243 / 1082
S230
ESTRO 36 2017
_______________________________________________________________________________________________
The phantom was scanned at different tube potentials (80
kV, 120 kV and 140 kV) with a novel SOMATOM
Confidence® RT Pro scanner (Siemens Healthcare GmbH,
Germany). Images were reconstructed both into HU and
ED for each tube potential. Next, the usability of the
reconstruction algorithm was evaluated in a clinical
workflow. Five patients with an abdominal lesion (e.g.
rectal or prostate cancer) were scanned using the
clinically used tube potential of 120 kV and an additional
dual-spiral dual-energy CT acquisition was made at 80 kV
and 140 kV. Dose distributions (Eclipse
TM
, Varian, USA) of
the ED images of the 80 kV, 120 kV, 140 kV acquisitions
using the novel reconstruction algorithm were then
compared with the clinical plan based on the 120 kV
acquisition using the clinical CT to ED curve with the
standard HU image of the 120 kV scan. The difference in
mean doses delivered to the planning target volume were
quantified (i.e. relative difference ± 1 SD).
Results
The CT to ED conversion curve for the HU images
depended on the tube potential of the CT scanner. The
novel reconstruction algorithm produced ED values that
had a residual from the identity line of -0.1% ± 2.2% for all
inserts and energies and is shown in Figure 1.
The dose distributions between the standard and the novel
reconstruction algorithm were compared for different
energies. The relative differences in target dose ranges
were small and ranged from -0.2% to 0.7% for 80 kV, -0.1%
to 1.1% for 120 kV, and 0.1 to 1.0% for 140 kV.
Figure 1: The linear conversion curve (fitted) of the
novel reconstruction algorithm.
Conclusion
A novel reconstruction algorithm to derive directly
relative electron density irrespective of the tube potential
of the CT scanner was evaluated. A single identity curve
for the CT to ED could be used in the treatment planning
system. This reconstruction algorithm may enhance the
clinical workflow by selecting an optimal tube potential
for the individual patient examination that is not
restricted to the commonly used 120 kV tube potential.
OC-0441 Dose Prescription Function from Tumor
Voxel Dose Response for Adaptive Dose Painting by
Number
D. Yan
1
, S. Chen
2
, G. Wilson
1
, P. Chen
1
, D. Krauss
1
1
Beaumont Health System, Radiation Oncology, Royal
Oak MI, USA
2
Beaumont Health System, Radiation Oncology, Royal
Oak, USA
Purpose or Objective
Dose-painting-by-number (DPbN) needs a novel Dose
Prescription Function (DPF) which provides the optimal
clinical dose to each tumor voxel based on its own dose
response. To obtain the DPF for adaptive DPbN, a voxel-
by-voxel tumor dose response matrix needs to be
constructed during the early treatment course. The study
demonstrated that the voxel-by-voxel tumor dose
response can be quantified and predicted using Tumor
Metabolic Ratio (TMR) matrix obtained during the early
treatment weeks from multiple FDG-PET imaging.
Material and Methods
FDG-PET/CT images of 15 HN cancer patients obtained
pre- and weekly during the treatment were used. TMR was
constructed following voxel-by-voxel deformable image
registration. TMR of each tumor voxel,
v
, was a function
of the pre-treatment SUV and the delivered dose,
d
, such
as TMR(
v
,
d
) = SUV(
v
,
d
)/SUV(
v
, 0). Utilizing all voxel
values of TMR in the controlled tumor group at the last
treatment week, a bounding function between the pre-
treatment SUV and TMR was formed, and applied in early
treatment days for all tumor voxels to model a tumor voxel
control probability (TVCP). At the treatment week
k
, TVCP
of each tumor voxel was constructed based on its pre-
treatment SUV and TMR obtained at the week
k
using the
maximum likelihood estimation on the Poisson TCP model
for all dose levels. The DPF at the week
k
was created
selecting the maximum TVCP at each level of the pre-
treatment SUV and TMR measured at the week
k
. In
addition, 150Gy was used as an upper limit for the target
dose.
Results
TVCP estimated in the early treatment week, i.e. week 2,
had their D
50
=13~65Gy; g
50
= 0.56~1.6 respectively with
respect to TMR = 0.4~1.2; Pre-treatment SUV = 3.5~16.
Figure 1 shows the TVCP estimated using the TMR
measured at the week 2 with different levels of pre-
treatment SUV, as well as TVCP at different weeks, the
week 2 ~ week 4. Large dose will be required to achieve
the same level of tumor control for the same level of TMR
appeared in the later week of treatment. Figure 2 shows
the corresponding DPF for the week 3 TMR, as well as the
prescribed tumor dose distribution for the 3 failures.
Figure
1