METHODS
This study included 10 patients with the nasal cavity and paranasal sinus tumors, and 7-field non-coplanar
IMRT plan and VMAT plans were generated with 6-MV photon beams specially selected for each
patient anatomy. The effects on planning target volume (PTV) and organ-at-risk (OAR) were evaluated
using AXB and AAA in each treatment technique to compare the accuracy of the calculation.
RESULTS
Conformity Index (CI) values for PTV were found to be 1.02±0.02 and 1.03±0.03 for VMATAAA and
VMATAXB plans, respectively and 1.18±0.03 and 1.20±0.02 for IMRTAAA and IMRTAXB plans, respectively.
Regarding heterogeneity index (HI) values, VMATAAA and VMATAXB plans (0.025±0.02; 0.029±0.02)
were found to have better HI values than IMRTAAA and IMRTAXB plans (0.246±0.02; 0.335±0.03). Depending
on the technique and algorithm used, a dose difference of 4%-14% was detected between PTV
Dmin values.
CONCLUSION
The selection of AXB algorithm in treatment regions with high tissue heterogeneity will give more accurate
dose calculation results for PTV and healthy tissues.
Keywords: Anisotropic analytical algorithm; Acuros XB; nasal cavity, radiotherapy for paranasal sinus; integral dose
The integral dose (ID) is the volume integral of the
dose stored in a medium and is equal to the average
dose received by the medium multiplied by its volume.
It is also the area under the differential absolute dosevolume
histogram curve. Published studies suggest
that a large number of beam and monitor units (MU)
used in intensity-modulated radiotherapy (IMRT) may
cause an increase in ID and high-energy photon beams
substantially reduce ID. D"Souza et al. reported that the
change in ID with four or more beams is a function of
the number of beams. High-energy beams reduce the
ID as expected. The reduction rate was reported to be
1.5%-1.7% for the nasopharynx, 0.9%-1.0% and 0.3%
for the pancreas and 0.4% for the prostate. In different
beam-weighted two, four, and eight-field plans, ID
was reported as 1.4%-2.1% for the nasopharynx, 0.2%-
1.3% for the pancreas and 0.5% for the prostate. These
results show that the ID decreases with increasing tumor
size for similar anatomical dimensions, whereas
it increases with the increasing size of the anatomical
region for similar tumor sizes.[
There are chemical structural elements in the human
body, and it is, therefore, a medium with different
density. Air, bone, adipose tissue and lungs measure
about -1000 Hounsfield unit (HU), +1000 HU, -50-100
HU and -500 HU, respectively. The reduction of radiation
in the tissue is calculated with the help of computed
tomography (CT) data and calibration curves
using HU values obtained from CT and tables specific
to predefined density ranges. The accuracy of the algorithms
(dose calculation mechanisms) that can include
tissue composition in determining the dose in each organ
may be different.
In Eclipse? Treatment Planning System version
13.0 (Varian Medical Systems, Palo Alto, CA), the analytical
anisotropic algorithm (AAA) method is widely
used for the calculation of dose distributions.
There are studies in the literature reporting that the
dose calculation made using AAA was significantly inaccurate.
In particular, it has been observed that it calculates
the dose inaccurate when near the two media
during the transition from tissue to air.[
To our knowledge, in the relevant literature, there
are no studies emphasizing the importance of using
calculation algorithms using VMAT and IMRT techniques
for nasal cavity and paranasal sinus tumors. The
most recent study on nasal cavity and paranasal sinus
tumors was conducted by Jeong et al.[
To contribute to the literature, the present study
aims to investigate the effects of the calculation algorithm
on treatment plans made using IMRT and
VMAT techniques in radiotherapy of nasal cavity tumors.
This study investigated the effects of calculation
differences between AXB and AAA algorithms on PTV
and critical organ doses for nasal and paranasal sinus
tumors with large air mass.
Analytical Anisotropic Algorithm
Acuros XB Algorithm
Instead of Boltzman Transport Equation (BTE),
which describes the macroscopic behaviour of radiation
particles, LBTE - its linear form - assumes that
interaction in the environment where radiation particles
penetrate occurs without the particles contacting
with each other in the medium and without an
external magnetic field.[
Although Monte Carlo and LBTE solution methods
provide similar results, they cannot produce clear
solutions and result in errors. Monte Carlo errors are
random and result from that a limited number of particles
are simulated. Systematic errors may occur when
the Monte Carlo method uses precise techniques to
speed up solution time.
The source model of the AXB algorithm used in the
Eclipse TPS uses the existing AAA source model. This
model includes primary photons, out-of-focus photons,
contaminant electrons and scattered photons.
Fogliata et al.[
The AXB algorithm can calculate the dose more
accurately than the AAA using the mass density information
obtained from the CT images in each voxel
for the dose calculation. The calculation difference
between the two algorithms is affected by parameters,
such as the energy of the incoming beam, the field size
and the electron density of the medium.
