To gate or not to gate - dosimetric evaluation comparing Gated vs. ITV-based methodologies in stereotactic ablative body radiotherapy (SABR) treatment of lung cancer
Kim et al. Radiation Oncology
To gate or not to gate - dosimetric evaluation comparing Gated vs. ITV-based methodologies in stereotactic ablative body radiotherapy (SABR) treatment of lung cancer
Joshua Kim 0
Qixue Wu 0
Bo Zhao 0
Ning Wen 0
Munther Ajlouni 0
Benjamin Movsas 0
Indrin J. Chetty 0
0 Department of Radiation Oncology, Henry Ford Health System , 2799 W. Grand Blvd, Detroit, MI 48202 , USA
Background: To compare retrospectively generated gated plans to conventional internal target volume (ITV)-based plans and to evaluate whether gated radiotherapy provides clinically relevant dosimetric improvements to organs-at-risk (OARs). Methods: Evaluation was performed of 150 stereotactic ablative radiotherapy treatment plans delivered to 128 early-stage (T1-T3 (<5 cm)) NSCLC patients. To generate gated plans, original ITV-based plans were re-optimized and re-calculated on the end-exhale phase and using gated planning target volumes (PTV). Gated and ITV-based plans were produced for 3 × 18 Gy and 4 × 12 Gy fractionation regimens. Dose differences between gated and ITV-based plans were analyzed as a function of both three-dimensional motion and tumor volume. OARs were analyzed using RTOG and AAPM dose constraints. Results: Differences between gated and ITV-based plans for all OAR indices were largest for the 3 × 18 Gy regimen. For this regimen, MLD differences calculated by subtracting the gated values from the ITV-based values (ITV vs. Gated) were 0.10 ± 0.56 Gy for peripheral island (N = 57), 0.16 ± 0.64 Gy for peripheral lung-wall seated (N = 57), and 0.10 ± 0. 64 Gy for central tumors (N = 36). Variations in V20 were similarly low, with the greatest differences occurring in peripheral tumors (0.20 ± 1.17 %). Additionally, average differences (in 2Gy-equivalence) between ITV and gated lung indices fell well below clinical tolerance values for all fractionation regimens, with no clinically meaningful differences observed from the 4 × 12 Gy regimen and rarely for the 3 × 18 Gy regimen (<2 % of cases). Dosimetric differences between gated and ITV-based methods did generally increase with increasing tumor motion and decreasing tumor volume. Dose to ribs and bronchial tree were slightly higher in gated plans compared to ITV-based plans and slightly lower for esophagus, heart, spinal cord, and trachea. Conclusions: Analysis of 150 SABR-based lung cancer treatment plans did not show a substantial benefit for the gating regimen when compared to ITV-based treatment plans. Small benefits were observed only for the largest tumor motion (exceeding 2 cm) and the high dose treatment regimen (3 × 18 Gy), though these benefits did not appear to be clinically relevant.
