A treatment-planning comparison of three beam arrangement strategies for stereotactic body radiation therapy for centrally located lung tumors using volumetric-modulated arc therapy
Journal of Radiation Research
A treatment-planning comparison of three beam arrangement strategies for stereotactic body radiation therapy for centrally located lung tumors using volumetric-modulated arc therapy
Kentaro Ishii 2
Wataru Okada 2
Ryo Ogino 2
Kazuki Kubo 2
Shun Kishimoto 2
Ryuta Nakahara 2
Ryu Kawamorita 2
Yoshie Ishii 1
Takuhito Tada 0
Toshifumi Nakajima 2
0 Department of Radiology, Izumi Municipal Hospital , 4-10-10 Futyu-cho, Izumi, 594-0071 , Japan
1 Department of Radiation Oncology, Yodogawa Christian Hospital , 1-7-50 Kunijima, Higashiyodogawa-ku, Osaka, 533-0024 , Japan
2 Department of Radiation Oncology, Tane General Hospital , 1-12-21 Kujo-minami, Nishi-ku, Osaka, 550-0025 , Japan
A B S T R AC T The purpose of this study was to determine appropriate beam arrangement for volumetric-modulated arc therapy (VMAT)-based stereotactic body radiation therapy (SBRT) in the treatment of patients with centrally located lung tumors. Fifteen consecutive patients with centrally located lung tumors treated at our institution were enrolled. For each patient, three VMAT plans were generated using two coplanar partial arcs (CP VMAT), two non-coplanar partial arcs (NCP VMAT), and one coplanar full arc (Full VMAT). All plans were designed to deliver 70 Gy in 10 fractions. Target coverage and sparing of organs at risk (OARs) were compared across techniques. PTV coverage was almost identical for all approaches. The whole lung V10Gy was significantly lower with CP VMAT plans than with NCP VMAT plans, whereas no significant differences in the mean lung dose, V5Gy, V20Gy or V40Gy were observed. Full VMAT increased mean contralateral lung V5Gy by 12.57% and 9.15% when compared with NCP VMAT and CP VMAT, respectively. Although NCP VMAT plans best achieved the dosevolume constraints for mediastinal OARs, the absolute differences in dose were small when compared with CP VMAT. These results suggest that partial-arc VMAT may be preferable to minimize unnecessary exposure to the contralateral lung, and use of NCP VMAT should be considered when the dose-volume constraints are not achieved by CP VMAT. K E Y WO R D S : lung tumor, stereotactic body radiation therapy, volumetric-modulated arc therapy, planning study
I N T R O D U C T I O N
Stereotactic body radiation therapy (SBRT) has proven to be an
effective treatment modality for medically inoperable early-stage
non–small cell lung cancer (NSCLC) and lung metastases [
However, the favorable data on lung SBRT are based mainly on the
treatment of peripheral lesions. The use of SBRT for centrally located
lung tumors still remains controversial due to the potential of severe
toxicity, such as bronchial stenosis, hemoptysis, and fistulas .
Therefore, reducing the radiation dose to organs at risk (OARs) in
the mediastinum and pulmonary hilum is essential when treating
centrally located lung lesions with SBRT.
Volumetric-modulated arc therapy (VMAT) is now widely used
clinically because it can deliver intensity-modulated radiation therapy
(IMRT) quality plans with superior treatment delivery efficiency. As
for lung SBRT, several treatment planning studies have shown that
VMAT achieved superior dose conformity and OAR sparing with a
shorter treatment time than 3D conformal radiotherapy (3D-CRT)
] or conventional IMRT [
]. With these promising results,
there has been an increase in the use of VMAT in lung SBRT. A
patterns-of-care survey for lung SBRT performed in 2012 reported that
47% of radiation oncologists have used rotational IMRT in their
practice in the USA .
