Evaluate dose distribution of IMRT and VMAT technique in radiotherapy for head and neck cancer using truebeam STx linear accelerator

Journal of military pharmaco-medicine no1-2019

EVALUATE DOSE DISTRIBUTION OF IMRT AND VMAT
TECHNIQUE IN RADIOTHERAPY FOR HEAD AND NECK CANCER
USING TRUEBEAM STX LINEAR ACCELERATOR
Pham Hong Lam1; Nguyen Thi Van Anh2; Pham Quang Trung2
SUMMARY
Objectives: To evaluate and compare dose distribution between IMRT and VMAT plan in
radiotherapy for head and neck cancer using TrueBeam STx accelerator. Subjects and
methods: CT imaging of head and neck cancer’s patients treated with VMAT on the TrueBeam
STx accelerator was used to replan using IMRT technique in TPS Eclipse v13.6. Conformity
index, gradient index and homogeneity index were used to compare plan quality and dose
distribution at planning target volume, organs at risk. Results: The dose distribution on planning
target volume of IMRT technique based on CI100 - Paddick, HIRTOG index (0.82 ± 0.04, 1.085 ±
0.014) gave the same value as the VMAT technique (0.81 ± 0, 44, 1.094 ± 0.014). Maximum
doses on organs at risk such as spinal cord, brainstem, and mandible received from IMRT
technique were a little lower than the doses from VMAT technique. Conclusion: The IMRT
technique is equivalent to the VMAT based on coverage, ability to focus dose on the planning
target volume and the ability to spare dose to critical organs. Both the IMRT and VMAT
technique on the TrueBeam STx Linac can be selected to treat head and neck cancer patients
in 108 Military Central Hospital.
* Keywords: Head and neck cancer; IMRT; VMAT; Conformity index; Homogeneity index;
Gradient index.

INTRODUCTION
Radiation therapy is one of the main
modalities for cancer treatment. The linear
accelerator (Linac) is an indispensable
device and it is the most basic component
of an external radiotherapy. Especially,
radiation therapy plays an important role

in head and neck cancer treatment. The
biggest difficulty with the treatment of
head and neck cancer by radiotherapy is
that it has a large number of critical

organs near treatment volume. Organs at
risk that need to be protected during
radiotherapy include: brainstem, spinal
cord, salivary glands, esophagus, larynx,
mucosa... Whereas the head and neck
area has relatively small surface area [1].
In most early 3D-CRT cases, it is inevitable
that these organs will be overdosed to
reach doses in the tumor. This can have
serious consequences for the quality of
life of the patient.

1. 103 Military Hospital
2. 108 Military Central Hospital
Corresponding author: Pham Hong Lam ()
Date received: 20/10/2018
Date accepted: 04/12/2018

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Journal of military pharmaco-medicine no1-2019
Nowadays, many generations of modern
linear accelerators have been introduced
and put into clinical applications, these

radiotherapy systems are equipped with
many advanced features and many new
techniques.

Advanced

radiotherapy

techniques, such as IMRT, VMAT have
been used to treat head and neck cancer,

SUBJECTS AND METHODS
1. Subjects.
A total of 30 patients with head and
neck cancer, who received radical
radiotherapy treatment on TrueBeam
STx accelerator at Radiotherapy &
Radiosurgery Department, 108 Military
Central Hospital, enrolled in the study
from January 2018 to September 2018.

in which, planning target volume (PTV)
doses can be optimized meanwhile organs
at risk (OARs) are protected.
In Vietnam, in recent years, many
oncology centers have been established
and equipped with modern accelerators.
In 2017, the TrueBeam STx Linear
Accelerator (Linac) has been installed
and put in use to treat patients at the

108 Military Central Hospital. The Linac
system is the latest generation of
radiotherapy accelerators from Varian
manufacture. It is integrated a number of
technologies that enable the implementation

2. Methods.
- 30 patients were assigned to receive
radiation therapy with VMAT. Simulated
imaging data of 30 patients were re-used,
plan using IMRT technique on Eclipse v13.6.
- In order to facilitate the comparison of
the quality of the plans, the prescribed
dose, the number of fraction is the same
70 Gy/35 Fx. This study focused on the
evaluation of dose distribution on the PTV
of 70 Gy.
- Setting of beam energy parameters,
specific field size for each plan.
Table 1: Beam summary report.

of radiation techniques with optimized

VMAT

IMRT

6 MV

6 MV

600 MU/min

600 MU/min

dosage. To understand the new techniques,

Energy

this report will focus on: Comparing and

Dose rate

evaluating the quality of the VMAT and

Prescription dose

70 Gy

70 Gy

IMRT plan to point out the optimal treatment

Number of fraction

35

35

3 arcs

9 fields

for patients with head and neck cancer.
The entire research process was performed
on the TrueBeam

STx

Accelerator

(Eclipse v13.6) at

Radiotherapy &

Radiosurgery Department - Cancer Institute,
108 Military Central Hospital.

