【免费】论文研究-TrueBeamSTx直线加速器的Winston-Lutz-Gao测试

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International Journal of Medical Physics, Clinical Engineering and Radiation Oncology, 2019, 8, 9-20

http://www.scirp.org/journal/ijmpcero

ISSN Online: 2168-5444

ISSN Print: 2168-5436

DOI:

10.4236/ijmpcero.2019.81002 Jan. 21, 2019 9

Int

. J. Medical Physics, Clinical Engineering and Radiation Oncology

Winston-Lutz-Gao Test on the True Beam STx

Linear Accelerator

Junfang Gao

, Xiaoqian Liu

Northeast Louisiana Cancer Institute, Monroe, LA, USA

Division of Pharmacy, The University of Texas MD Anderson Cancer Center, Houston, TX, USA

Abstract

In the linear accelerator-based stereotactic radio surgery (SRS) and stereotac-

tic body radiotherapy (SBRT) programs, single isocenter-multiple metastases’

treatment has become more and more popular due to their

high efficiency in

treatment time. However, the

absence of a comprehensive quality assurance

program is still the challenge for medical physicists. The Winston-Lutz-

Gao

test, which we developed two years ago, was performed for the first time on a

True Beam STx (Varian Medical System) linear accelerator in this study.

Beams were designed by Eclipse with gantry, collimator, and couch full rota-

tions, and a 200-pound weight was placed on the couch to mimic real treat-

ment. The “frameless SRS QA target pointer” from the Brainlab company,

with a 3.5-mm metallic ball embedded in the center,

was used as a phantom.

Images were acquired by the portal imager built-

in linear accelerator and

analyzed directly by the Image browser in ARIA. We found that the farther

the metastases were from the linac isocenter, the worse

the congruence was

between the beam mechanical and the radiation center. The farthest metas-

tases should be within 6 cm from the linac isocenter per the AAPM TG-

142

and

American Society for Radiation Oncology (ASTRO) white paper criteria.

To the best of our knowledge, this is the first off-isocenter Winston-

Lutz test

performed on a True Beam STx linear accelerator.

Keywords

Winston-Lutz-Gao Test, off-Iso Winston-Lutz Test,

Single Iso-Multiple Mets’ Treatment

1. Introduction

In recent decades, stereotactic radio surgery (SRS) and stereotactic body radio-

How to cite this paper:

Gao, J.F. and Liu,

Q. (2019) Winston-Lutz-Gao Test on the

True Beam STx Linear Accelerator

Inter

national Journal of Medical Physics

Cli

nical Engineering and

Radiation Oncology

, 9-20.

https://doi.org/10.4236/ijmpcero.2019.8100

Received:

December 20, 2018

Accepted:

January 18, 2019

Published:

January 21, 2019

9 by author(s) and

Scientific

Research Publishing Inc.

This work is licensed under the Creative

Commons Attribution International

License (CC BY

4.0).

http://creativecommons.org/licenses/by/4.0/

Open Access

J. F. Gao, X. Q. Liu

DOI:

10.4236/ijmpcero.2019.81002 10

Int

. J. Medical Physics, Clinical Engineering and Radiation Oncology

therapy (SBRT) have been heavily implemented and researched in the radiation

oncology field due to their unique medical and financial merits to cancer pa-

tients. Among SRS and SBRT practice the multiple cranial metastases treatment

advanced faster than did any other kind of lesion treatment in both clinical and

technique aspect. From Sahgal

et al.

’s recent comprehensive review in

Neo-

ro-Oncology

[1], the SRS alone has a superior patient survival rate compared

with adjuvant whole brain radiation therapy (WBRT) for patients with up to

four brain metastases (mets); even the adjuvant WBRT has a better distant con-

trol rate than does SRS alone. This will trigger more SRS treatment for multiple

cranial mets in the future. Sahgal

et al.

reviewed some of the pros and cons for

various type of modern SRS technologies, including the Gamma knife, cyber

knife, and linear accelerator (linac). Each modality has a unique mechanical and

geometrical character, but the clinical data do not support one modality’s supe-

riority over the others.