Treatment Planning of Nasal Cavity and Paranasal
Sinus Tumors
The selected dose calculation algorithm and techniques
were compared. In the IMRT technique, model
IMRTAAA was created for the AAA algorithm, and
model IMRTAXB was created for the AXB algorithm.
Similarly, in the VMAT technique, model VMATAAA
was created for the AAA algorithm, and model
VMATAXB was created for the AXB algorithm.
a- IMRT Planning Technique
b- VMAT Planning Technique
Treatment planning was performed for each patient
using AAA and AXB algorithms. In all planning,
the calculation grid size (CGS) of 1 mm was selected to
reduce the effects of CGS on dose distribution.
Dosimetric Evaluation of the Treatment Plans
Quality Assurance of the Treatment Plans
The AAA dose calculation model is a three-dimensional
(3D) pencil beam and convolution superposition
algorithm consisting of separate models for
primary photons, scattered photons, and electrons
scattered from beam regulating devices (primary collimator,
beam straightening filter, and wedge filter). The
functional forms forming the basic physical quantities
initiate a process by considering the device properties.
This usually results in a significant reduction in the
computational time required for such algorithms. Tissue
heterogeneities are anisotropically taken into account
in the 3-dimensional neighborhood using multiple lateral photon scattering kernels. The final dose
distribution occurs by overlapping the contribution
of photon and electron beams. The AAA algorithm
calculates the dose behind the airspace to some extent
due to an error that arises from modelling the scattered
dose.[
The AXB algorithm was developed for two strategic
needs-accuracy and speed-in external photon beam
treatment planning. Acuros XB uses a complex technique
to solve Linear Boltzmann Transport Equation
(LBTE) and fully exploits patient dose calculation for
heterogeneities due to lung, bone, air and non-biological
implants.[
In this study, CT data with a 2 mm cross-sectional
thickness of 10 patients with the nasal cavity and
paranasal sinus tumors admitted to our clinic were
used. Varian TrueBeam STx using 6 MV beams was
used for treatment planning. Non-coplanar IMRT and
VMAT plans were made through the Eclipse treatment
planning system.
For each patient's anatomy and tumor location, 7-field
non-coplanar treatment areas were selected. Table
angle was chosen as 90° for non-coplanar areas in a
way that the selected treatment areas were not parallel
to each other. A collimator angle of 5-10° was used
to minimize the tongue-and-groove effect created by
treatment areas.
The beam angles were selected as follows: counterclockwise
from 179.90-180.10 with a collimator angle
of 300, a couch angle of 0° and clockwise from 180.10-
179.90 with a collimator angle of 330°, a couch angle
of 0°.
Each treatment plan was evaluated in terms of PTV
and organ-at-risk (OAR) using dose-volume histograms
(DVH) and taking into account the criteria of
the Radiation Therapy Oncology Group (RTOG). In
each treatment planning, 95% of PTV was ensured to
receive at least 50 Gy as the primary dose limitation.
The followings were calculated: PTVD98, which was
considered a low dose zone for PTV, PTV D2, which
was a high dose zone for PTV, minimum dose of PTV
(PTV Dmin), mean dose values received by PTV (PTV
Dmean), and heterogeneity index (HI) and conformality
index (CI) for PTV.
Arc CHECK (Sun Nuclear Corporation, FL-USA)
phantom providing 3D comparison was used for quality
assurance (QA) of the patient treatment plans. Four
different QA plans were prepared for each patient treatment
plan using the IMRT and VMAT techniques. Dose
difference (DD) and distance-to-agreement (DTA) were
selected as 2% and 2 mm in gamma analysis.
When Table
In terms of PTV Dmin doses, the highest difference was observed between the IMRTAAA and VMATAXB plans, which was 14%. The least difference was between the IMRTAAA and VMATAAA plans, which was 4%. This difference was due to the calculation algorithm, not the treatment technique used.
Concerning PTV Dmean doses, the highest difference was found to be between the IMRTAAA and IMRTAXB plans, which was 4%, and the least difference was between the IMRTAAA and VMATAAA plans, which was <1%.
Regarding PTV D2 doses, the highest difference was between the IMRTAAA and VMATAXB plans, which was 6%, and the least difference was between the IMRTAAA and VMATAAA plans, which was <1%.
In terms of PTV D98 doses, the highest difference was between the IMRTAAA and IMRTAXB plans, which was 3%, and the least difference was between the IMRTAAA and VMATAAA plans, which was <1%. There was a difference between VMATAAA and VMATAXB.
When evaluated in terms of left and right optic nerve, there was a significant difference between IMRT and VMAT in all plans (IMRTAAA, IMRTAXB, VMATAAA, VMATAXB).