SABR; Gating; ITV-based planning; Treatment planning
Stereotactic ablative radiotherapy (SABR), also called
stereotactic body radiation therapy, is a radiotherapy
treatment method for delivering high dose in few
fractions (~1–5) [
]. It is essential that the high doses
used in SABR treatments be highly conformal to the
tumor volume and delivered with high accuracy. For
lung cancer, SABR has been used as the primary
treatment in prospective trials for medically inoperable, early
stage non-small-cell lung cancers (NSCLC) [
] and has
provided much greater local tumor control after three
years relative to conventionally fractionated radiotherapy
(~90 % [
] vs. <55 % ) leading to a higher overall
3-year survival rate (~56 % [
] vs. 20–40 % [
Due to respiratory motion of the tumor, immobilization
and motion management for lung cancer is important for
accurately defining and treating the tumor. The
immobilization devices used are somewhat dependent on
the motion management strategy used. They come in
many forms including cradles that hold a polyurethane
foam that conforms to the patient’s body as it cures [
cushions that conform to the patient’s contours and
become rigid as air is evacuated (e.g. BodyFIX®) [
abdominal compression plates that constrict abdominal
motion . Several types of strategies have been
introduced to mitigate the effects of motion. These techniques
include: motion encompassing methods, respiratory
gating, breath-hold control, forced compression approaches,
and tumor-tracking [
]. Many of these methods employ
four-dimensional computed tomography (4DCT), where
the patients’ respiratory waveforms are monitored during
very low pitch helical CT acquisitions [
waveforms are then used to retrospectively sort the
projections into a predetermined number of phases of the
breathing cycle, and the binned projections are used to
reconstruct a CT of the patient at each phase. These phase
images can be further used to generate derived images
such as the average, maximum intensity, and minimum
intensity image sets. One type of motion-encompassing
method involves the use of 4DCT phase and derived
images to define the full range of motion during respiration
and to include that full range within an internal target
volume (ITV) [
] (or alternatively using a mid-ventilation
approach with a MidV volume [
]), which is then
expanded to the planning target volume (PTV) for
treatment planning. Treatment planning is then typically
performed on the average CT. Respiratory gated
radiotherapy involves monitoring an internal [
] (e.g. implanted
fiducial markers) or external [
] (e.g. abdominal surface
motion) surrogate for the tumor. The beam is activated
only when the surrogate lies within some predefined gating
window on the waveform. This enables the use of the
reconstructed image of only a single phase of the waveform
and the use of smaller treatment planning margins that are
expected to allow for reduced dose delivered to
surrounding organs-at-risk (OARs). In tumor tracking modalities,
real-time image guidance is employed to dynamically
modify the beam delivery so that the radiation delivered
tracks with the motion of the target. This can be
accomplished through moving the linac itself through use of a
robotic arm [
] or using the multi-leaf collimators (MLCs)
to track with the tumor [
]. While the previous methods
allow for the patient to breath normally, breath-hold
methods seek to limit the tumor motion by only treating
during the time patients are holding their breath [
Forced compression methods mechanically limit the range
of motion by using a plate to physically compress the
abdomen during delivery [
]. Patients with inoperable
lung tumors already suffer from compromised breathing
and often suffer from comorbidities, and, therefore, only a
subset are able to hold their breath for sufficiently long
times for breath-hold methods. Similarly, some patients are
not able to handle forced compression of their abdomen,
and accurate repositioning of the abdominal compression
plate is sometimes difficult.
The American Association of Physicists in Medicine
(AAPM) Task Group 76 [
] recommends use of a
5 mm motion threshold above which motion
management is needed. In this study, respiratory gated
treatment plans were retrospectively generated for a large
cohort of SABR treatment plans (n = 150) that had been
planned and treated using a motion encompassing
method for managing tumor motion based on the use of
an internal target volume (ITV), which is the standard
method employed in our clinic. Clinically relevant
dosimetric indices between gated and ITV-based plans
were compared to evaluate benefits of gating with
particular regard to normal lung tissue sparing.
Retrospective analysis was performed for 150 SABR
plans that were each generated for individual tumors
and were used to treat 128 medically inoperable NSCLC
patients between 2010 and 2014. Patients with multiple
tumors presented either with synchronous or
metachronous primary tumors or both. All tumors were
earlystage (T1-T3 (<5 cm)) to match criteria outlined in the
report of Radiation Therapy Oncology Group (RTOG)
]. Of 128 patients, 57 were male and 71 were
female, and median patient age was 73 years (range:
32–96). Patients were immobilized using the BlueBAG
BodyFIX® (Elekta, Stockholm, Sweden) immobilization
system. Median tumor volume was 10.9 cm3 (range:
0.3–75.6 cm3). The distribution of tumors, sorted
according to position relative to the proximal
bronchial tree and according to lobe, is given in Table 1.
