Does breathing training reduce residual motion for
respiratory-gated radiotherapy?
Rohini George, Paul Keall, Theodore Chung, Sastry Vedam and Radhe Mohan
Department of Radiation Oncology, Virginia Commonwealth University Health System, USA.
Abstract
Respiratory gating is used during CT imaging and treatment to account for respiration motion. The efficacy of respiratory gating is
increased when gating is done at exhale. Also a typical duty cycle of 30-50 % is used to avoid patient movement during delivery due
to increase in treatment time. Increasing the duty cycle more than this will result in large residual motion within the gating window.
Verbal or visual feedback may help improve the periodicity and displacement of the respiration signal. The hypothesis is the audio
and audio-visual feedback will improve the efficacy of respiratory-gated treatments. The respiration data obtained from 300 data
sets for 20-lung cancer patients taken over 5 days with 3 training types on each day were analyzed to observe the effects of training
prompts on the reproducibility of the respiration signal. The residual motion was quantified by the standard deviation of the
respiration signal that occurs within the gating window. . The data of all the patients for all the sessions was concatenated for
analysis and divided into three types. (1) Free breathing, (b) Audio training and (c) Audio-visual training. Phase gating was used to
obtain the standard deviation of residual motion as a function of duty cycle. Results show that standard deviations of exhale for all
three breathing types are lower than the standard deviations of their respective inhales. Comparing the three training types, audio
visual has the lowest statistically significant variation as compared to audio and free breathing. Audio has a lower statistically
significant variation as compared to free breathing in the 30-50 % duty cycle range. The prime gain with using training during
treatment is that respiratory gating can be performed at inhale thus giving us the benefit of treating at inhale in addition to benefit
from respiratory gating.
Keywords
Respiratory gating, duty cycle, breathing training
Introduction
Respiratory gating is a method to limit the deleterious
effects of respiration motion during CT imaging and radiation
delivery. Two important parameters that affect respiratory
gated treatments are (1) the stage of the breathing cycle during
which to gate (inhale/exhale) and (2) the ratio of the beam on
time (also called the duty cycle). To acquire maximum benefit
from a gated treatment, gating should be performed at inhale or
exhale since mobility of internal anatomy is at its minimum at
these positions. Vedam at al 1 have shown that gating during
exhale is more reproducible than gating during inhale. As the
ratio of the beam on time (duty cycle) decreases the radiation
delivery time increases.2 This may result in patient movement
during delivery, which will negate the effect of the gating. 1, 2
Large duty cycles may result in a large residual motion within
the gating window. Typical beam duty cycles during gated
treatment varies between 30% and 50 %.3, 4
Another important factor for respiratory gating efficacy
is minimum variation of patient breathing within a treatment
fraction and from fraction to fraction. There have been many
studies performed that suggest that verbal prompts help
improve reproducibility of breathing. 3-6 Kini et al5 in their
study have concluded that audio prompts improve the
frequency or rate of the patients breathing while visual prompts
control the regularity of the displacement.
The study described in this paper was performed to
determine the reproducibility of patient breathing when
different training prompts were given as a function of the ratio
of the beam on time (duty cycle). The aim of this study was to
investigate that verbal and visual prompts could improve the
reproducibility within a single treatment and from day to day
treatment.
The hypotheses of this study were: For respiratorygated techniques
o Audio breathing coaching decreases residual
motion compared to normal respiration.
o Audio-visual breathing coaching decreases
residual motion compared to normal respiration
o Audio-visual breathing coaching decreases
residual motion compared to audio breathing
coaching
Material and methods
Since May 2003, 20 lung cancer patients have been
enrolled in an IRB approved breathing training protocol and
more than 300 data sets have been collected to evaluate the
reproducibility of patient breathing using various types of
breathing coaching. The patients breathing trace is recorded
using the RPM system (Real Time Position Management
system, Varian Medical Systems, Palo Alto, CA 94304).
On the first day of the study, each patient was initially
asked to breathe normally without any prompts (called free
breathing). This was used as a control and also to determine the
rate at which audio (verbal) instructions should be given to the
patient. The patient’s breathing during free breathing is
recorded for four minutes. After this the patient is given audio
instructions – breathe-in/breathe out – and the breathing is
recorded for four minutes (called audio).
Figure 1: Sequence of events during the measurement of respiration
motion measurement.
