Comparison of Two RapidArc Delivery Strategies in Stereotactic Body Radiotherapy of Peripheral Lung Cancer with Flattening Filter Free Beams
Comparison of Two RapidArc Delivery Strategies in Stereotactic Body Radiotherapy of Peripheral Lung Cancer with Flattening Filter Free Beams
Bao-Tian Huang 0 1 2
Jia-Yang Lu 0 1 2
Pei-Xian Lin 0 1 2
Jian-Zhou Chen 0 1 2
Yu Kuang 0 1 2
Chuang-Zhen Chen 0 1 2
0 1 Department of Radiation Oncology, Cancer Hospital of Shantou University Medical College , Shantou, Guangdong , China , 2 Department of Nosocomial Infection Management, the Second Affiliated Hospital of Shantou University Medical College , Shantou, Guangdong, China, 3 Medical Physics Program, University of Nevada Las Vegas , Las Vegas, NV, 89154, Unites States of America
1 Funding: This study was supported in part by the Shantou University Medical College Clinical Research Enhancement Initiative (201424) and NIH/ NIGMS grant (U54 GM104944). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
2 Academic Editor: Eric Deutsch, Institut Gustave Roussy , FRANCE
Competing Interests: The authors have declared
that no competing interests exist.
To investigate the performance of using partial arc (PA) and full arc with avoidance sectors
(FAAS) in stereotactic body radiotherapy (SBRT) of peripheral lung cancer with flattening
filter free (FFF) beams.
Eighteen patients with primary (T1 or T2) non-small-cell lung cancer (NSCLC) or lung
metastatic were selected for this study. Nine patients with a gross tumor volume (GTV) <= 10 cc
were designated as the small tumor group. The other nine patients with a GTV between 10
cc and 44 cc were assigned to the large tumor group. The treatment plans were generated
in eighteen patients using PA and FAAS techniques, respectively, and delivered with a
Varian TrueBeam Linac. Dosimetry of the target and organs at risk (OARs), monitor unit (MU),
out-of-field dose, and delivery time were statistically analyzed. Delta4 and portal dosimetry
were employed to evaluate the delivery accuracy.
For the small tumor group, compared with the PA plans, the FAAS plans significantly
achieved a lower MU/fraction, out-of-field dose and a shorter treatment time (p<0.05), but
the target dose was slightly higher than that delivered by PA plans (p<0.05). For the large
tumor group, the PA plans significantly attained a shorter treatment time (p<0.05), whereas
MU/fraction, out-of-field dose and dose to OARs were comparable between the two plans
(p>0.05). Furthermore, all plans generated from the eighteen patients achieved a high pass
rate in patient-specific quality assurance, with all the gamma indices greater than 97% at
This study suggests that the FAAS technique is more beneficial for the small tumor patients
undergoing lung SBRT with FFF beams because of its higher treatment efficiency and MU
reduction. However, for the large tumor patients, the PA technique is recommended due to
its higher treatment efficiency.
Lung cancer remains the most frequent cause of death from cancer in both men and women
worldwide [1, 2]. Clinical studies have indicated that stereotactic body radiation therapy
(SBRT) is effective for both primary and metastatic lung cancer. To patients with medically
inoperable early stage peripheral non-small-cell lung cancer (NSCLC), SBRT has achieved a
favorably high local control rate, up to 88–92% .
More recently, RapidArc combined with flattening filter free (FFF) beams has become an
extraordinarily attractive dose delivery technique in lung SBRT with high dose per fraction,
which leads to a clinically meaningful reduction in treatment time, consequently improving
patient stability and treatment accuracy during the course of lung cancer treatment [4–6].
SBRT with RapidArc and FFF beams involving one or more full arcs rotation strategy
appears to be suboptimal for peripheral lung cancer as it increases the disadvantageous dose to
the contralateral lung, which potentially increases the incidence of radiation induced
pneumonitis (RIP) . Therefore, partial arc (PA) and full arcs with avoidance sectors (FAAS) which
could maintain lower pneumonitis rate in the contralateral lung are the most common used
techniques in lung SBRT [8–11]. However, the dosimetric effect and treatment efficiency
between the two delivery techniques remains unknown and needs further investigation.
