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ESTRO 36 2017
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Conclusion
Thanks to this analysis, the steps of increased risk have
been discovered and new quality management tools have
been proposed. However, the multidisciplinary team will
perform this analysis periodically to increase the safety
and quality of TSEI treatment.
EP-1745 EPID and Gantry sag characterization in
Elekta LINAC
F. Tato de las Cuevas
1
, J. Yuste Lopez
1
1
Hosp. Univ. de Canarias, Medical Physics Dept., Santa
Cruz de Tenerife, Spain
Purpose or Objective
The EPID (Electronic Portal Imaging Device) is a well-
known useful tool for LINAC QA (like MLC QA). The position
of the EPID radiation center (
RC
) is crucial for this kind of
tasks. The purpose of this study is analyse the mechanical
performance of EPID and Gantry for LINAC QA with EPID.
Material and Methods
The LINAC used is an Elekta Synergy equipped with Agility
MLC and iViewGT EPID and 6 MV photons energy.
All fields acquired in the study have 20x20 cm size, 5 MU
each. To obtain the RC of the EPID at 0º gantry, four fields
are used, at 0, 90, 270 and 180 º of collimator and 0º
gantry with a radiopaque crosshair attached to the LINAC
head. RC is calculated with two methods: using the
radiation field limits and with the radiopaque crosshair
center.
Second series of measures are acquired with a
BB
(bearing
ball) placed in laser isocenter and with a tray with four
smaller BB fixed in it, in the periphery of the field. Images
are obtained over a 360º arc, with 15 º gantry steps at 0º
collimator angle.
Cross reference of the 4 smaller BB positions with the RC
(determined in the first step) are made at 0º collimator
and 0º gantry. This gives to the 4 BB the ability to
determine RC position in subsequent gantry angles.
EPID sag is calculated for all gantry angles taking into
account the laser isocenter BB position in each EPID image
and compared with 0 º gantry angle. EPID + Gantry sag is
determined taking into account the mean position of the
BB fixed in the tray. The Gantry sag is obtained after
subtraction of EPID sag from EPID + Gantry sag.
Changes in SDD (Source-Detector Distance) are obtained
measuring the distance between two tray BB (d) and
comparing then to the distance (d
0
) for SDD for 0º gantry
angle (SDD
0
) in the way as Eq. reflects.
ΔSDD = SDD
0
· (d/d
0
-1)
A MATLAB in-house software is developed to make the
image analysis. The BBs and the center of radiopaque
crosshair is determined in each direction (in-plane and
cross-plane) with sub-pixel accuracy, 3 profiles near de BB
are obtained and fitted to Gaussian curves, the mean
maximum of the 3 curves is calculated. Radiation field
center is obtained calculating the 50% pixel value of a
vertical and horizontal profile displaced from the center
in case of BB in the image center.
Results
The LINAC measurements take no longer than 2 hours.
The RC for the EPID at 0º gantry obtained with radiopaque
crosshair is 1.11 and -1.02 mm for cross-plane and in-plane
directions, respectively. The RC using radiation field limits
is less than 0.3 mm away from this.
EPID RC is not plotted in Fig. 1 for clarity but is obtained
from the EPID + gantry sag measurements after adding the
RC for 0º gantry angle.
The major change in SDD is less than 1.4 cm (for 180º) from
SDD at 0º gantry angle (see Fig. 2).