Thirty-six tumors were located centrally, defined as
being within 2 cm of the proximal bronchial tree [
or mediastinal structures, and 114 lesions were
located peripherally. Peripheral lesions were
subdivided into “island” (n = 57) or lung wall-seated tumors
(n = 57) using definitions given by Altman et al. [
Island tumors are enclosed by lung parenchyma, and
lung wall-seated tumors abut the lung wall. Tumors
were fairly evenly split between upper (n = 75) and
lower (n = 65) lobes with 10 tumors located in the
right middle lobe.
Prior to planning, patient 4DCT scans were performed
using a Philips Brilliance Big Bore CT simulator (Philips
Health Care, Cleveland, OH), and four breathing phases
were reconstructed using a phase-based binning method.
For island tumors, the physician used the maximum
intensity projection of the reconstructed phase images to
define the tumor volume as shown in Fig. 1a. For
tumors near or abutting high intensity structures that
overlap the ITV in the maximum intensity projection
such as the liver or chest wall, the relevant phase images
are used to define the outline of the ITV near the high
intensity structures as shown in Fig. 1b. To account for
setup uncertainties, the ITV-based PTV, PTVconv, used
for treatment was generated by isotropically expanding
the ITV by 5 mm. For gated radiotherapy, one 4DCT
phase is typically chosen to define the gating window.
Free breathing gating about the end-exhalation (50 %)
] phase is the most common choice for gating
because of the smaller residual motion during
endexhalation and higher duty cycle. Therefore, the
endexhale phase was used to define the gating window for
this study. As outlined in RTOG 0915, the gross tumor
volume (GTV) was defined as the visible tumor within
the CT pulmonary window, and the clinical target
volume (CTV) was identical to the GTV [
planning target volume (PTV) for gating, PTVgate,
was determined by 5 mm, isotropic expansion of the
CTV, as shown in Fig. 1c, where 2 mm defined a
narrow gating window consistent with free-breathing
Fig. 1 Gating and ITV-based volume definition. a For island tumors,
ITVs were contoured using the maximum intensity projection of all
reconstructed 4DCT phases and expanded isotropically to generate
the PTVconv (red line). b For tumors close to high intensity structures
such as the liver, the structure overlaps the range of tumor motion
and, therefore, contours of the individual phase images are used in
addition to the maximum intensity projection to define the boundary
of the ITV. c For gating, the 50 % phase of the 4DCT was chosen as
the CTV and expanded isotropically to produce the gating PTV, PTVgate
gating using internal surrogates [
] and 3 mm
accounted for setup uncertainties [
Patient plans for treatment were generated in the
Eclipse® Treatment Planning System v11.0 (Varian
Medical Systems, Palo Alto, CA) using an ITV
approach by members of the physics and dosimetry
team, who each had several years of planning SBRT
cases. Of the 150 plans, 109 were intensity modulated
radiation therapy (IMRT) plans, 36 were 3D conformal
plans, and 5 were volume modulated arc therapy (VMAT)
plans. The standard dose regimen at our institution was
12 Gy per fraction in 4 fractions, with optimization
starting from normal tissue constraints consistent with
the recommendations of RTOG protocol 0915 [
Optimization target and OAR objectives were made
progressively more stringent in order to improve plan
quality. To generate gated plans, the ITV-based treatment
plan was copied, and the isocenter for the new gating
plane was moved to the geometric center of the PTVgate
contour as determined in Eclipse. Treatment fields were
then realigned to the new isocenter position. Optimization
of the new PTVgate plan was performed beginning with
constraints derived from RTOG 0915 with more stringent
lung constraints introduced later that were considered to
be appropriate due to the use of the gating window. As
with the conventional plans, the OAR and PTV objectives
for the gated plans were made more stringent in a way
that was unique to each patient in order to minimize
OAR dose while maintaining plan quality. Dose was then
recalculated based on the optimized fluence. The
endexhalation phase of the 4DCT was used as the treatment
planning CT. Another commonly used high dose
fractionation regimen explored in this study was 3 fractions of
18 Gy per fraction. To evaluate the 3 × 18 Gy schedule,
original clinical treatment plans were modified with
respect to fraction number and dose delivered per
fraction. Modified plans were re-optimized using scaled
target volume objectives and OAR constraints derived
from the AAPM Task Group 101 [
] report with
optimization objectives that were made more stringent
just as with the 4 × 12 Gy schedule. A single physicist with
experience in generating SBRT plans was responsible for
both optimizing modified conventional plans and
generating the gated plans. To establish plan quality independent
of the planner, all plans were required to achieve
standardized constraints with respect to plan conformity, target
coverage, dose heterogeneity, and dose to OARs.