In this study, we investigated, for the first time to the best of our knowledge, the dosimetric
effects of two RapidArc delivery techniques, PA and FAAS, on SBRT in peripheral lung cancer
with FFF beams. Dosimetric analysis was performed to determine which planning technique
(PA vs FAAS) is optimal for MU, out-of-field dose reduction and the improvement of
treatment efficiency according to different tumor sizes.
Materials and Methods
The protocol was approved by the Ethics Committees of Cancer Hospital of Shantou
University Medical College. Since this is not a treatment-based study, our institutional review board
waived the need for written informed consent from the participants. But the patient
information was anonymous to protect their confidentiality.
Eighteen patients previously diagnosed with primary (T1 and T2) NSCLC or lung metastatic
with single peripheral lesion no larger than 5 cm and treated with IMRT or RapidArc at Cancer
Hospital of Shantou University Medical College were retrospectively selected for this study. All
patients were chosen by a radiation oncologist with lung SBRT expertise to present different
challenge levels for different tumor sizes and peripheral locations that had needed an optimal
lung SBRT treatment strategy with RapidArc and FFF beams in the clinic.
According to the volume-adapted dosing strategy described below, based on different
tumor volumes, the patients were separated into small and large tumor groups, respectively.
Nine patients with a gross tumor volume (GTV) < = 10 cc were designated as the small tumor
group . The remaining nine patients with a GTV between 10 cc and 44 cc were assigned to
the large tumor group.
Immobilization and CT scanning
All patients were treated in supine position with arms crossed above their heads. A vacuum
bag (Medtec Medical, Inc, Buffalo Grove, IL) or a thermoplastic mask (Guangzhou Klarity
Medical & Equipment Co., Ltd, Guangzhou, China) was used to immobilize the thoracic
regions. Of the eighteen patients, two patients were received contrast-enhanced CT scan followed
by four-dimensional computed tomography (4DCT) scans using Brilliance CT with Big Bore
(Cleveland, OH, USA). The remaining sixteen patients were only received the conventional
contrast-enhanced CT scans. The contrast-enhanced CT thickness was set to 3 mm per slice.
The CT images were then transferred to Eclipse treatment planning system (V10, Varian
Medical System, Inc., Palo Alto, CA) for target volumes and organs at risk (OARs) delineation and
Target and OARs delineation
For 4DCT images, the gross tumor volume (GTV) accounting for tumor motion on all ten
phases of the 4DCT images were contoured within the CT pulmonary windows by one
radiation oncologist with expertise in lung SBRT. The GTV of the ten phases were then combined to
form internal target volume (ITV). To account for the set-up uncertainties and potential
baseline tumor shift, the planning target volume (PTV) was created by adding a uniform 5 mm
margin expansion from ITV.
For conventional contrast-enhanced CT images, the GTV was also contoured within the CT
pulmonary windows and the PTV was created accounting for tumor motion under the
guidance of fluoroscopic examination using a simulator.
OARs contouring include aorta, bronchial tree, esophagus, spinal cord, chest wall, heart,
trachea and superior vena cava (SVC). The OARs were contoured by the same radiation
oncologist according to the guidelines of the RTOG 0915 protocol .
The volume-adapted dosing strategy
All plans were created on the contrast-enhanced CT images. A biologically effective dose
(BED) of 100 Gy could achieve high rates of local control in SBRT for both primary and
metastatic lung tumors . The rate-limiting factor for local control is tumor volume, with the
evidence that eleven-month locate control was 93–100% for tumors up to 12 cc but only 47% for
tumors >12 cc with dose range of 15–30 Gy per fraction [12, 15]. Thus, a volume-adapted
dosing strategy for lung tumor SBRT was used in this study.
