pre-
dicted by the AXB was lower by about 4% [23] for lo-
cally persistent nasopharyngeal carcinoma cases treated
with intensity modulated stereotactic radiotherapy (IMSRT).
Similar to that study [23], the results found in our study
indicate that the AXB calculations predicted a lower
minimum PTV dose by average 4.3% with a statistical
significance (P = 0.00004).
3.2.2. D ose C o v er a g e, Conformi t y and Heterogeneity
Ofthe PTV
The V90 to the PTV was slightly lower in the AXB plans
than in the AAA plans with a statistical significance (P =
0.02465) but the difference was not large (average Dc =
0.1%). In comparison to the V100 values in the AAA
plans, the V100 values of the AXB plans were lower by
average 8.2% indicating an inferior PTV coverage from
the AXB calculations, and the difference was statistically
significant (P = 0.00017). Similar to our V100 result,
Kan et al. [23] showed that the AXB provided lower
coverage to the PTV by about 4% than the AAA for na-
sopharyngeal carcinoma treated with IMSRT.
Similar trend was obtained for plan conformity as the
averaged dose conformity index value in the AXB plans
was lower by average 5.8% with a statistical significance
(P = 0.00186) compared to the AAA plans. The AXB
plans showed higher target heterogeneity by average
4.5% and a statistical significance was reached (P =
0.00000).
3.2.3. D ose t o L un gs
The maximum dose to the lungs was slightly higher in
the AXB plans compared to the AAA plans. Specifically,
the difference in the maximum dose to the ipsi-lung and
contra-lung was average up to 0.5% without a statistical
significance (P = 0.05128 for ipsi-lung and P = 0.83808
for contra-lung). The averaged mean dose to both the
lungs in the AAA and AXB plans was comparable with-
out a statistical significance (P = 0.11243 for ipsi-lung
and P = 0.82813 for contra-lung).
The values of V20 and V5 for the ipsi-lung were
higher in the AXB plans by average 1.1% (P = 0.01003)
and 2.8% (P = 0.00000) respectively showing statistical
significances. For the contra-lung, the V20 value was not
achieved for any of the patients, whereas the V5 value
was present for only 9 patients in this study. The avera-
ged V5 values were comparable in the AAA and AXB
plans without a statistical significance (P = 0.38310).
The low doses to the contra-lung in the AAA and AXB
plans were mainly contributed from the exit doses since
the beam entrance through the contra-lung was avoided
as shown in the Figure 2.
3.2.4. Dose to Heart and Spinal Cord
The maximum doses to the heart and spinal cord pre-
dicted by the AXB were lower by average 1.3% and
2.6% respectively. The difference was statistically sig-
nificant for the spinal cord (P = 0.00012) but not for the
heart (P = 0.29230). Because the locations of the heart
and spinal cord were distant from the target and smaller
field sizes were used to cover the target, the maximum
doses to the heart (AXB: 7.5 Gy vs. AAA: 7.6 Gy) and
spinal cord (AXB: 7.6 Gy vs. AAA: 7.8 Gy) were well
below the planning dose limit (maximum heart dose < 30
Gy and maximum spinal cord dose < 18 Gy).
3.2.5. MU Difference
The AXB_Norm plans required a higher number of MUs
in order to cover the 95% of the PTV by the 100% of the
prescribed dose compared to the AAA plans. Specifically,
the number of MUs in the AXB_Norm plans (3934 ± 845)
were higher by about 85 MUs than in the AAA plans
(3849 ± 839) and the difference was statistically signifi-
cant (P = 0.00007).
From the results analysis in this study, it is clear that
the discrepancies occurred between the AAA and AXB,
and the dose prediction errors can be made when an in-
sufficiently accurate dose calculation algorithm is used
for the dose computations of clinical radiation treatment
plans, especially for the NSCLC cases. It is essential that
the dose calculation algorithm accounts the different tis-

