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together with iMAR. The delivered dose was measured
with EBT3 Gafchromic films, inserted in three sagittal
planes of the phantom included in the PTV area, and was
compared with the dose calculated on the different CTs
from machine log files. Local dose differences and gamma
maps were used to evaluate the results, taking into
account residual positioning errors, daily machine
dependent uncertainties and film quenching.
Results
We restricted the analyses to the 50% isodose and defined
A
+10%
and A
-10%
as the percentage area having percentage
differences higher (lower) than 10% (-10%). In general,
A
+10%
between calculated and measured dose distributions
were below 10% for plane 1 and 2 with the DE approach
combined with iMAR (Table 1). Maximum differences were
mainly located in the areas of steep dose gradients.
Focusing on the SAFIRE algorithms, the three methods
showed comparable results to the corresponding FBP
algorithms for plane 2 and 3. For plane 1, A
+10%
increased
to 24.8% for uncorrected approach, but SAFIRE was again
comparable to FBP when iMAR is used.
Conclusion
DE combined with iMAR shows potential for predicting SP
values and reducing metal artefacts. However, all
approaches provided comparable, and clinically
acceptable, results in terms of dosimetry accuracy. This
could be related to the uncertainties in the experimental
setup and in the measurements method (mainly use of
gafchromic films), which might be comparable to the
differences introduced by the metal artefacts correction
approaches. The planning approach with multiple fields
was robust against errors introduced by metal implants.
EP-1675 Influence of CT contrast agent on head and
neck VMAT dose distributions
L. Obeid
1
, J. Prunaretty
1
, N. Ailleres
1
, L. Bedos
1
, A.
Morel
1
, S. Simeon
1
, P. Fenoglietto
1
1
Institut Régional du Cancer de Montpellier,
Radiotherapy, Montpellier, France
Purpose or Objective
Intravenous contrast agent injection during the patient CT
simulation facilitates radiotherapy contouring in the case
of head and neck cancers. However, the image contrast
enhancement may introduce discrepancy between the
planned and delivered dose. The aim of this retrospective
study is to quantify the variations of Hounsfield unites
(HU) and to investigate their effect on Volumetric
Modulated Arc Therapy (VMAT) dose distributions.
Material and Methods
Ten patients previously treated by VMAT techniques with
identical dose levels (70/60/50 Gy) were selected. For
each patient, two CT scans were performed, 2 min. (CT
inj
)
and 12 min. (CT
delay
) after Iomeron® 350 biphasic
intravenous injection (60 mL, 1mL/s followed by 90 mL, 2
mL/s after 30 s). The treatment planning (optimization
and calculation) was performed with CT
inj
using the Eclipse
TPS and two calculation algorithms (AAA® and Acuros
XB®). Two other treatment plans were recalculated with
the same parameters and CT
delay
. The mean HU and the
iodine distribution were compared between the two scan
images in the PTV50, the parotids and the thyroid. A
dosimetric comparison using dose-volume histograms in
target volumes and OAR (thyroid, parotids) was
performed. The maximum (D
2%
), minimum (D
98%
) and
median (D
50%
) doses were registered.
Results
The maximum HU average difference over all the patients
was observed in the thyroid (81.37 ± 36.01 HU) followed
by the PTV50 (10.76 ± 15.70 HU) and the parotids (9.39
±16.01 HU). The differences found with the AAA®
algorithm were below 0.1% for D
2%
, D
98%
and D
50%
in target
volumes and between -0.11 and 0.36% in OAR. The
differences observed with Acuros XB® Algorithm were less
than 0.2% in target volumes and 0.31% in OAR. Moreover,
the differences between two algorithms were statistically
insignificant (p > 0.4).
Conclusion
This study shows that the use of intravenous contrast
during CT simulation does not significantly affect dose
calculation in head and neck VMAT plans using AAA and
Acuros XB algorithms.
EP-1676 Comparison of accuracy of Hounsfield units
obtained from pseudo-CT and true CT images
N. Reynaert
1
, P.F. Cleri
1
, J. Laffarguette
1
, B. Demol
1
, C.
Boydev
1
, F. Crop
1
1
Centre Oscar Lambret, PHYSIQUE MEDICALE, Lille,
France
Purpose or Objective
Quality of pseudo-CT (pCT) images used for MRI-only
treatment planning is often evaluated using the so-called
MAE (Mean Average Energy) curve. Furthermore, a
dosimetrical comparison is performed by comparing DVHs
using pCT and true CT (tCT). The tCT is always considered
as the reference, while uncertainties on these images are
neglected. The purpose of the current work is to compare
MAE curves for tCT images by varying different scanning
parameters and to compare the results with uncertainties
on our pCTs.
Material and Methods
A Toshiba Large Bore CT was used. Different IVDT curves
were determined, for different energies (100-135 kV),
FOVs, reconstruction kernel, phantom size, insert
positions, using an in-house phantom, with variable size.
The IVDT curves were used in our in-house Monte Carlo
platform for recalculation of Cyberknife and Tomotherapy
plans. pCT images were generated from MRI images (3D T1
sequence) using an atlas-based method. Image quality was
determined using MAE, ME and gamma curves.
Results
Three parameters for tCT had an important impact on the
HUs, namely the energy, patient size and reconstruction
kernel. These parameters individually modified image
values with up to 300 HUs in bone inserts. Furthermore,
patient size and energy are often correlated as, it is
specifically for small patients that lower energies are
used, both leading to higher HUs in bone. The impact of
the reconstruction kernel was a surprise (e.g. comparing
the FC64 and FC13). For the energy and the reconstruction
kernel one can consider introducing specific IVDTs. It
becomes more complicated when the IVDT should be
modified as a function of patient diameter though.
Furthermore, in some TPSs (e.g. Masterplan, Nucletron)
only one predefined IVDT is used. Another important
problem is the fact that the HUs in the air surrounding the
patient are increased when using large phantom sizes
(changing from -1000 HU to -910 HU). Depending on the
IVDT, this can lead to a largely overestimated air density
around the patient (0.2 g/cm
3
) with a possible dosimetric