For small tumor group, the patients were prescribed with 25 Gy in single fraction regimens
with BED < 100 Gy. For the large tumor group, the patients were prescribed with 48 Gy in
four factions according to RTOG 0915 protocol with BED > 100 Gy. The purpose of this
dosing strategy used was to balance locate control and toxicities for patients with smaller tumors.
The patients’ characteristics were summarized in Table 1.
DPTV = diameter of PTV in greatest dimension;
FZ = field size; FS = fraction scheme.
For all patients, two different treatment strategies, PA and FAAS, were used to implement the
lung SBRT plans with FFF beams, respectively. The PA plans were generated through using
two coplanar arcs which rotates from 179° to 320° (the stop angle is slightly different from
patient to patient to prohibit the beams from entering contralateral lungs) clockwise and
counterclockwise if targets locate at the left lung. The FAAS plans were generated through using two
360° coplanar arcs with avoidance sector to exclude entrance of beams through contralateral
lungs. Avoidance sectors are ranges of gantry rotation where no MU are delivered (i.e., the
beam is turned off in this avoidance sector areas). The mirror treatment strategies were also
applied to the tumor on the right lung.
The collimator settings were the same in both strategies. Collimator angles for all plans were
set to 30° in one arc and the complementary angle 330° for the other. Schematic diagrams for
the two types of arcs were shown in Fig 1. The grouped fields were aligned to the center of
PTV. To ensure a steep dose fall-off outside the PTV, a 6 mm thick ring structure was created
surrounding the target. The dose constraints for the target volume and different OARs followed
the guidelines of the RTOG 0915 protocol .
The plans using PA or FAAS strategies were optimized using the same optimization
constraints. During the process of the optimization, we utilized 114 and 178 controls points for PA
and FAAS techniques, respectively. Dose calculations were carried out using the anisotropic
analytical algorithm (AAA_10028) with a grid resolution of 2.5 mm, with the heterogeneity
correction taking into account. The final dose calculation was normalized to ensure at least 95% of
the PTV volume received the prescription dose. The 6 MV FFF photon beams was used for
treatment and delivered by a TrueBeam Linac (Varian Medical Systems, Inc, Palo Alto, CA)
equipped with a millennium multileaf collimator (MLC, spatial resolution of 5 mm at isocenter
for the central 20 cm and 10 mm for the outer 20 cm). A maximum dose rate on the central
beam axis of 1400 MU/min was employed in the optimization process. The plan calculated at
the first time was used as a basedose plan for further optimization to compensate any underdose
or “dose cloud” areas in the previously calculated plan by giving or reducing extra dose.
Different dosimetric metrics were used to evaluate the dosimetric effects of PA and FAAS
plans on SBRT in peripheral lung cancer with FFF beams.
D98%, D2% and Dmean were used to evaluate PTV. D98%, D2% represented the dose received
by 98% and 2% of the target. Dmean represented the mean dose received by the target.
Conformity index (CI) was used to compare the plan conformity in the two treatment strategies.
CI80%, CI60%, CI50% and CI40% were defined as the volumes encompassed by the 80%, 60%,
50% and 40% isodose lines divided by the volumes of PTV encompassed by the same isodose
levels, respectively .
The maximum dose and various dose parameters (Vx) to specific OARs were generated for
the plans to assess their effectiveness in OAR sparing. The maximum dose was used to evaluate
the effectiveness of maintaining OARs’ sparing profiles by both treatment strategies in aorta,
spinal cord, esophagus, heart, trachea, bronchial tree and SVC. In addition, four dosimetric
metrics of lung V5, V10, V20, and mean lung dose (MLD) and three metrics of chest wall (V45,
V30 and V20) were also included [3, 16].