Number of field

From the obtained plans, the dose
volume histogram is studied to compare
and evaluate the damage at critical organs.
The dose for each organ is recommended
by the Radiation Therapy Oncology Group
(RTOG) (table 2).
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Journal of military pharmaco-medicine no1-2019

Table 2: Dose tolerance of organs at risk.
Organs at risk

Volume (cc)

Spinal cord

Dtotal (Gy)

Dmax (Gy)

Reference

-

45

RTOG 0623 [2]

Brainstem

1%

60

54

RTOG 0225 [3]

Optic chiasm

1%

60

54

RTOG 0225

Parotid (ipsilateral)

Mean

26

-

RTOG 0912 [4]

Esophagus

Mean

35

-

RTOG 0920 [5]

Mandible

-

-

70

RTOG 0225

Lens

-

-

25

RTOG 0615 [6]

Mean

50

-

RTOG 0225

Cochlea

Coverage index (CI), conformity index (CI), gradient index (GI) and homogeneity

index (HI) are included to compare the quality of VMAT and IMRT plans.
Table 3: Formulas for calculating plan evaluation indicators.
Index
Coverage

CI100

Formula

Ideal value

Reference

Dmin
DP

0,9 ≤ A < 1

RTOG (1993) [7]

V100
VPTV

A=1

ICRU - 62 [8]

(VPTV 100 )2
VPTV × V100

A=1

Paddick [9]

D5 − D95
DP

A=0

Wu Qiuhen [10]

Dmax
DP

A ≤ 1,1

RTOG (1993)

HI

GI

V 50
V1 00

Paddick [11]

(Dmin: Minimum dose value; Dmax: Maximum dose value; DP: Prescription dose;
VPTV: PTV Volume; VPTV100: Volume of PTV receiving 100% prescribed dose; V50, V100:
the volume is covered by 50% and 100% isolines;D5%, D95%: Minimum dose delivered

to 5 and 95% volume of PTV; A: ideal value)
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RESULTS
1. Mean doses.
Table 4: Mean doses value.
DP (Gy)

Fx

Dmax (%)

Dmin (%)

Dmean (%)

VMAT

70

35

109.4

80.4

103.9

IMRT

70

35

108.5

83.2

103.4

Dmax, Dmean, and Dmin dose values are averaged over all plans. The VMAT and IMRT
plans are guaranteed at least 95% of the tumor volume received 100% of the
prescribed dose. The maximum dose (Dmax) of the techniques was 109.4% (VMAT);
108.5% (IMRT).
2. Plan evaluation index.
Table 5: Plan evaluation index.
Coverage

CI

RTOG 1993

CI - ICRU

CI100 - Paddick

VMAT

0.8 ± 0.2

1.08 ± 0.04

0.811 ± 0.045

IMRT

0.83 ± 0.14

1.09 ± 0.04

0.817 ± 0.042

GI

HI

Paddick

Quihen Wu

RTOG (1993)

VMAT

27.0 ± 15.6

0.063 ± 0.009

1.094 ± 0.014

IMRT

30.2 ± 13.6

0.050 ± 0.004

1.085 ± 0.014

With prescribed dose of 70 Gy/35 Fx, the coverage, CI, HI and GI values are shown.
On average, coverage, CI and HI of the IMRT plans are closed to ideal values.
According to the formula given by Paddick (2000), the CI100 index shows the
intersection between the volume receiving 100% of the prescribed dose (V100) and the
volume of PTV (VPTV). The VPTV100/VPTV ratio is used to evaluate the volume of tumor
receiving 100% the prescribed dose. The CI100 - Paddick values for the two subjects
were 0.811 ± 0.045 (VMAT); 0.817 ± 0.042 (IMRT). The specific value of each
component ratio is given in figure 1.
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Journal of military pharmaco-medicine no1-2019

Figure 1: Value of CI100 Paddick index.
3. Tolerance dose of organs at risk.
Table 6:
VMAT

IMRT

Spinal cord

D1% (Gy)

39.3 ± 2.7

35.7 ± 11.6

Brain stem

D1% (Gy)

39.7 ± 12.4

35.1 ± 14.9

Optic nerve (left)

D1% (Gy)

35.0 ± 23.8

38.2 ± 25.9

Optic nerve (right)

D1% (Gy)

29.0 ± 22.2

37.1 ± 25.2

Parotid gland (left)

Dmean (Gy)

22.1 ± 4.6

22.2 ± 7.7

Parotid gland (right)

Dmean (Gy)

24.4 ± 6.1

24.1 ± 9.3

Esophagus

Dmean (Gy)

16.4 ± 11.8

16.7 ± 11.7

Mandible

Dmax (Gy)

69.2 ± 5.1

68.8 ± 23.3

Len (left)

Dmax (Gy)

2.61 ± 2.96

2.53 ± 2.82

Len (right)