The main focus of the evaluation is lung toxicity.
Analysis for lung toxicity will primarily be using mean lung
dose (MLD) and lung volume percentage receiving doses
of >20 Gy (V20). MLD [
] and V20 [
] have been
shown to correlate well with incidence of radiation
pneumonitis in conventional fractionation regimens.
Trends between the total tumor motion (defined as the
magnitude of the three-dimensional difference in
position between the tumor’s center of mass at
endinhalation (0 % phase) and end-exhalation (50 % phase))
and the calculated difference between gated and
ITVbased plans were investigated. For each methodology
(ITV vs. Gated), lung was defined to be the total lung
minus the PTV used for each method.
Additionally, we wanted to determine how the use of
gating-based margins would affect doses calculated for
other OARs. For each plan, only those OARs that were
determined by the physician to be proximate enough to
the tumor to receive significant dose were contoured
and then included in the study (e.g. heart was contoured
for central lesions, ribs for peripheral tumors). For this
patient population, the most commonly contoured
nonlung OARs were spinal cord (n = 133), heart (n = 80),
esophagus (n = 70), and ribs (n = 53). Additionally, the
bronchial tree and trachea were contoured and analyzed
for thirteen centrally located tumors. Delivered doses as
well as doses calculated using gating margins for both
regimens were compared to recommended values of
RTOG 0915 and AAPM TG101.
For all metrics, a two-tailed paired t-test was used
to evaluate the significance of the results from using
the two planning methodologies on the same patient
population. A difference was considered to be
statistically significant if the p-value was ≤0.05.
MLD and V20 results for the 3 × 18 Gy fractionation
regimen are shown in Table 2. Only results for the
3 × 18 Gy regimen, where the largest differences were
observed, are included in the table. Table 2 was
divided into two halves with results sorted by position
relative to the central region in the top half and
results sorted by lobe in the bottom half. Dosimetric
values were determined for both gated and ITV-based
plans, with differences calculated by subtracting gated
from ITV-based values. The range of dose values was
reported in parentheses. For both gated and
ITVbased plans, centrally located tumors showed the
highest average absolute values for all metrics (e.g.
gating central MLD = 4.05 ± 1.63 Gy vs. 2.94 ± 1.35 Gy
for peripheral-lung wall and 3.35 ± 1.16 Gy for
peripheral-island tumors) due to the increased volume
of lung receiving radiation exposure for centrally
located tumors. For all regions, the average dose
reduction was small (<0.5 Gy and <0.7 % for MLD
and V20, respectively) and statistically insignificant.
The largest average dose reductions from gating plans
relative to ITV plans were observed in the peripheral
regions (particularly the lung wall). Average
differences (ITV vs. Gated) were: 0.16 ± 0.64 Gy (lung wall)
and 0.20 ± 1.17 % (lung wall) for MLD and V20,
respectively, though none of the differences were
statistically significant (p = 0.08 and p = 0.21, respectively).
Interestingly, MLD and V20 differences between gating and
ITV-based methods were essentially the same for both
peripheral island tumors and peripheral lung wall tumors.