Peripheral doses outside the treatment fields
AAA was widely used in the dose calculation of treatment planning, but uncertainties also existed
due to its accuracy for estimating the peripheral dose . The method of choice for peripheral
dose assessment is phantom measurements or Monte Carlo (MC) simulations. Peripheral dose
cannot be easily calculated with a high degree of accuracy due to the restricted CT scans to the
treated region, defective head-scatter models without taking the treatment head leakage into
account and lack of models for deriving the peripheral dose from fluence information .
To compare peripheral doses outside the treatment field delivered by PA and FAAS plans, a
thorax phantom (CIRS, Inc, Norfolk, VA) combined with a FC-65G ionization chamber (0.6
cm3) with buildup cap (Standard Imaging, Middleton, WI) was used to measure the ionization
of the photon beams as a function of distance from the isocenter. The thorax phantom was
constructed of tissue equivalent epoxy materials to simulate photon scattering effects in the
patient during treatment. The thorax phantom with a tumor rod (3 cm in diameter) was placed at
the isocenter and the ionization chamber was placed at 20, 40, and 60 cm away from it,
respectively, to measure the out-of-field doses. Because the head leakage is the predominant
contributor to the out-of-field dose at a distance far from the treatment field (> 15 cm) , the tip of
the chamber was placed to face towards the gantry to ensure the accuracy of measurement.
DOSE-1 electrometer (IBA, Munich, Germany) was used to record the measurement by
connecting itself to the chamber using extension cable.
The absorbed dose was calculated as follows. The out-of-field dose was then converted to
mGy/Gy for comparison.
Dair = Mu×Nx×0.876×Katt×Km Where Dair (cGy) was the absorbed dose in the air, Mu was
the readout on the electrometer and Nx was the exposure calibration factor of an ionization
chamber (equal to 1.033 in this study). 0.876 (cGy/R) was the coefficient of exposure to
absorbed dose in the air. Katt was the correction factor of absorption and scattering of an
ionization chamber, Km was the factor to take account of non-air equivalence of the ionization
chamber wall and buildup cap material (Katt×Km was equal to 0.987).
To compare the treatment efficiency delivered by PA and FAAS plans, the treatment time for each
plan was recorded by performing the dry-run function on the Linac. It was recorded from the
start of the first arc and the end of the second arc, including the intervals between the two arcs and
the gantry rotation time in the avoidance sectors. The actual measured treatment time was also
cross-checked with the estimated one according to the empirical equations as follows:
For PA plan,
Each plan was verified to assess the agreement between calculated and delivered doses using
both 3D detector array delta4 (ScandiDos, Uppsala, Sweden) and electronic portal imaging
devices (EPID) mounted on the TrueBeam Linac. For Delta4 measurement, we implemented
1069 p-type silicon diodes for gamma analysis in a 20 cm × 20 cm detection area. The spatial
resolution was 5 mm for the central 6 cm × 6 cm area, and 10 mm for the outer area. The
results were evaluated in terms of gamma index (Γ3mm, 3%), which is calculated using spatial and
dosimetric limits of 3 mm distance-to-agreement and a 3% dose difference, respectively.
All reported values are expressed as mean ± standard deviation of the mean. The data were
compared using paired t-test when the data obey normal distributions; otherwise Wilcoxon
signed-rank test was used. A p-value < 0.05 was regarded as statistically significant. All
statistical analysis was performed in SPSS 19.0 (SPSS, Inc, Chicago, IL).
The statistical analysis of dosimetric metrics comparison for PTV and different OARs in all
patients were summarized in Table 2. All the plans met the dose constraints described in the
RTOG 0915 protocol and achieved a similar level of PTV coverage. For the small tumor group,
a higher D2% and Dmean of PTV was observed in the FAAS plans (p < 0.05). The FAAS plans
attained a lower maximum dose in aorta compared to the PA plans (p < 0.05). The conformity
index CI80% and CI60% of FAAS plans were inferior to those of PA plans (p < 0.05). Above all,
MU/fraction delivered by the FAAS plans were significantly reduced compared with those
delivered by PA plans (p < 0.05). In the large tumor group, both FAAS and PA plans had a
similar PTV and OARs dose. However, the conformity index CI80% and CI60% seem to be inferior
to those of PA plans. Unlike the small tumor group, MU/fraction delivered by both plans was
PA = partial arc; FAAS = full arc with avoidance sector; PTV = planning target volume; MLD = mean lung dose; CI = conformity index; SVC = superior
vena cava. Values are mean ± SD.