The treatment beam arrangements used for lung SBRT are either
coplanar, non-coplanar or a mixture of both. Many institutions use
multiple non-coplanar beams for lung SBRT using 3D-CRT [
A study comparing beam arrangements for 3D-CRT-based lung
SBRT concluded that non-coplanar beams were best when compared
with coplanar beams or arcs (volumes at high doses were equivalent
between all, but volumes at low doses were better with non-coplanar
beams) . VMAT has the ability for non-coplanar arc delivery, but
the current implementations of VMAT have focused on coplanar
4–6, 9, 10, 15–19
]. In addition, the range of arc used for
VMAT-based lung SBRT varies among institutions: some institutions
use full arc [
4, 6, 8, 10, 16, 17
] and others use partial arc to avoid the
contralateral lung [
5, 9, 15, 16, 18
]. However, data for optimal beam
arrangements are limited to date.
This study was undertaken to determine appropriate beam
arrangement for VMAT-based SBRT in the treatment of patients
with centrally located lung tumors that require better OAR sparing
when compared with patients with peripheral lesions.
OARs = organs at risk, MLD = mean lung dose, VnGy = percentage or absolute
structure volume receiving ≥n Gy, D0.1cm3 = maximal dose received by 0.1 cm3 to
M AT E R I A L S A N D M E T H O D S
Patient selection and contouring
This study was approved by the institutional ethics committee, and
written informed consent was obtained from all patients. Computed
tomography (CT) datasets of 15 consecutive patients with centrally
located lung tumors who were treated with VMAT-based SBRT in
our institution between November 2011 and February 2015 were
included in this study. Central location was defined as the area within
2 cm of critical structures, such as the bronchial tree, great vessels,
esophagus, and heart, but without direct involvement of the
mediastinal structures. Those OARs located within 2 cm of the tumor were
defined as immediately adjacent OARs.
Four-dimensional CT images were acquired in the supine position,
with 1.25-mm slice thickness after immobilization with the BodyFix
double vacuum immobilization system (Elekta, Stockholm, Sweden).
Scans were performed using the Varian Real-time Position Management
(RPM) system (Varian Medical Systems, Palo Alto, CA). The gross
tumor volume (GTV) was contoured at the lung window level on all 10
respiratory phase CT datasets. These 10 GTVs were fused to form the
internal target volume (ITV). The planning target volume (PTV) was
generated by adding a 5-mm margin to the ITV in all dimensions to
account for set-up errors. Contouring of OARs, including the whole
lung, esophagus, heart, spinal cord, bronchial tree, and great vessels
(aorta, vena cavae, pulmonary artery, and pulmonary vein), was defined
according to the Radiation Therapy Oncology Group (RTOG)
protocol 0813 [
]. The bronchial tree consisted of the trachea and the
proximal bronchial tree. The contralateral lung was also contoured as a
separate structure. Dose constraints used for OARs are listed in Table 1
and were based primarily on the MD Anderson Cancer Center
experiences using 70 Gy in 10 fractions [
All plans prescribed 70 Gy delivered in 10 daily fractions to the PTV.
They were normalized so that at least 95% of the PTV received the
prescribed dose. Three VMAT plans were generated with two
coplanar partial arcs (CP VMAT), two non-coplanar partial arcs (NCP
VMAT) and one coplanar full arc (Full VMAT). CP VMAT and
NCP VMAT plans consisted of two ipsilateral arcs of 179° with
collimator angles of 30° and 330°, and Full VMAT plans consisted of one
arc of 358° with a collimator angle of 30°. The couch was set at ± 15°
to the straight position for the NCP VMAT plans. The arrangement
of the fields for a representative patient is shown in Fig. 1. Mean field
sizes by jaws in CP VMAT, NCP VMAT, and Full VMAT were
5.7 × 5.4 cm2, 5.6 × 5.5 cm2, and 5.9 × 5.6 cm2, respectively, where
results were reported as the average of the two arcs for CP VMAT
and NCP VMAT. All VMAT plans were developed in the Eclipse v.10.0
treatment planning system using 6-MV photons in a Novalis Tx
treatment unit (Varian Medical Systems, and BrainLAB AG, Feldkirchen,
Germany). The same optimization template was utilized for the three
VMAT techniques (Table 2); however, these objectives were modified
during the optimization process based on the real-time updated dose–
volume histograms (DVHs) of the PTV and OARs. To achieve the
highly steep gradient dose to the PTV, we used the normal tissue
objective (a parameter that limits dose as a function of distance from the outer
border of the PTV) with a priority of 150 and a 0.15 fall-off between the
start dose of 105% and the end dose of 60%. In addition, avoidance
structures were used for optimization to minimize the dose to OARs. The
optimization constraints and priorities of these avoidance structures were
not initially applied and were real-time adjusted during the progression
of optimization. During the optimization process, the primary goal was
to achieve similar PTV coverage for the three VMAT plans, and the
secondary goal was to decrease the OAR doses as much as possible.
