ESTRO 36 Abstract Book

S986

ESTRO 36 2017

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Varian Standard Couch(VSC) by modeling with Treatment

Planning System (TPS)

Material and Methods

VSC was scanned by CBCT (Cone Beam Computed

Tomography) of the Linac(Clinac IX, VARIAN, USA),

following the three conditions of VSC, Side Rail Out

Grid(SROG), Side Rail In Grid(SRIG), Side Rail In Out Spine

Down Bar(SRIOS). After scan, the data was transferred to

TPS and modeled by contouring Side Rail, Side Bar Upper,

Side Bar Lower, Spine Down Bar automatically.

We scanned the Cheese Phantom(Middelton, USA) using

Computed Tomography(Light Speed RT 16, GE, USA) and

transfer the data to TPS, and apply VSC modeled

previously

with

TPS

it.

Dose was measured at the isocenter of Ion Chamber(A1SL,

Standard imaging, USA) in Cheese Phantom using 4 and 10

MV radiation for every 5° gantry angle in a different filed

size(3X3cm², 10X10cm²) without any change of MU(=100),

and then we compared the calculated dose and measured

dose. Also we included dose at the 127° in SRIG to

compare the attenuation by Side Bar Upper.

Results

The density of VSC by CBCT in TPS was 0.9g/cm³, and in

the case of Spine Down Bar, it was 0.7g/cm³. The radiation

was attenuated by 17.49%, 16.49%, 8.54%, and 7.59% at

the Side Rail, Side Bar Upper, Side Bar Lower, and Spine

Down Bar. For the accuracy of modeling, calculated dose

and measured dose were compared. The average error was

1.13% and the maximum error was 1.98% at the 170°beam

crossing the Spine Down Bar.

Conclusion

To evaluate the usability for the VSC modeled by TPS, the

maximum error was 1.98% as a result of compassion

between calculated dose and measured dose. We found

out that VSC modeling helped expect the dose, so we think

that it will be helpful for the more accurate treatment.

EP-1826 Analysis of dose distribution with change of

the air gap when proton therapy using line scanning

S. Seo

samsung medical center, radiation oncology, Seoul,

Korea Republic of

Purpose or Objective

When proton therapy of cranio-spinal irradiation (with

prone position) using line scanning technique, there is a

motion of spinal code caused by breathing. According to

our clinical experience, we could find out the length of

breathing motion is up to 20mm. Because of this motion,

the air gap(defined as the distance from nozzle to surface)

could be changed. In this study, we are going to find out

the target dose distribution in various air gap.

Material and Methods

Sumitomo proton therapy machine(SHI, JAPAN) and

robotic couch(Forte, USA) have been used for this study.

CT scans were performed using GE Discovery CT-590 RT.

In order to measure the target dose, glass dosimeter(AGC

Techno, JAPAN) and atom phantom(Norflok, CIRS, USA)

were used. For treatment planning, Ray Station(Ray

search ver. 5.0, USA) has been used. Mean and D95 were

analyzed shifted points from isocenter in PTV of lower

spine which has significant effect from patient’s

respiratory motion. The shifted pitch was ±10mm, ±20mm.

Results

As a result of analysis, D95 dose at each depth are as

follows:

iso-center:

2347.4(100%),

iso-10mm:

2302.3(98.1%), iso+10mm: 2341.9(99.7%), iso-20mm:

2281.4(97.2%), iso+20mm: 2361.7(100.6%). And mean dose

at each depth are as follows: iso-center: 2389.2(100%),

iso-10mm: 2355.1(98.6%), iso+10mm: 2394.7(100.2%), iso-

20mm: 2335.3(97.7%), iso+20mm: 2415.6(101.1%).

Conclusion

The main purpose of this study is to confirm that the air

gap changes may affect the target dose or not when

proton therapy using line scanning. The difference of

prescribed dose due to a change of air gap is only D95: -

2.8% ~ 0.6%, mean: -2.3% to 1.1%. So even though the air

gap changes, it dose not affect to target dose. Therefore,

line scanning proton therapy can be seen clinically useful.

EP-1827 Dosimetric comparison of 3D-CRT, IMRT and

VMAT for bilateral breast irradiation

H.M. Hung

Princess Margaret Hospital, Department of Oncology-

Radiotherapy Centre, Kowloon, Hong Kong SAR China

Purpose or Objective

Breast cancer is the most common cancer worldwide

amongst females and ranked the 5th cause of cancer death

in 2012. There were 1.67 million new breast cancer cases

diagnosed, and contributed to more than 25% of the total

number of new cases of cancer diagnosed. The incidence

rate of female breast cancer in Hong Kong has been tripled

in the past twenty years. Some of the patients may

suffered from synchronous bilateral breast cancer which

involved a more complex radiotherapy planning as both

lungs, the heart and a large irradiated volume are

involved. The aim of this study was to evaluate the

dosimetric difference between three breast irradiation

techniques: 3-Dimensional Conformal Radiotherapy (3D-

CRT), Intensity-modulated Radiotherapy (IMRT) and

Volumetric-modulated Arc Therapy (VMAT) on bilateral

breast radiotherapy.

Material and Methods

20 breast cancer patients were previously treated with

adjuvant radiotherapy, with the prescription of 50Gy over

25 daily fractions to 100% isodose level. 3-dimensional CT-

based treatment planning of the three bilateral breast

irradiation techniques was performed and all plans were

calculated by Acuros External Beam (AXB) algorithm. The

cardiac dose, lung dose, conformity, homogeneity, low

dose spillage, the overall planning, simulation and

treatment time were analyzed and compared.

Results

IMRT and VMAT significantly improved the conformity to

the Planning Target Volume (PTV) and increased the

volume of PTV covered with 50Gy. This effect was clearly

illustrated in the region of the lateral field edges since it

is difficult to achieve in 3D-CRT due to the dose building

up effect. The lung volume received 20Gy in IMRT and

VMAT were higher than that of 3D-CRT but the result was

found to be not significant. The treatment time in terms

of number of monitor unit (MU) was found to be

significantly lower in 3D-CRT but the time required for

setting up the patient to the treatment position was

almost doubled to other two techniques.

Conclusion

IMRT and VMRT are well-accepted techniques and able to

provide encouraging dosimetric results for bilateral breast

irradiation. VMAT could further reduce the overall

treatment time because of the simple setup procedure

and the relatively small number of MU. This study

suggested that VMAT technique is feasible and is

recommended for bilateral breast irradiation.

EP-1828 Mean Dose in healthy lung for chest tumors

treated with Stereotactic Body Radiation Therapy

(SBRT)

J.l.Monroy Anton

, M. Asensio Martinez

, V. Borreda

Talon

, J. Hernandis Barbera

, M. Marco Collado

Hospital universitario de la ribera, radiation oncology,

alzira, spain

hospital universitario de la ribera, radiation protection

and radiophysics dpt, Alzira, Spain

Purpose or Objective

Stereotactic body radiotherapy (SBRT) is considered

treatment of choice in patients with pulmonary lesions not

candidates for surgical treatment.

High doses of radiation, both total and per fraction