* stands for statistically significant.
Fig 2. A representative DVH from the PA and FAAS plans in the two groups of patients. (a) small tumor group; (b) large tumor group. BT = bronchial
tree; Eso = esophagus; CW = chest wall; SVC = superior vena cava.
comparable. A representative dose-volume histogram (DVH) from the PA and FAAS plans in
the small and large tumor groups are shown in Fig 2. MU/fraction from individual patient was
displayed in Fig 3.
The peripheral doses at lateral distance 20, 40 and 60 cm from the isocenter delivered by both
plans were also evaluated in Fig 4. In the small tumor group, the FAAS plans show significantly
reduced peripheral doses along the longitudinal direction from the isocenter than that
contributed by the PA plans (p < 0.05). In contrast, in the large tumor group, no significant differences
were observed in the peripheral doses delivered by both FAAS and PA plans (p > 0.05).
The treatment efficiency of both plans was also investigated via measuring the delivery time.
It can be observed from Fig 5 that the estimated treatment times were in an excellent agreement
with the measured ones, regardless of small tumor or large tumor group. The mean value of
actual treatment time in the small tumor group was 6.2±0.7 minutes for the PA plans and only
5.7±0.5 for the FAAS plans (p < 0.05). In contrast, the PA plans attained a shorter treatment
time compared to the FAAS plans (2.6±0.1 vs 3.1±0.2 minutes on average, p < 0.05) in the
large tumor group.
Table 3 summarized the γ analysis for both plans using delta4 and portal dosimetry. Both
verification techniques show very high agreement between calculated doses and measured
Fig 3. MU/fraction in the two groups of patients investigated. (a) small tumor group; (b) large tumor group.
Fig 4. Out-of-field dose comparison in the two groups of patients. (a) small tumor group; (b) large tumor group. * stands for statistically significant.
Fig 5. Treatment efficiency in PA and FAAS plans. (a) small tumor group; (b) large tumor group. E-PA = estimated treatment time of PA; A-PA = actual
treatment time of PA; E-FAAS = estimated treatment time of FAAS; A-FAAS = actual treatment time of FAAS.
doses. Less than 2% of the analyzed areas exceeded γ value > 1. Meanwhile, the maximum γ or
mean γ value showed similar results for both verification techniques.
In this study, we found that the MU/fraction delivered by FAAS plans in the small tumor
group were significantly reduced than that delivered by PA plans. Consequently, the peripheral
dose and the treatment time achieved by FAAS plans were considerably superior for these
patients. By contrast, the comparison of MU/fraction delivered between FAAS and PA plans
reveals no significant difference in the large tumor group. What’s more, the treatment time of the
FAAS plans was longer than that of PA plans in large tumor patients. Our results optimize the
selection of different radiation techniques during lung SBRT treatment and can provide
valuable information for clinical implement.
It was reported that the scattered radiation to patients was at first-order directly
proportional to the MU , and the increase in peripheral dose could theoretically increase the risk of
secondary malignancies [21, 22]. Since peripheral dose assessment can’t be easily calculated
with a high degree of accuracy , we employed a setup of dynamic thorax phantom to
measure the peripheral dose as a function of longitudinal distance from isocenter. Our data showed
that the peripheral doses were obviously reduced along the longitudinal direction from the
isocenter using the FAAS plans in the small tumor patients, suggesting its potential to lower the
risk of secondary malignancies induced by radiotherapy.
In the present study, we used PA and FAAS techniques to accomplish SBRT treatment
because full arcs rotation strategy appears to increase the disadvantageous dose to the
contralateral lung. Lower dose to it was of concern because it was a risk factor to the incidence of RIP .