Dmax (Gy)

2.41 ± 2.61

2.25 ± 2.38

Cochlea (left)

Dmean (Gy)

22.9 ± 19.6

24.7 ± 21.9

Cochlea (right)

Dmean (Gy)

24.9 ± 21.7

25.1 ± 22.2

477 ± 83

1864 ± 623

MUs

Comparison of tolerance dose at critical organs between VMAT and IMRT, for spinal
cord, brainstem and optic chiasm we consider the value of D1% (dose at 1% of organ
volume). The data obtained were compared with the tolerance dose range recommended
by RTOG.
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DISCUSSION
The biggest demand for radiotherapy
in cancer treatment is how to focus the
dose on the target volume and minimize
the dose to the surrounding normal tissues.
However, for head and neck cancer
radiotherapy, the organs at risk are closed
to the location of the tumor so that the
requirement becomes more difficult to
achieve. Using evaluation indicators, we

can compare and evaluate the quality of
each plan, selecting the best treatment for
head and neck cancer patients.
* Coverage:
With the data obtained, the study
demonstrated that both plans VMAT and
IMRT achieved a TV coverage greater
than 0.8: 0.8 ± 0.2 and 0.83 ± 0.14,
respectively.
* CI:
In term of conformity, the VMAT and
IMRT plans both give the same CI index
and it is also close to the ideal value:
CIICRU (1.08 ± 0.04 and 1.09 ± 0.04), CI100
- Paddick (0.81 ± 0.44 and 0.82 ± 0.04).
This may be because the TrueBeam
STx uses a high resolution multi leaf
collimator (MLC) HD120, which offers
flexible dose modulation, with 32 pairs of
central leaves of 2.5 mm thickness and 28
pairs of mini-leaves 5 mm thickness.
The CI100 index given by Paddick is
calculated by the intersection between
volumes received prescribed dose and
PTV. Based on the VPTV100/V100 ratio,
normal tissue areas receiving high doses

are also considered. This ratio averaged
over 30 patients (0.869) (VMAT) and
0.862 (IMRT).

* HI:
In a study by Q. Shamsi et al [12], the
analysis and evaluation of the IMRT plan
for treating head and neck cancer on a
varian clinac DHX, the study provided the
HIRTOG value (1.15 ± 0.05). Meanwhile,
the plans on the TrueBeam STx in this
study provide near-ideal results: VMAT
(1.094 ± 0.014), IMRT (1.085 ± 0.014).
We also compared the homogeneity in
dose distribution in the treatment volume
by the HI index given by Quihen Wu (2003).
Specifically, the HI with IMRT plans
(0.050 ± 0.004) was closer to the ideal
value than the VMAT (0.063 ± 0.009).
This suggests that, with the TrueBeam
STx, the IMRT technique could provide
better uniformity in dose distribution at PTV.
* GI:
In terms of the possibility of reducing
the dose when going out of the tumor
volume, our study also showed that the
dose-reduction value - GIPaddick (2006)
with VMAT (27.0 ± 15.6) better than the
value with IMRT plan (30.2 ± 13.6). These
results showed that in radiotherapy for
head and neck cancer, VMAT can reduce
the dose from 100% to 50% better than
IMRT.
* Doses in organs at risk:

According to statistics, with the plans
on the TrueBeam STx, the normal tissues
receive quite small dose, the mean dose
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in the two salivary glands and mandible
was below the tolerance dose range.
In a previous study by Braam et al [13],
referring to the comparison of the quality
of head and neck cancer treatment between
IMRT and conventional radiotherapy, the
authors point out that the Dmean dose at
each salivary gland which are higher than
26 Gy can cause xerostomia for patients
after radiation therapy.
* MUs and delivery time:
Number of MUs in VMAT plan (477 ±
83 MU) was 2.9 to 3.6 times fewer than
IMRT plan (1864 ± 623 MU). Small MUs
help to reduce delivery time in VMAT plan,
minimizing fatigue for patients and increasing
treatment outcome.
CONCLUSION
- In term of dose distribution on tumor,
IMRT technique had CI100-Paddick (0.82 ±
0.04) and HIRTOG (1.085 ± 0.014) were
similar to those of the VMAT technique
(0.81 ± 0.44 and 1.094 ± 0.014).

- The IMRT technique also offers
better protection based on the ability to
protect the organs at risk. The mean dose
to the spinal cord, brainstem and mandible
are 35.7 Gy, 35.1 Gy, 68.8 Gy for IMRT,
and 39.3 Gy, 39.7 Gy and 69.2 Gy for VMAT,
respectively.
- Both IMRT and VMAT on the TrueBeam
STx Linac are guaranteed to meet the
treatment planning criteria for head and
neck cancer. The results also show that
IMRT plans are equivalent to the VMAT
186

plans based on coverage, the ability to
focus dose on the tumor and the ability to
minimize the dose to organs at risk.
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