Multiple plans for each VMAT technique were generated as we performed
optimization several times, further tightening dose–volume constraints
of the OARs each time, and the most preferable plan was selected for a
fair comparison. The final dose calculation was performed using the
anisotropic analytical algorithm with a 1.25-mm grid resolution.
DVHs for the PTV, whole lung, contralateral lung, esophagus, heart,
spinal cord, bronchial tree, and great vessels were calculated for each
subject. Parameters chosen for comparison were mean dose, D2%,
and D98% for the PTV, where Dn% is the minimal dose delivered to
n% of the PTV. The homogeneity index (HI) and conformation
number (CN) for the PTV were calculated as below [
HI ¼ ðD2%
D98%Þ=Dp × 100%
CN ¼ ðPTV encompassed by 95% isodose=PTVÞ
× ðPTV encompassed by 95% isodose=95% isodose volumeÞ;
where Dp is the prescription dose. For the CN, the first portion is an
assessment of target volume coverage by 95% of the prescription
dose, and the second portion is an assessment of normal tissue
sparing. The CN values range between 0 and 1, with 1 representing
the ideal conformity. Mean lung dose, V5Gy, V10Gy, V20Gy and V40Gy
for the whole lung and V5Gy for the contralateral lung were compared,
where VnGy is the percentage structure volume receiving at least n Gy.
The maximal dose received by 0.1 cm3 (D0.1cm3) of the esophagus,
heart, spinal cord, bronchial tree, and great vessels was also compared.
The number of Monitor Units (MU) needed for each plan was
collected to determine the dose-delivery efficiency of each treatment.
Patients were divided into two groups based on whether all VMAT
techniques met the dose–volume constraints (Group 1) or not
(Group 2). The distance from the PTV to its immediately adjacent
OAR was recorded for each patient.
Dose–volume parameters, HI, CN and MU were compared by
Wilcoxon matched-pair signed-rank tests for non-parametric data. The
Mann–Whitney U test was used to analyze the difference in the
closest distance to the PTV between Groups 1 and 2. Significance
was defined at P < 0.05. All reported P-values are two-tailed.
R E S U LT S
The characteristics of the 15 patients are listed in Table 3. The
comparison of dose distributions is illustrated in Fig. 2 for the example
case shown in Fig. 1 for each of the three VMAT techniques. A
comparison of mean values of the dosimetric parameters of the PTV and
the OARs is provided in Table 4.
The three VMAT techniques provided highly conformal dose
distributions in terms of target coverage. All plans were almost identical
with respect to the resulting dosimetric parameters of the PTV, HI
and CN. The NCP VMAT and CP VMAT plans showed better
sparing of the whole lung than the Full VMAT plans. The whole lung
V10Gy was significantly lower in the CP VMAT plans than in the
NCP VMAT plans, whereas no significant differences were observed
in the mean lung dose, V5Gy, V20Gy or V40Gy. NCP VMAT plans
achieved the best contralateral lung sparing. On the other hand, Full
VMAT increased mean contralateral lung V5Gy by 12.57% and 9.15%
when compared with NCP VMAT and CP VMAT, respectively. NCP
VMAT plans showed better sparing of mediastinal OARs than CP
VMAT plans. Although NCP VMAT plans resulted in significantly
lower D0.1cm3 to the esophagus, spinal cord, aorta, and vena cavae
compared with CP VMAT plans, the absolute differences in mean
dose were small: 1.9 Gy, 1.24 Gy, 2 Gy and 1.68 Gy, respectively. CP
VMAT consistently provided equivocal or favorable mediastinal OAR
sparing when compared with Full VMAT, although the difference was
very small in absolute terms. The mean MUs for CP VMAT, NCP
VMAT and Full VMAT were 1889 ± 197, 1850 ± 216 and 1832 ± 71,
respectively, showing that there were no statistically significant
differences between the three techniques.