The linac based SRS/SBRT technique is more popular than the Gamma knife

and cyber knife because almost every radiation oncology site is equipped with a

linac. The modern digital controlled linac (such as the Varian True Beam STx

Edge series) has a variety of beam energies and a very accurate mechanical con-

trol system. It makes the SRS/SBRT program less expensive and faster treatment

than other modalities. However, there are two challenges ahead of us when we

treat multiple targets by linac. As Sahgal

et al.

noted, one challenge is the

small-field dosimetry, which requires accurately measured beam data to model

the beam in the computer planning system. The other challenge is the complex-

ity of the mechanical and geometrical quality assurance (QA). It is well known

that the poor QA program comprises coverage for multiple targets and easy

cause radionecrosis. Roper

et al.

[2] and Faught

et al.

[3] demonstrated that cur-

rent standard linac QA tolerance (recommended byTG-142) is not appropriate

for the single iso-multiple met treatment. However, single iso-multiple mets

treatment planning systems such as Element and Eclipse are already on the

market. A standard QA procedure really needs to be established for these sys-

tems to ensure the same accuracy as that is attained with single iso-single met

treatment in the medical physics community.

This study tried to meet one of the needs of the above-mentioned QA proce-

dures. We will illustrate one practical QA test, which is called the Wins-

ton-Lutz-Gao test [4], on the True Beam STx linac for single iso-multiplemets

treatment. This test is testing the congruence accuracy between the field me-

chanical center and radiation center when the asymmetric beam is implemented

to treat off-iso multiple mets. It is different from the standard Winston-Lutz test

which only tests the congruence accuracy between field mechanical center and

radiation center for symmetric beam treating the single met at isocenter. We

developed Winston-Lutz-Gao test two years ago for single iso-multiple mets

treatment with use of an old Varian 21EX machine. Since then, several relevant

articles [5] [6] [7] were published that started to verify either the dosimetric or

geometric accuracy of the single iso-multiple mets treatment technique. We will

J. F. Gao, X. Q. Liu

DOI:

10.4236/ijmpcero.2019.81002 11

Int

. J. Medical Physics, Clinical Engineering and Radiation Oncology

discuss these articles later in this article. The standard Winston-Lutz test was re-

viewed by Rowshanfarzad

et al.

[8] and carefully researched by Du

et al.

[9] [10].

Current tests are performed with full gantry, collimator, and couch three-axis

rotation, and a 200-pound weight is used on the couch to mimic real clinical sit-

uations. Four locations, including one at the isocenter and three off from the

isocenter (off-iso), have been investigated. We directly measured the linac robust

specifications instead of its dosimetry delivery accuracy. Our results clearly in-

dicated that the farther the locations from the isocenter, the worse the congru-

ence between the field mechanical center and radiation center. We found that

with our new True Beam STx with high-definition 120 multileaf collimator (HD

120 MLC) linac, the maximum off-iso distance limit from which to perform sin-

gle iso-multiple mets treatment is 6 cm. To make this procedure easier for use in

the user clinical setting, we will elaborate the details of the test. The Eclipse plan

is offered at no charge on request.

2. Materials and Methods

In the linac based SRS/SBRT program, the multileaf collimator (MLC) and Cone

are two radiation beam collimators that conform the beam onto the planning

target volume (PTV) or gross tumor volume (GTV). Therefore, the Wins-

ton-Lutz test should be performed on either the MLC or cone beam collimator,

depending on the clinical need for each individual patient. During the single

iso-multiple mets treatment, the MLC is the only option to date that can be used

to conform the beam onto multiple targets. The Winston-Lutz-Gao test is per-

formed only on the MLC in the Varian product environment in our clinic. A

field size of 1 cm × 1 cm square is created at four different locations where the

off-isodistance from the isocenter (d) is 0, 3, 6, 9 cm (

Figure 1). The field shown

Figure 1 labeled “d = 0 cm” is a symmetric field, and the remaining three

fields are asymmetric fields. All of these squares were generated in the Eclipse

V.13.7 by either a 2.5-mm micro-MLC or a 5-mm MLC. The red mark in the

figure indicates the ISO position. There are several ways to create a 1 cm × 1cm

micro-MLC or MLC square, such as creating a block and then copying the MLC

to the block, or using the MLC Shape tool, or using Shaper directly. However,

the easiest and most practical way to create each square is to type the MLC posi-

tion in the beam MLC property after resetting all MLC positions; an example

case is shown in

Figure 2. We listed in Table 1 those MLC leaf pairs that needed

to be adjusted to generate a corresponding square in

Table 1 (only for a HD 120

MLC system such as the Varian True Beam STx, Edge and Novalis).

d = 0 cm d = 3 cm d = 6 cm d = 9 cm

Figure 1. Off-iso distance between beam mechanical center and linac isocenter.

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