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

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.

EVALUATE DOSE DISTRIBUTION OF IMRT AND VMAT TECHNIQUE IN RADIOTHERAPY FOR HEAD AND NECK CANCER

USING TRUEBEAM STX LINEAR ACCELERATOR

Pham Hong Lam 1 ; Nguyen Thi Van Anh 2 ; Pham Quang Trung 2

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 CI 100 - Paddick, HI RTOG 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 (phamhonglam@vmmu.edu.vn)

Date received: 20/10/2018

Date accepted: 04/12/2018

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,

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

of radiation techniques with optimized

dosage To understand the new techniques,

this report will focus on: Comparing and

evaluating the quality of the VMAT and

IMRT plan to point out the optimal treatment

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

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

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

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)

Table 2: Dose tolerance of organs at risk

Organs at risk Volume (cc) Dtotal (Gy) Dmax (Gy) Reference

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

P

D

100

PTV

V

CI 100

2 100 100

PTV

V

V ×V

P

D

Wu Qiuhen [10]

HI

max

P

D D

1 0 0

V

(Dmin : Minimum dose value; D max : Maximum dose value; D P : Prescription dose;

to 5 and 95% volume of PTV; A: ideal value)

RESULTS

1 Mean doses

Table 4: Mean doses value

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

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

Figure 1: Value of CI100 Paddick index

3 Tolerance dose of organs at risk

Table 6:

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

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

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

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

REFERENCES

1 B N T T Huong Mean Characteristics

of Head and Neck Cancer 2014

2 R Lilenbaum, R Komaki, M.K Martel

Radiation Therapy Oncology Group Rtog 0623: A Phase Ii trial of combined modality therapy with growth factor 2008

3 N Lee, A Kramer, P Xia A phase II

study of intensity modulated radiation therapy (IMRT) +/- chemotherapy for nasopharyngeal cancer Radiotion Ther Oncol Gr RTOG 0225)

2005

4 C Drug, P Nsc, E Sherman, N Lee

Rtog 0912 Protocol: A randomized phase II study of concurrent intensity modulated radiation therapy (Imrt), paclitaxel and pazopanib (Nsc 737754)/placebo, for the treatment of anaplastic thyroid cancer no Nsc 737754, 2010

5 I Hazell et al Rtog 0920 a Phase Iii

study of postoperative radiation therapy (Imrt) +/- cetuximab for locally-advanced resected head and neck cancer 2016, Vol 17, No 1

6 N Lee, J Kim RTOG 0615 Protocol:

Radiation Therapy Oncology Group Rtog

0615 2008

7 S.C.Oliveira Comparison of three

linac-based stereotactic radiosurgery techniques

2003

8 D Zentralbibliothek ICRU Report 62

2018, February

9 I Paddick A simple scoring ratio to

index the conformity of radiosurgical treatment plans Technical note J Neurosurg 2000, Vol 93, Suppl 3, pp.219-222,

10 Q Wu, R Mohan, M Morris, A Lauve,

R Schmidt-Ullrich Simultaneous integrated

boost intensity-modulated radiotherapy for

locally advanced head-and-neck squamous

cell carcinomas I: Dosimetric results J Radiat

Oncol Biol Phys 2003, Vol 56, No 2, pp.573-585

11 I Paddick, B Lippitz A simple dose

gradient measurement tool to complement the

conformity index J Neurosurg 2006, Vol 105,

Suppl, pp.194-201

12 Q Shamsi, M Atiq, A Atiq, S Ahmad

Analysis of dosimetric indices for evaluating intensity modulated radiotherapy plans of head and neck cancer patients 2017, Vol 5, pp.1-6

13 P È M B Raam, M D C H H J T Erhaard, J U M R Oesink, C O P J R Aaijmakers Intensity-modulated radiotherapy

significantly reduces xerostomia compared with conventional radiotherapy 2006, Vol 66,

No 4, pp.975-980