All dose–volume constraints were met satisfactorily by the NCP
VMAT plans for all patients. However, four immediately adjacent
OARs in three patients and five immediately adjacent OARs in four
patients failed to meet the dose–volume constraints by CP VMAT
plans and Full VMAT plans, respectively. Table 5 shows a list of the
dosimetric parameters of the immediately adjacent OARs that
PTV = planning target volume.
received more than the threshold value and the distance from the
PTV to each OAR in Group 2 patients. The mean PTV to its closest
OAR distance was significantly shorter in Group 2 than in Group 1
(0.13 cm vs 0.46 cm, P = 0.004). In 10 cases where the distance
between the PTV and closest OAR was 0.25 cm or greater, optimal
OAR sparing was achieved through all VMAT techniques. In cases
where the distance was shorter than 0.25 cm, it was not possible to
achieve optimal OAR sparing through CP VMAT, Full VMAT or
both in four of five cases.
D I S C U S S I O N
VMAT has recently played an important role in SBRT of lung
tumors. In the present study, appropriate beam arrangement for lung
SBRT using VMAT was investigated. Since most OARs are removed
from the high-dose area in the treatment of peripheral lung tumors,
the dose constraints are easily met using 3D-CRT. Thus, there is
controversy as to whether VMAT should be routinely used to treat
peripheral lung tumors [
]. On the other hand, it is more challenging
to generate an acceptable SBRT plan using 3D-CRT for centrally
located lung tumors because of the close proximity of critical
structures, which can be better handled with IMRT or VMAT. Therefore,
15 consecutive patients with centrally located tumors were selected
for this dosimetric study. As for SBRT for centrally located lung
cancer, most institutions have adopted more conservative
fractionation schemes [
21, 24, 25
]. Recently, a report from the MD Anderson
Cancer Center showed that SBRT with 70 Gy in 10 fractions
achieved excellent local control and acceptable toxicity for clinically
challenging cases, including centrally located lesions, compared with
50 Gy in 4 fractions [
]. In the current study, this dose fractionation
schedule, as well as recommended dose–volume constraints, was
followed. However, differing from the recommended dose–volume
constraints, we used D0.1cm3 instead of maximum point dose, because
maximum point doses are very sensitive to many factors such as dose
calculation grid size and algorithm.
Several recent studies have compared VMAT with 3D-CRT for
lung SBRT. In general, VMAT has shown better conformity to the
PTV than 3D-CRT [
]. The increase in the volume of normal
lung that receives a low dose of radiation is a concern associated with
VMAT. However, lung V20Gy/12.5Gy/10Gy/5Gy values were
significantly lower with VMAT than with 3D-CRT . A recent study
indicated that VMAT improved mean lung dose, lung V5Gy and V20Gy
]. For the contralateral lung, Rauschenbach et al. showed a lower
mean dose with VMAT than with 3D-CRT [
], whereas Ong et al.
reported that VMAT led to a small increase in V5Gy [
the results of these studies showed that VMAT outperformed
3DCRT in most dosimetric aspects. When compared with IMRT,
coplanar VMAT achieved treatment plan qualities comparable with the
non-coplanar IMRT technique and better than those of coplanar
Many institutions have reported promising results with SBRT
using VMAT in patients with lung tumors. However, there are very
few data in the literature on dosimetric comparisons between various
beam arrangements. To the best of our knowledge, there is only one
planning study comparing coplanar VMAT with non-coplanar
VMAT in the setting of lung SBRT. Zhang et al. showed that the
dosimetric differences between coplanar and non-coplanar VMAT were
not significant [
]. However, the angle separation of the
non-coplanar arc was uniformly set at 10°. As they discussed, it was presumed
that the dosimetric differences are expected to increase, favoring the
non-coplanar VMAT plans, with a larger angle separation. In the
present study, the couch was offset ± 15°; thus, the angle separation
was set at 30°. This large angle separation may lead to better
mediastinal OAR sparing compared with coplanar VMAT.