For SBRT treatment of lung cancer, the internal scatter of the patient also contributed to the
contralateral lung dose. It may play a more significant role especially when the distance from
the radiation central axis is usually less than 15 cm in lung cases. Since we found that the FAAS
technique achieved significant MU reduction in the small tumor group, its contribution to the
contralateral lung dose needs further investigation.
The treatment efficiency was evaluated via measuring the treatment time. The total
treatment time includes three parts in RapidArc based lung SBRT: MU delivered time (equal to
total MU divided by maximum dose rate), interval time between two arcs (about 5 seconds
when the collimator was set to 30° and 330° rotations on the TrueBeam Linac), and the gantry
rotation time (6°/s on TrueBeam) when the FAAS technique is used [6, 23, 24]. Our novel
calculation model was demonstrated to be in an excellent agreement in treatment time with the
measures ones. However, it is worthy of note that the calculated treatment time is noticeably
shorter (ranged from 1–5 seconds) than the actual measured one. This is partially due to the
shutter effect of Linac, where the unsaturated dose rate was generated at the beginning and the
end of beam on time.
As illustrated in Fig 3, the FAAS plans achieved a particularly higher MU reduction in small
tumor group. Both of the two techniques investigated possess the constant dose rate (1400MU/
min) during the treatment process and the treatment time was therefore about thirty seconds
shorter in average than that of the PA plans. Thirty seconds reduction of treatment time
achieved by the FAAS plans is of utmost importance for SBRT . Because 7 Gy dose could
have been delivered within thirty seconds when FFF beams with the maximum dose rate
(1400MU/min) are applied. On the other hand, the shorter treatment time generally introduces
substantially superior patient stability and treatment accuracy, simultaneously reduces the
likelihood of intrafractional baseline shifts in tumor position [25, 26]. Previous research regarding
the target motion as a function of treatment time found the average time needed to maintain
the target motion within 1 mm of translation or 1 degrees of rotational deviation was 5.9 min
for thoracic tumors, implying an inevitable target motion beyond the threshold of 5.9 min .
As the mean delivery time in the small tumor group was 6.2±0.7 minutes for the PA plans and
only 5.7±0.5 for the FAAS plans, we thought thirty seconds reduction in delivery time was
critical for SBRT treatment of lung cancer. Although a reduction of delivery can be interesting in
regards to tumor movement and patient positioning, its biological consequences are limited.
Delta4 and Portal dosimetry were also employed to verify the delivery accuracy. Table 3
lent agreement between the calculated and measured dose. However, there are some limitations
associated with the use of delta4 and portal dosimetry for plan validation. For small field,
Christian et al  recommended the use of films for dose verification measurements in
stereotactic radiosurgery due to its high resolution than other tools, and they found a good agreement
with estimated data by Monte Carlo algorithm for films. Thus, the experimental verification
with films for small field size, such as those implemented in lung SBRT, will be an interesting
topic for our future studies.
We have demonstrated that the FAAS delivery strategy is more beneficial for small tumor
patients undergoing lung SBRT with FFF beams due to the reduction of MU, peripheral doses,
and the improvement in treatment efficiency. In contrast, for large tumor patients, the PA
delivery strategy is recommended because it required less treatment time with similar target
coverage, OARs sparing and peripheral doses compared to that achieved by FAAS plan. It remains
to be determined whether the treatment schemes investigated would improve the local control,
limit the late toxicity, and ultimately prolong patient survival.
The work presented as the oral presentation at the 56th Annual Meeting of The American
Association of Physicists in Medicine (AAPM), AUSTIN, Texas, July 20–24, 2014.
Conceived and designed the experiments: BTH CZC. Performed the experiments: BTH JYL
JZC. Analyzed the data: PXL. Contributed reagents/materials/analysis tools: JYL PXL JZC.
Wrote the paper: CZC YK.
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