Sorted by lobe, the highest MLD values were seen in
lower lobe tumors, but the highest V20 values were seen
for upper lobe tumors. This was due to higher average
motion and, therefore, larger PTVs seen for lower lobe
tumors that resulted in a larger amount of healthy lung
receiving a lower dose. At the same time, upper lobe
tumors tended to have a smaller range of motion that
resulted in small changes to PTV volume. With the PTV
volume staying relatively constant combined with the
effects of the smaller healthy lung volume in end-exhale
images, the V20 values tended to be higher for upper
lobe tumors. Consequently, dosimetric differences
between gated and ITV-based plans were larger for
lower lobe tumors than for upper lobe tumors, where
average tumor motion was smaller (2.9 ± 3.4 mm
(range: 0.1–17.0 mm) compared to 8.1 ± 6.1 mm
(range: 0.1–28.1 mm) for lower lobe tumors). In fact,
for upper lobe tumors, the average MLD and V20
were greater for the gating plans than for the
ITVbased plans due to the effect of the smaller lung
volume in the end-exhale image washing out any benefit
from the slightly smaller target volume. The average
tumor motion for middle lobe tumors (7.1 ± 6.7 mm) was
less than for lower lobe tumors, resulting in slightly lower
dose improvements for gating-based methods. Overall,
while dosimetric reductions in the gating plans for
any specific region were not statistically significant,
the MLD reduction for the population as a whole
was small (0.13 ± 0.60 Gy) but statistically significant
(p = 0.012). No statistically significant differences
were observed for V20. The same trends, but smaller in
magnitude, in MLD and V20 results were observed for the
4 × 12 Gy regimen (as shown in Table 3).
Dose reduction as a function of tumor motion and volume
The relationship between tumor motion and MLD for
both fractionation regimens is displayed in Figs. 2 and 3,
respectively. In Figs. 2a and 3a, the reduction in MLD
from implementing gating-based plans for 4 × 12 Gy and
3 × 18 Gy regimens, respectively, is plotted as a function
of tumor motion with data organized by lobe. Upper
lobe tumors (circles) were mainly clustered in the region
of <5 mm motion while both lower lobe (diamonds) and
middle lobe tumors (triangles) showed greater variability
in the magnitude of motion. As can be seen in the
figures, there was little difference between MLD values.
Figures 2b and 3b display plots of absolute MLD
calculated for both gated (crosses) and ITV-based (circles)
plans for each patient, showing the trend of greater
separation between the two methods as tumor motion
increases. This can also be visualized in Fig. 4a, where
the MLD reduction from using gating-based plans are
plotted as a function of both tumor motion and volume
for all 3 × 18 Gy (circles) and 4 × 12 Gy (crosses) patient
plans. Just as in Figs. 2 and 3, Fig. 4b shows dose
difference increasing as tumor motion increases. Overall, no
benefit is seen in using the gating-based method for
tumor motion less than 1.5 cm, with a small but
increasing dose benefit past that point. Since tumor motion
generally decreased as tumor volume increased, we
expected that the dose reduction would be greatest for
the smallest GTV. This was reflected in Fig. 4c where
the percentage of plans with dose reduction exceeding
0.5 Gy decreased as GTV volume increased. To evaluate
3.05 ± 1.27
3.35 ± 1.40
3.16 ± 1.34
the magnitude of dosimetric effects for different
ranges of motion, Fig. 5 displays absolute reduction
in V20 from using gated plans with tumor motion in
the intervals from 5 to 10 mm, 10–15 mm, and
greater than 15 mm given as a percentage of the total
number of patient plans. Approximately 29 % (n = 44)
of plans had motion between 5 and 10 mm, 9 % (n = 13)
between 10 and 15 mm, and 7 % (n = 11) greater than
15 mm. Within each group, absolute difference in
V20 exceeded 2 % between gated and ITV-based
plans for 9 % (n = 6), 13 % (n = 3), and 22 % (n = 2)
of plans, respectively.
