RP
Evaluation of the Effectiveness of Analytical Anisotropic Algorithm
in Flattened and Flattening-Filter-Free Beams for High Energy Dose
Delivery Using the Radiological Physics Center Lung Phantom
C
R Repchak1, A Molineu1, R Popple3, S Kry1, R Howell1, D Followill1*
(1) MD Anderson Cancer Center, Houston, TX, (2) University of Alabama at Birmingham, Birmingham, AL,
Introduction
Results
The Radiological Physics Center (RPC) provides QA
services to over 1,800 institutions (~14,000 beams) in the
United States and internationally that consist of:
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The measured-to-predicted dose ratio criteria used by the
RPC to credential institutions is 0.92-1.02, however for this
work, a criteria of 0.95-1.05 was used to compare the
measured results with the calculated doses. In addition, the
gamma index analysis criteria for this work was set to be
±5%/3mm with 90% of the average number of pixels
passing this criteria. All of the target measured-to-predicted
dose ratios fell within the ±5% criterion. In addition, all of
the gamma analyses also met the 90% criterion even for
the highest energies.
Evaluating radiotherapy programs
Developing protocols and QA procedures
Helping to correct institutional deficiencies
Making sure that prescribed radiation doses that
are being delivered are clinically comparable,
accurate and consistent.
The Lung RPC phantom is used to verify dose delivery
from 3D conformal (3D CRT) and intensity-modulated
(IMRT) radiation therapy techniques used in clinical trials.
It contains structures simulating human organs, such as
heart, spine, lungs, and tumor. One of its major purposes
is to evaluate an institution’s ability to deliver
heterogeneity corrected doses. It is becoming increasingly
important to correct for heterogeneities due to dose
escalation and high dose gradients using IMRT or
Stereotactic Body Radiation Therapy (SBRT) for treatment
of lung tumors. The 3D superposition-convolution
techniques that take into account electron disequilibrium
due to increased lateral electron scattering in low density
medium result in a better agreement with measured dose
distributions and should be used instead of those
algorithms that ignore lateral scattering component such
as pencil beam or ray tracing algorithms. While most dose
calculation algorithms work well in a homogeneous
medium, many have been shown to not work well in
heterogeneous medium using higher photon beam
energies and small field sizes.
The hypothesis for this work was that there would NOT be
a difference of greater than ± 5% or 3 mm distance to
agreement on average between SBRT treatments using 6
MV beam and energies greater than 10 MV using
flattened and flattening-filter-free (FFF) photon beams as
calculated with the Eclipse TPS using the Analytical
Anisotropic Algorithm (AAA) and measured with the RPC’s
Lung phantoms.
Fig. 1 The RPC Lung phantom: Fully assembled phantom (top left);
Phantom tumor, heart, and spine inserts (top right); Axial slice of the
phantom CT scan (bottom left); Beam arrangements used for planning in
Eclipse TPS (bottom right).
Dosimetry
 Four TLD capsules (two in the target, one in spine, and
one in heart) and three orthogonal Radiochromic EBT2
films in axial, coronal, and sagittal planes were used to
verify the accuracy of the dose delivered during each
phantom irradiation
Treatment Planning
 The CTV was equal to GTV and the PTV was created by
expanding the CTV by 0.5 cm axially and by 1 cm in
superior-inferior direction
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The structures that represent major organs have
simplified shapes
Hollow body of the phantom is made of PVC and is
filled with water during CT simulation and dose
delivery (L:39cm x W:41cm x H:27/32cm)
The tumor in the shape of rounded cylinder (3cm in
diameter, 5 cm in length) is located in the middle of
the left lung
Care should be taken when considering the RPC limits for
energies higher than 12 MV, since the original RPC criteria
was established based on the statistical analysis of the
large number of irradiations in 6-12 MV range, its validity
has not been verified for energies greater than 12 MV.
There was a divergence in the measured profiles from the
calculated outside the PTV in superior-inferior direction for
all plans and beam energies. This deviation represents an
increased dose to the tissues outside the planned
treatment volume that leads to a higher dose to a normal
lung (by 4-5% of the prescribed dose on average) than
what was predicted by AAA calculation in the Eclipse TPS.
AAA v.8.9.08 heterogeneity correction was applied for
volume dose calculations in all plans
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A single fraction of 6 Gy was prescribed and normalized
to at least 95% of the PTV
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Normal Tissues Constraints
Methods and Materials
The RPC Lung Phantom
 All the structures are made of material with similar
radiological properties to that of the human body
*RPC Gamma index using 5%/5mm around 0.97
Conclusion
The results from both flattened and flatted-filter-free plans
in 6-10 MV range are in a good agreement with the RPC
data and show that AAA algorithm is capable of
calculating treatment plans in a complex heterogeneous
environment consistently and accurately using ±5%/3mm
gamma index and ±5% point dose criteria. This AGREES
with the existing recommendations of only using photon
beams energies of 12 MV for lung treatments.
However, regarding the use of higher photon energies for
lung treatments, neither 15 MV or 18 MV are
recommended to be used in radiation therapy treatments
of lung tumors due to a larger penumbra (Wang et al) and
potential underdose of the tumor (Klein et al) which can
significantly compromise the effectiveness of the radiation
treatment and local tumor control.
Fig. 2 Deviation of measured dose profiles from the calculated in superiorinferior direction
A total of six phantom irradiation plans were developed:
 Two SBRT plans (6 MV and 18 MV) were delivered on
the Varian Clinac 2100CD and 21EX linear accelerators

Fig. 4 Summary for all plans: Measured-to-predicted dose ratios (top); 2D
Gamma index results using ±5%/3mm criteria (bottom)
Four SBRT plans (6 MV, 6 MV FFF, 10 MV FFF, and 15
MV) were delivered on a Varian TrueBeam STx linear
accelerator
The results from 15 MV and 18 MV plans calculated using
AAA delivered to the RPC anthropomorphic lung phantom
do not show a decreased dose to the tumor and
demonstrate a good agreement between the calculated
and delivered doses despite an increased electronic
lateral disequilibrium and a larger penumbra for higher
energies. Our evaluation of the AAA heterogeneity
corrected dose calculations using the RPC lung phantom
DISAGREES with the recommendation to only use 12
MV for lung treatments, specifically for the AAA algorithm.
References
1) L. Wang, E. Yorke, G. Desobry, and C. Chui, Dosimetric advantage of using 6 MV over 15 MV
photons in conformal therapy of lung cancer: Monte Carlo studies in patient geometries, Journal of
Applied Clinical Medical Physics 3 (1), 51-59 (2002)
2) E. Klein, A. Morrison, J. Purdy, M. Graham, and J. Matthews, ‘‘A volumetric study of measurements
and calculations of lung density corrections for 6 and 18 MV photons’’, Int. J. Radiat. Oncol., Biol., Phys.
37, 1163–1170 (1997)
The analysis of the data was performed in a Computational
Environment for Radiotherapy Research (CERR) v.3.3
using software developed specifically for the RPC
Fig. 3 Change in measured-to-predicted dose ratio with the beam energy
This investigation was supported by PHS grants CA10953
and CA81647 awarded by the NCI, DHHS.
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