S. RANA ET AL.
12
Table 2. Comparisons of dose-volume parameters and dosimetric indices in the AAA and AXB plans.
AAA AXB
(Avg. ± SD) (Avg. ± SD)
P
Max. Dose (Gy) 66.7 ± 1.9 68.2 ± 1.8 0.00004
Mean Dose (Gy) 62.6 ± 0.9 62.4 ± 1.0 0.21053
Min. Dose (Gy) 53.7 ± 6.6 51.3 ± 6.3 0.00004
V100 (%) 95.0 ± 0.0 87.2 ± 6.3 0.00017
V90 (%) 99.88 ± 0.49 99.77 ± 0.51 0.02465
CIPaddick 0.82 ± 0.07 0.77 ± 0.06 0.00186
PTV
HI 1.13 ± 0.07 1.18 ± 0.06 0.00000
Max. Dose (Gy) 64.8 ± 1.8 65.1 ± 2.0 0.05128
Mean Dose (Gy) 4.8 ± 1.3 4.8 ± 1.3 0.11243
V20 (%) 7.1 ± 3.1 7.2 ± 3.2 0.01003
Ipsi-Lung
V5 (%) 23.3 ± 6.1 23.9 ± 6.1 0.00000
Max. Dose (Gy) 6.5 ± 3.3 6.6 ± 3.4 0.83808
Mean Dose (Gy) 0.8 ± 0.4 0.8 ± 0.4 0.82813 Contra-Lung
V5 (%) 0.9 ± 1.3 0.9 ± 1.3 0.38310
Heart Max. Dose (Gy) 7.6 ± 10.8 7.5 ± 10.9 0.29230
Spinal Cord Max. Dose (Gy) 7.8 ± 4.7 7.6 ± 4.6 0.00012
Abbreviations: PTV = Planning Target Volume, Ipsi-lung = Ipsilateral lung excluding PTV, Contra-lung = Contralateral lung, AAA = Anisotropic Analytical
Algorithm, AXB = Acuros XB Algorithm, Avg. = Average, SD = Standard Deviation, Max. = Maximum, Min. = Minimum, CIPaddick = Paddick Conformity
Index, HI = Heterogeneity Index, V100 = Percentage of PTV covered by 100% of the Prescribed Dose, V90 = Percentage of PTV covered by 90% of the Pre-
scribed Dose, V20 = Percentage of lung volume receiving at least 20 Gy, V5 = Percentage of lung volume receiving at least 5 Gy. (The values are averaged
over the 16 analyzed patients. The P-values were obtained from paired two-sided student’s t-test).
sue densities along the beam path as well as model the
lower attenuation of photon beams within the tissue ac-
curately so that the dose overestimation or underestima-
tion due to miscalculation of MUs can be avoided. Fur-
thermore, if lower attenuation of photon beams within
lung tissue is not considered accurately and the effect of
the electronic disequilibrium is not taken into account,
the dose to the tissues downstream will be underesti-
mated [31-34].
While both the AAA and AXB take into account pa-
tient heterogeneities, the dose to the PTV and the PTV
coverage depend on the volume of the PTV, the location
of the target such as in the lung or attached to the chest
wall, the lung volume surrounding the PTV and the
lung/air volume included in the PTV [27]. The clinically
significant reduction in the PTV coverage produced by
the AXB raises the issue whether it is essential to in-
crease the prescribed dose such that PTV coverage in the
AXB plans is same as in the AAA plans. If the AXB is
used instead of AAA for the dose computations of SBRT
lung plans and an equivalent dose coverage is expected
such that 95% of the PTV covered by the 100% pre-
scribed dose, the average number of MUs in the AXB_
Norm plans would be increased by about 2.3%. However,
the increase in the number of MUs in the AXB_Norm
plans is dependent on the individual patient anatomy.
Figure 4 shows the MU difference in percentage be-
tween the AXB_Norm and AAA plans for 16 individual
patients, and the MU difference varies from 0.4% (pa-
tient #14) to 5.3% (patient #9). The effect of tumor size
and its position in the lung on doses to the PTV and its
coverage due to the AXB calculations will be an inte-
resting topic for future studies.
Currently, the RTOG 0813 allows using the AAA for
the dose calculations of SBRT lung plans and the dose
compliance criteria were established based on the super-
position algorithm dose calculations. In the future, we
plan to conduct study on SBRT lung plans to ascertain
whether the dose compliance criteria of RTOG 0813
needs to be adjusted for the AXB dose calculations.
While the AXB was found to be more accurate than the
AAA in this study, further validation studies on the AXB
are warranted to determine its limitations before the cli-
nical implementation.

S. RANA ET AL. 13
Figure 4. The percentage difference in the number of MUs
between the AAA and AXB_Norm plans for 16 patients.
Abbreviations: MU = Monitor Unit, AAA = Anisotropic
Analytical Algorithm, AXB_Norm = Acuros XB Normali-
zed;






AAA 100 AXB_Norm
D MUAAA .
4. Conclusion
The experimental verification study using a heterogene-
ous rectangular slab phantom containing two air gaps
showed that the AXB is more accurate and provides the
better agreement to the measurements than the AAA. For
16 NSCLC patients, in comparison to the AAA, the AXB
predicted lower mean and minimum PTV doses by aver-
age 0.3% and 4.3% respectively but a higher maximum
PTV dose by average 2.3%. The averaged maximum
doses to the heart and spinal cord predicted by the AXB
were lower by 1.3% and 2.6% respectively; whereas
doses to the lungs predicted by the AXB were up to 0.5%
higher compared to the AAA. The values of V20 and V5
for the ipsi-lung were higher in the AXB plans by aver-
age 1.1% and 2.8% respectively. The AXB plans pro-
duced higher target heterogeneity by average 4.5% and
lower plan conformity by average 5.8% compared to the
AAA plans. Furthermore, using the AXB, the V100 of
PTV was reduced by average 8.2% than using the AAA,
and the planning criteria of getting at least 95% of the
PTV receiving 100% of the prescribed dose (V100 ≥ 95)
was not achieved for any of the patients in this study.
Thus, the appropriateness of switching from the AAA to
the AXB under V100 ≥ 95 planning criteria should be
further evaluated since lower dose coverage of the PTV
may decrease the tumor control probability.
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