Results for different fractionation regimens
Overall, differences between ITV and gated plans for
MLD and V20 dose indices were found to be well below
clinical tolerance values, considering radiation
pneumonitis as the endpoint [
]. As observed in Table 3,
this was borne out in the dosimetric results for both
fractionation schedules. Figure 4 also showed that the
magnitude of dosimetric difference between gated and
ITV-based plans for the 4 × 12 Gy fractionation schedule
followed the same patterns as for the 3 × 18 Gy
fractionation schedule, but with a smaller magnitude. For the
4 × 12 Gy schedule (standard of care regimen in our
department), the differences between ITV and gated
treatment plans for all dose indices were small and deemed
clinically irrelevant as lung doses already fell well below
clinical tolerances and the calculated equivalent dose in
2 Gy fraction dose difference was rarely greater than
2 Gy (1 fraction) [
]. Moreover, no patient for any
fractionation regimen approached clinically relevant
V20 values (>10–15 % [
] of normal lung) based on
RTOG 0915 recommendations [
Dose to non-lung organs at risk
Mean dose to the spinal cord, heart, esophagus, and
trachea remained within the recommended criteria given
in RTOG 0915 [
], while the ribs and proximal
bronchial tree infrequently exceeded the criteria due to
unavoidable overlap with the PTV. OAR results as well
as clinical endpoints are provided in Table 4. Additionally,
the dose limits taken from RTOG 0915 and TG 101 are
listed for the 4 × 12 Gy and 3 × 18 Gy regimens,
respectively. Statistically significant differences between gating
and ITV-based plans were only observed for the dose to
0.35 cc for the spine (p = 0.05) and dose to 5 cc for the
esophagus (p = 0.038), both for 3 × 18 Gy regimens.
Overall, average dose in gated plans tended to be slightly
lower for the 4 × 12 Gy regimen but slightly higher for the
3 × 18 Gy regimen. While results were mixed with no
overall benefit seen for non-lung OARs from using a
gating-based method, less time was focused on optimizing
dose to the OARs as was devoted to reducing dose to the
healthy lungs for this study.
In this study, gated plans were retrospectively generated
for a large patient cohort in order to perform dosimetric
comparisons with original ITV-based SABR plans used
for treatment. Results indicate that dosimetric
differences in equivalent dose in 2 Gy fractions between
gating and ITV-based SABR plans for either
fractionation regimen did not approach clinical relevance as
defined by Ten Haken et al. [
] where a difference in
4 Gy (2 fractions) could be considered clinically relevant.
The results indicate that this threshold may possibly be
reached only for a population with very large (>4 cm)
tumor motion, particularly for the 3 × 18 Gy regimen.
The dose values were derived using the linear quadratic
model for radiobiological effect [
]. Different groups
have proposed alternative models (e.g. the universal
survival curve ) for evaluating the effects of
SABRbased treatments. Dosimetric differences between ITV
and gated plans should be considered in combination
with the absolute MLD or V20 values to determine
whether they are clinically relevant, so that even a small
reduction in dose to the lung can be important when the
MLD or V20 values are close to the constraint.
Conversely, a relatively large reduction in dose could be
irrelevant if the absolute dose fell well within
constraints. Of all 150 treatment plans, only one patient (at
the 3 × 18 Gy regimen) had an MLD value reduced by
nearly 3 Gy in the gated plan. This was for a slightly
below average volume (8.6 cc), peripheral tumor in the
right middle lobe that had large tumor motion (>2 cm).
For this patient, ITV-based MLD was only slightly over
8 Gy. This case emphasizes that the greatest benefit
would be seen in patients with large tumor volumes that
have greater than 2 cm motion, which tend to be
concentrated in the peripheral regions. Typically, the target
volume must be large to result in a lung dose that
approaches constraints. However, as target volume
increased, the target motion tended to decrease, limiting
the benefit of using this gating methodology. Additionally,
the added complexity associated with gating and the
significantly increased patient compliance and treatment
time should be factored into determining the utility of
implementing a gating-based method.