During this time the displacement of the patient’s
breathing is observed and noted. Once the audio session is
complete, the patient is shown a TV screen, which facilitates
visual feedback. The visual feedback shows residual motion as
determined from the audio session. In addition to this visual
feedback the patient was given audio instructions (called audiovisual). The breathing trace during this audio-visual session
was recorded for four minutes. These three training prompts
were conducted on one day and thereafter repeated on four
more days generally spaced a week apart. A flowchart is shown
of the events that occur on each day.
The settings of the audio and audio-visual were
maintained for the remaining four days. The aim was to
determine how well the patient maintained his/her breathing
trends as compared to the first day. A typical patient set-up is
shown in Figure 2.
The breathing trace data file that was obtained contains
information about the position of the patients breathing, the
phase of the breathing cycle (inhale or exhale) at that particular
position and the time (0-240 secs) for the particular training
prompts. Example breathing traces for all three training types,
each for 30 seconds is shown in Figure 3.
Other demographic data was also collected such as
patient age, sex, smoking history, performance status, stage of
disease etc for future multivariate analysis.
Residual motion as a function of duty cycle
Though respiratory gating improves the efficacy of
radiotherapy there is some amount of residual motion within
the gating window. Decreasing the gating window will result in
increase in treatment time and will negate the benefit
respiratory gating.
The patient data collected was analyzed to study the
effects of the different training prompts on the residual motion.
of the breathing trace. Figure 4 shows the distribution of the
three breathing types at inhale and exhale for 40 % duty cycle
(published range3, 4). It can be seen from the histogram that the
distributions of all these data sets are approximately normal.
a
Exhale
Inhale
TV screen
(a)
b
Marker
block
Exhale
Inhale
(b)
Upper limit
c
Exhale
Current
respiration
position
Inhale
Lower limit
Figure 2 : a) The patient lying on the couch in treatment position. (b)
The visual feedback on the TV screen that is given to the patient
Figure 3: A respiratory trace of 30 secs for each breathing type.(a)
free breathing (b) audio and (c) audio-visual.
Therefore residual motion was quantified by the standard
deviation of the respiration signal that occurs within the gating
window.
The 0 position of each data set was the average of the
residual motion of the first three cycles. The 0 position is also
consistent with the procedure followed during treatment since
tracking is done for approximately 15 secs (~3 cycle) before
gating is enabled. The patient is setup at the beginning of
treatment and thus the 0 position is the average of the first 15
secs (approximately) of data.
Since the main aim of this paper was to compare the
effect of different training types on the variation within the
gating window, the data was divided into three types. (1) Free
breathing, (b) Audio training and (c) Audio-visual training.
The data of all the patients for all the sessions was
concatenated for analysis for each training type.
 = 0.34
 = 0.45
(a)
 = 0.30
(b)
 = 0.46
(c)
The F test is used to test the hypotheses. This test
returns an F value which is the ratio of the variances of the two
populations. If variance of one residual motion is significantly
greater than that of another residual motion, the F value will be
less than 0.05. If variance of one residual motion is less than
that of another residual motion, then a F-value of 1 is returned.
Results
The plots in Figure 4 show that the distribution of the
residual motion for free breathing, audio and audio-visual
training are approximately normal. Thus we can use the
standard deviation of the distributions to quantify the residual
motion.
The results of the analysis are shown in Figure 5. This
figure plots duty cycle versus the standard deviation of the
residual motion. It can be seen from this figure that the
standard deviations of the residual motion of exhale for all
three breathing types are lower than the standard deviations of
their respective inhales. This supports the conclusion made that
the patient is more relaxed and spends more time at exhale than
at inhale.
Comparing the three training types at exhale, audiovisual training has the lowest residual motion standard
deviation as compared to the other training types. Since the
typically used gating duty cycle is between 30 and 50 %, in this
range for exhale audio training has a lower standard deviation
as compared to free breathing.
The inhale curve of the audio-visual training is lower
than free breathing exhale curve up to a duty cycle of 40 %
indicating that gating with audio-visual training at inhale is at
least as good as gating at exhale for free breathing. For the
inhale curves audio training has the largest standard deviation
of residual motion compared to free breathing and audio-visual
training.