When evaluated concerning optic chiasm doses, there was a significant difference between IMRT and VMAT in all plans (IMRTAAA, IMRTAXB, VMATAAA, VMATAXB).
Concerning the left eye, there was a significant difference between VMATAXB and IMRTAAA plans (p=0.027), whereas no significant difference was observed between VMATAXB and IMRTAXB plans (p=0.062).
In terms of the right eye, there was a significant difference between VMATAXB and IMRTAAA plans (p=0.039), whereas no significant difference was observed between VMATAXB and IMRTAXB plans (p=0.058).
Left and right lens doses were found to be higher in VMAT technique than in IMRT technique. In both techniques, the AXB algorithm determined a higher dose than AAA. This increase was due to the increase in small doses in the VMAT technique, which leads to some dose increase on critical organs with a small volume.
It can be further seen in Table 1 that there is some increase in the brainstem and spinal cord doses in the VMAT technique.
b- Evaluation of the Quality Assurance of the Patient
Treatment Plans
Four different QA plans were prepared for each patient treatment plan using the IMRT and VMAT techniques.
Dose difference (DD) and distance-to-agreement
(DTA) were selected as 2% and 2 mm in gamma
analysis. Gamma analysis evaluations are shown in
Table
When Table
In the phantom study with 6 MV photon beams by
Bush et al.,[
In our study that investigated the effects of the
AAA algorithm and AXB algorithm on critical organ
doses in breast radiotherapy, the findings showed that
AAA calculated 2%, 2%, 8%, and 4% more dose for the
left lung, heart, contralateral breast, and contralateral
lung, respectively.[
In a study conducted by Padmanaban et al.,[
The most remarkable side of our study was that the
dose of PTV, which started after the air cavity, was calculated
higher with the AAA algorithm. There was a
14% between AAA plans and AXB plans in determining
PTV Dmin dose. A higher dose than should be in
the build-up area between air and tissue was obtained with the AAA algorithm. The higher dose in PTV will
increase the maximum dose effect in the hot dose regions
as a result of the normalization of the plan to the
treatment dose.
The literature review has shown that there are no
studies emphasizing the importance of using calculation
algorithms with the VMAT and IMRT techniques
for nasal cavity and paranasal sinus tumors. The most
recent study on nasal cavity and paranasal sinus tumors
was conducted by Jeong et al. in 2014 in which
the dosimetric results of the VMAT and IMRT techniques
were compared. In this study, the IMRT and
VMAT treatment techniques were compared only in
terms of PTV and critical organ doses.[
Compared to the IMRT technique, the VMAT
technique provides great convenience concerning
optimization. Variable gantry speeds, simultaneous
multi-leaf collimator (MLC) motion, and dose rate
variability allow the dose to be adjusted at the desired
site. However, the VMAT technique shows an increase
in some low dose sites compared to the IMRT technique.
This increase leads to an increase in critical organ
doses, particularly in healthy tissues with a small
volume.[
In daily patient set-ups, two-dimensional (2D) image registration using kV-kV/MV-kV or imageguided radiation therapy (IGRT) methods like three- -dimensional cone beam computerized tomography (CBCT) allow the correction of changes to occur in patient anatomy. The applicability of non-coplanar IMRT plans is more difficult than the VMAT technique. There is no possibility of image acquisition for each treatment area and table angle. Taking these difficulties into consideration, the VMAT technique will be more appropriate for both patient positioning and treatment.
In conclusion, radiotherapy for nasal cavity tumors and the accuracy of dose delivery are quite difficult due to the anatomical structure of the region, where we are pushing critical dose limits for critical organs, and different density tissues. Similar to the different tissue densities within the treatment area, many devices increasing the dosimetric uncertainty due to the patient stabilizing devices also affect the dose in the patient. It should be noted that treatment planning algorithms do not have the ability to accurately calculate the dose during air-to-tissue transitions. The AAA algorithm calculates the dose behind the airspace to some extent due to an error resulting from modelling the scattered dose. It should be further kept in mind that the VMAT technique provides similar and even better results with the IMRT technique regarding HI and CI evaluation. The selection of the AXB algorithm in the VMAT technique is of great importance for the accuracy of the calculation and for evaluating the doses to be received by the critical organs.
Peer-review: Externally peer-reviewed.
Conflict of Interest: None declared.
Financial Support: None declared.
Authorship contributions: Concept - A.Ç., Z.A..; Design - A.Ç., Z.A.; Supervision - A.Ç., Z.A.; Materials - A.Ç., Z.A.; Data collection &/or processing - A.Ç., Z.A.; Analysis and/ or interpretation - A.Ç., Z.A.; Literature search - A.Ç., Z.A.; Writing - A.Ç., Z.A.; Critical review - A.Ç., Z.A.
