ESTRO 36 Abstract Book

S887

ESTRO 36 2017

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posture at the linac is slightly in better

agreement with the posture at the CT for

absolute couch positioning.

On the ML images, a residual deviation in lung

wall position of 5 mm or more was present for

5.1% of the fractions for relative couch

positioning and for 2.7% for absolute couch

positioning. This also implies a slightly improved

patient setup using absolute couch positioning.

Conclusion

The patient posture as well as patient setup for breast

cancer patients is slightly improved using the predicted,

or absolute, couch position.

W.J. de Kruijf, R.J. Martens, Reducing patient posture

variability using the predicted couch position. Med.

Dosim, 40:218-21; 2015.

EP-1656 The inter-fraction variation of the

supraclavicular- and the axilla-area in breast cancer

patients

S. Gerrets

, L. Kroon- van der

, M. Buijs

, P. Remeijer

Netherlands Cancer Institute Antoni van Leeuwenhoek

Hospital, Radiotherapy, Amsterdam, The Netherlands

Purpose or Objective

Currently a volumetric modulated arc technique (VMAT)

for whole breast irradiation, including the supraclavicular-

and axilla-area is being implemented at our institute. In

contrast to our currently used tangential fields, VMAT

requires a CTV and PTV with corresponding margins. From

our clinical experience we know that the setup of the

shoulder can be very challenging..The purpose of this

study was to quantify the inter-fractional variation of the

supraclavicular- and the axilla-area in order to quantify

CTV to PTV margins.

Material and Methods

So far 6 right sided and 6 left sided breast cancer patients,

were randomly selected in this ongoing study. Patients

were positioned on a Macromedics MBLXI breastboard with

upper- and lower- arm trays. During the acquisition of the

planning CT skin marks were drawn extended to the

humerus to improve reproducibility of the arm positioning.

Setup verification and correction was performed based on

bony anatomy registration (ribs and sternum) using Cone

beam CT and an offline shrinking action level (SAL)

protocol. Retrospectively, the residual inter-fraction

errors of the supraclavicular area and the axilla were

measured by performing bony anatomy registrations using

a rectangular region of interest representative for these

areas/regions (see Figure 1 'Region of interest”), and

determining their difference from the registration on ribs

and sternum. From these residual errors, the random and

systematic errors were computed and corrected for the

use of a SAL protocol (N=3 and α=9mm). Using previously

determined setup data from Topolnjak et al [1],

Subsequently, the CTV to PTV margins were determined

according to the standard margin recipe: 2.5∑+0.7σ.

Results

In total 88 Cone beam CT were analyzed; 5-10 scans per

patient. Computed residual errors for the supraclavicular

region and axilla region are shown in Table 1.The random

and systematic residual errors for the axilla regions are

larger than the supraclavicular region, as expected.

Notable is the small residual error for supraclavicular in

LR-direction. The total margins are 0.59cm LR, 0.76cm CC

and 0.81cm AP for the supraclavicular region and 0.84cm

LR, 0.89cm CC and 0.98cm AP for axilla region.

Conclusion

For the introduction of a VMAT planning technique for

breast and axilla irradiation specific PTV margins adapted

for supraclavicular and axilla inter-fraction motion need

to be introduced.

Topolnjak, et al, IJROBP, Volume 78, Issue 4, 15

November 2010, Pages 1235–1243

EP-1657 Clinical use of transit dosimetry to analyze

inter-fraction motion errors

F. Ebrahimi Tazehmahalleh

, C. Moustakis

, U.

Haverkamp

, H.T. Eich

Universitaetsklinikum Muenster, Klinik fuer

Strahlentherapie, Münster, Germany

Purpose or Objective

The aim of this work was to inquire the correlation

between the target and organ at risks motions and volume

discrepancy with the dosimetric variations at hospital

levels. The high resolution, large active area, and

effectiveness of the Electronic portal imaging devices

offers it to be used for in vivo dosimetry during radiation

therapy as an additional dose delivery check. The transit

dosimetry has the potential of testifying dose delivery, the

accuracy of MLC leaf positioning and the calculation of

dose to a patient or phantom.

Material and Methods

In total 42 patients with stereotactic plans were

evaluated. Delivery was carried out on a Varian TrueBeam

linac equipped with an aS1000 EPID. Continuous portal

imaging was performed at each treatment fraction during

the delivery of treatment for all beams. To validate the

method, we executed treatment plans on a commercial

respiratory motion phantom containing plastic spheres as

target. Phantom CT scans were made in different phases.

First phase were done by applying sinusoidal breathing

cycle in different motion amplitudes (-20, -10, 0, 10, 20

mm) in superior/inferior direction and second phase was

done by pre-defined breathing simulation with a short

pause after exhalation in oscillation mode. Three

techniques: 3D-CRT, IMRT and VMAT-SBRT were generated

and on board transit dose was collected by EPID during the

treatment. The daily obtained portal image were

compared with the reference image using the gamma

evaluation method with criterion 2% dose difference and

2 mm distance to agreement (DTA) criteria with a

threshold value of 5% of maximum value.

Results

The area gamma passing rate per arc in most of the plans

was higher than the acceptable limit but in some arcs it

had lower agreement, the lowest value was 3.7%. Besides

irradiating phantom in planned respiratory motion, we re-

irradiated the same plans due to displacement of the

target by stopping the movement or changing the

breathing speed. Gamma parameters such as maximum

gamma, average gamma, and percentage of the field area

with a gamma value>1.0 were analyzed. For all the VMAT

arcs in phantom measurements, the gamma evaluations

were within the tolerance limits (γmax = 3.5, γavg = 0.5

and γ% >1 = 2%) tough in some measurement 20 mm target

displacement was applied. For IMRT fields, measurements

were not in good agreement in different tumor motion.

3DCRT fields showed poorest gamma agreement in portal

dosimetry analysis.

Conclusion

This research increases the need of a tool for monitoring

inter-fraction errors by confirming the tumor position

within the treatment field over the course of therapy.

Using daily EPID images over the course of treatment could

potentially provide accurate verification of dose delivery

to heterogeneous anatomical regions in patients receiving

3D-CRT and IMRT radiation therapy treatments. However,

further studies are required to assess 3D IN VIVO dose