OAR doses in the gated plans generally showed
variable results. Dose to trachea and esophagus easily met
the criteria for both methods, while dose to the heart
dropped somewhat in central regions, mainly due to the
lower percentage of heart volume being irradiated in
gating plans. Meanwhile, dose to the ribs in gated plans
were slightly higher than those for ITV-based plans. This
is understandable since those patients with ribs
contoured were typically lung wall seated patients where
movement of the tumor was small and the PTV
overlapped with the ribs. Therefore, the volume differences
between the PTVs on the ITV and gated plans were small.
Interestingly, the bronchial tree also received a slightly
higher average dose in gated plans so that no benefit was
seen in gating for those OARS where plans often failed to
meet dose constraints.
Several motion management techniques are used
clinically including free-breathing gating [
inspiration breath hold [
], and deep expiration breath hold
]. While free-breathing with a gating window defined
around the 4DCT end-exhalation phase is the most
common clinical approach, due to better reproducibility
], free-breathing with a gating window around the
end-inhalation phase has possible advantages in terms of
dose to the lung due to the larger lung volume
during inspiration. However, gating on the end-inhale
phase is also known to have significantly longer
treatment times [
]. Additionally, deep expiration
has been shown to be advantageous in terms of scan
time and reproducibility [
], though it is quite
limited for lung cancer patients, who often have other
significant co-morbidities, such as COPD, making it
difficult for them to undergo breath-hold treatment.
Margins used in this paper are consistent with those
used for free-breathing using an internal surrogate
reported by Shirato et al. [
]. Berbeco [
that when considering gating,
ation is necessary to properly
motion, which could exceed 6
For clinical relevance, absolute delivered values for V20
and MLD must be taken into account to justify the
added logistical requirements of gated radiotherapy. In
this study, average MLD and V20 reduction was very
small (~0.20 Gy and ~0.3 %, respectively) with only a
few ITV-based plans where MLD reduction exceeded
even 2 Gy. Few of these patients had total MLD or V20
values that approached dosimetric constraints.
Differences were even smaller for the 4 × 12 Gy regimen. OAR
results did not show meaningful improvement in gated
plans. This is especially true for those sites (ribs and
bronchial tree) most likely to exceed dose constraints
where average dose to those sites increased compared to
ITV-based plans. For tumors with motion of 2 cm or
less, which is the range of tumors observed in this study,
no significant dose reduction was observed by
implementing a gating-based method.
AAPM: American Association of Physicists in Medicine; CTV: Clinical target
volume; GTV: Gross tumor volume; IMRT: Intensity modulated radiation
therapy; ITV: Internal target volume; MLD: Mean lung dose; NSCLC: Non
small cell lung cancer; OAR: Organ at risk; PTV: Planning target volume;
PTVgate: Planning target volume for gating; RTOG: Radiation Therapy
Oncology Group; SABR: Stereotactic ablative body radiotherapy;
V20: Percent volume receiving 20 Gy; VMAT: Volume modulated arc
Research partially supported by Varian Medical Systems, Palo Alto, CA. Varian
Medical Systems did not play a role in the design or execution of the study
or writing of the manuscript.
Availability of data and materials
The data in this study is not shared since it contains information obtained
partially under the funding of a private industry grant and due to liability
related to sharing information regarding patient treatments.
JK, NW, and IJC were involved in the conceptual design of the project. JK
and QW were active in the modification of treatment plans, contouring of
gating targets and modification of OAR contours for gating plans,
generation and optimization of gating plans, and analyzing data. MA and BM
were involved in patient care, OAR and target contouring, and plan approval.
BZ was active in developing methods to be used for sorting, processing, and
analyzing data. All authors contributed to writing, editing, and giving final
approval to the manuscript.
Research partially supported by Varian Medical Systems, Palo Alto, CA.
Consent for publication
Ethics approval and consent to participate
All data was collected and analyzed as part of an institutional review board
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