(d)
 = 0.34
 = 0.27
(e)
(f)
Figure 4 : The distribution for the three breathing types at inhale and
exhale for 40 % duty cycle. (a) free breathing at exhale (b) free
breathing at inhale (c) audio training at exhale (d) audio training at
inhale (e) audio-visual training at exhale and (f) audio-visual training
at inhale.
Respiratory gating – Phase gating
Gating can be performed by two methods - phase or
displacement gating.
In this paper, phase gating was used (as is used
clinically at our institution) to obtain the residual motion as a
function of duty cycle. The percentage of the duty cycle was
increased from 10 to 100 % in intervals of 10 and the value of
standard deviation for residual motion of each duty cycle was
determined.
Analysis of residual motion as a function of duty cycle
Figure 5: This figures shows the standard deviation of the residual
motion as a function of duty cycle. The inhale and exhale data have
been plotted separately. FB – Free breathing, A=Audio instructions
and AV – Audio-Visual instructions
Hypothesis test results
The results of the statistical test are shown in Table 1.
The 0’s indicate that one population is statistically significantly
greater than the other. The 1’s indicate that one population is
statistically significantly not greater than the other.
From the Table 1 we can say that for exhale, free
breathing is statistically significantly greater than audio upto
70 % duty cycle. Both free breathing and audio training have
variances that are statistically significantly greater than audiovisual training.
For inhale, free breathing is statistically significantly
not greater than audio. Both free breathing and audio-visual
have variances that are statistically significantly greater than
audio-visual.
Table 1: The results of the F test for variances. The table is given
seprately for inhale and exhale and within each table the three
training types are listed separately.
Discussion
This study was performed on more than 300 data sets
for 20 lung cancer patients to investigate the improvement in
efficacy of respiratory gating using verbal and visual feedback.
From the results it is evident that audio-visual training if used
during treatment can increase the gain of using repiratory
gating during radiotherapy imaging planning and treatment.
Audio training can benefit respiratory gating at exhale as
compared to free breathing up to duty cycle of 40 %.
One limitation of this study is that we used external
marker motion as the respiration signal rather than the tumor
motion. However external marker motion has been correlated
to the diaphragm motion and therefore is useful for treatment
of patients with liver or lower lobe lung cancer.7 With respect
to the system flexibility, though the Varian RPM system allows
audio-visual training, it does not provide the neccessary
hardware to support this training type.
Most institutions gate at exhale for two reasons (1)
patients spend more time in exhale, therefore decreasing
overall treatment time and (2) the exhale position is more
reproducible than inhale position. The advantage of treating at
inhale as opposed to exhale is that the lung volume is larger
than at exhale and therefore the mass of lung receiving
radiation is less at inhale as compared to exhale. According to
the study conducted in this paper the audio-visual inhale curve
has lower residual motion than exhale for free breathing up to
40 % duty cycle. Thus by using audio-visual training,
respiratory gating can be safely performed at inhale.
Future work include the multivariate analysis of the
data. This will assist in predicting which patients are suitable
for audio or audio-visual training based on their age, sex,
disease stage etc. The study performed above was only for
phase gating. Therefore another potential aspect of this analysis
would be to look at the benefits of phase gating as compared to
displacement gating which is also supported by the RPM
system.
Conclusion
Respiratory gating effectively reduces motion during
CT imaging and radiation treatments. However as the residual
motion increases the gating efficiency decreases.
The residual motion was found to be a normal
distribution for free breathing audio and audio-visual training.
Thus standard deviations can be used to quantify residual
motion.
Three hypotheses were tested in this paper. According to
the results obtained it can be concluded
1. Audio breathing coaching decreases residual compared
to normal respiration from 0 to 70 % duty cycle for
exhale though at inhale the residual motion is greater
for audio training as compared to free breathing.
2. Audio-visual breathing coaching decreases residual
motion compared to free breathing for both inhale and
exhale at all duty cycle values.
3. Audio-visual breathing coaching decreases residual
motion compared to audio breathing coaching for both
inhale and exhale at all duty cycle values.
From the plot shown in Figure 5 it can also be concluded
that the variation of exhale for each of the three training type is
less than the corresponding inhale. Another important
observation is that the variation between inhale and exhale is
the lowest for audio-visual coaching compared to the other two
breathing types.
Therefore audio-visual training should be implemented
where possible to improve the efficacy of respiratory
gating.
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