ESTRO 36 Abstract Book

S232

ESTRO 36

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Results

The average beam-on delivery time was 3min, ranging

from 1.22min to 8.82min. The average

(3%,3mm) passing

rate for film measurements was 97.0 ±1.6% (range from

93.3 to 98.8%), while the Delta

measurements presented

an average

(2%,2mm) of 97.7±1.4% (range from 95.3 to

99.6%) respectively. The average isocentre point dose

ratio was 99.9±1.2% (range from 98.0 to 102.8%). For the

lung SBRT verifications with the CIRS phantom, an average

local

of 97.0±1,0% and 93.1±2.0% was obtained during

the coronal and sagittal film analysis, whereas the average

isocentre point dose ratio was 100.0±1.4%. An overall

mean gantry-ring geometric deviation of 0.04° ± 0.46° and

0.19° ± 0.26° was obtained, respectively.

Conclusion

DWA has been successfully added to the non-coplanar

rotational IMRT techniques’ arsenal, allowing additional

flexibility in dose shaping while preserving dosimetrically

robust delivery. In a short period of time, it has become a

standard treatment technique on the Vero system in our

department.

OC-0440 Characterization and clinical evaluation of a

novel CT reconstruction to derive electron densities

B. Van der Heyden

, M. Ollers

, C. Loon Ong

, F.

Verhaegen

, W. Van Elmpt

School for Oncology and Developmental Biology-

Maastricht University Medical Centre, Department of

Radiation Oncology MAASTRO- GROW, Maastricht, The

Netherlands

Purpose or Objective

Radiotherapy dose calculations are almost exclusively

performed on CT images. In a clinical workflow, the

Hounsfield Units (HU) are converted into electron density

(ED) by using a CT to ED conversion curve calibrated for a

typically fixed tube potential (e.g. 120 kV). Recently, a

novel CT image reconstruction algorithm (DirectDensity

Siemens Healthcare GmbH, Germany) was developed that

directly reconstructs the ED, independent of the tube

potential of the CT scanner. This allows the elimination of

a conversion curve for each tube potential. Our study

describes the accuracy in terms of dose calculation of the

reconstruction

algorithm

based

phantom

measurements and shows the application in a clinical

radiation therapy workflow for different tube potentials.

Material and Methods

The accuracy of the novel reconstruction algorithm to

derive ED was validated in a Gammex RMI 467 phantom

(Gammex, USA) using different tissue mimicking inserts.

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

, 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

, S. Chen

, G. Wilson

, P. Chen

, D. Krauss

Beaumont Health System, Radiation Oncology, Royal

Oak MI, USA

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