All three VMAT techniques produced highly conformal dose
distributions around the PTV. Li et al. suggested that compromised PTV
coverage contributed to local failure after lung SBRT [
respect to PTV coverage, VMAT seems to be an appropriate
technique for lung SBRT.
The use of Full VMAT, compared with CP VMAT or NCP
VMAT, resulted in increased doses to most of the OARs. In
particular, contralateral lung V5Gy was markedly increased with Full VMAT.
Ong et al. reported that contralateral lung V5Gy strongly correlates
with radiation pneumonitis after lung SBRT using VMAT [
addition, Full VMAT offered only a small advantage over CP VMAT
in terms of treatment time, because coplanar beams were used for
both techniques. Therefore, it is conceivable that partial-arc VMAT is
more suitable for lung SBRT to minimize unnecessary dose
deposition to the contralateral lung.
Lung dosimetric parameters in CP VMAT were comparable with
those of NCP VMAT: whole lung V5Gy, V20Gy, V40Gy and mean lung
dose were identical for the two techniques, whereas whole lung V10Gy
was better with CP VMAT, and contralateral lung V5Gy was lower
with NCP VMAT. Optimal OAR sparing was achieved in all cases
by NCP VMAT, but in only 12 cases by CP VMAT. However, the
changes in the quantitative dosimetric differences for mediastinal
OARs were small. Furthermore, NCP VMAT generally requires
more time to deliver treatments than CP VMAT due to the time
required for positioning of the couch. The European Organisation
for Research and Treatment of Cancer (EORTC) recommends
coplanar beam arrangements for lung SBRT, as long as the dose
distribution criteria are met, to keep the treatment time as short as
]. With respect to this study, all CP VMAT plans
fulfilled the dose requirements for OARs in all cases where the
distance between the PTV and closest OAR was 0.25 cm or greater.
Based on the present results, CP VMAT is sufficient for SBRT for
centrally located lung tumors when all dose–volume constraints can
be achieved. NCP VMAT may be more appropriate only in patients
who require better mediastinal OAR sparing of OARs that are very
close to the PTV.
This study had several limitations. In practice, the applicable area
of the non-coplanar beam direction is limited in lung SBRT using
VMAT due to a risk of collision between the couch and the gantry. In
the present study, the angle separation of the non-coplanar arc was
uniformly set at 30°, the maximum angle at our institution. However,
it is likely that the angle separation is smaller depending on the linear
accelerator and immobilization system used. In such cases, the benefit
of using the non-coplanar beam becomes less. Additionally,
evaluating the optimal number of beams is beyond the scope of this study.
However, Chi et al. compared OAR dose parameters between 2 and
8-arc VMAT plans in delivering SBRT for centrally located lung
tumors; they concluded that increasing the number of arcs in VMAT
cannot significantly improve OAR sparing [
In conclusion, VMAT techniques provided highly conformal target
coverage for SBRT for lung tumors. Considering the increase in dose to
the contralateral lung without improvement in mediastinal OAR sparing
with Full VMAT, beams that avoid the contralateral lung may be
preferable. Although NCP VMAT plans best achieved the dose–volume
constraints for mediastinal OARs for centrally located lung tumors, the
absolute differences in doses were small when compared with CP
VMAT. Therefore, we suggest that adoption of NCP VMAT be
considered, in practice, when the dose–volume constraints are not achieved by
CP VMAT due to shorter distances between the PTV and OARs.
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