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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