1) The study evaluated setup errors in radiotherapy patients by comparing portal images to reference images (digitally reconstructed radiographs or simulation films). 2) Setup errors were analyzed in two orthogonal directions for anterior and lateral fields in 43 patients receiving pelvic, prostate, abdominal or thoracic radiotherapy. 3) Transfer errors between the planning and treatment positions ranged from 0.3-2.8mm depending on the reference image and treatment site. Using digitally reconstructed radiographs as references resulted in slightly smaller transfer errors compared to simulation films.

1. Comparison of a portal image with reference images (simulation
films or digitally reconstructed radiographs – DRRs)
Scepanovic D, Lederleitner D, Stros I, Griflikova P, Lukacovicova M, Pobijakova M, Lukacko P,
Masarykova A, Dolinska Z, Stehlikova A
National Cancer Institute of Slovakia, Bratislava

2. 1. Background
• The aim of curative radiotherapy
To deliver a high dose of radiation to the tumor tissue; and at the
same time, to keep the dose to the surrounding normal tissues to the
minimum.
• Accurate and routine target localization is necessary for successful
outcome in radiation therapy treatments. Electronic portal imaging
devices (EPIDs) provide an advanced tool with digital technology to
improve target localization and maintain clinical efficiency.

3. 2. Background
• A patient set-up error is the difference between the actual
and intended position of the part of the patient that is irradiated,
with respect to the treatment beam(s) during treatment
• The intended or reference patient position is recorded
on a reference image, being either a simulator image or a
digitally reconstructed radiograph (DRR)
• The process of simulation may introduce
a transfer error (a systematic difference between
the patient’s setup position at CT and at simulator)

4. Purpose
To evaluate set-up errors of radiotherapy patients starting from
the comparison of a portal image with a reference images
(simulation films or DRRs).

5. 1. Material and methods
• From January 2010 to June 2010, the study population included 43
patients (pts) with pelvic, prostate, abdominal and thoracic cancers
• We performed portal imaging control (PIC) several times at the start
of a course of treatment and afterwards on a regular interval (weekly)
• All of pts were CT scanned (5mm slice spacing/width)

6. 2. Material and methods
• DRRs (23pts) and a simulator image (20pts) of an anterior and a
lateral field were used as reference images
• DRRs were generated for 5mm separations

7. 3. Material and methods
• The setup error evaluation was performed using the ELEKTA –
iView software
• Analysis of the precision of the setup error evaluations between the
reference images was performed in the two orthogonal directions of
each field
(left-right [LR], superior-inferior [SI] for the anterior
field; superior-inferior [SI] and antero-posterior [AP] for the lateral
field)

8. 4. Material and methods
• The transfer error was evaluated by calculating the mean
difference in setup error evaluation using either both DRRs
or simulation films as reference images
• DRRs group of pts (23) had 6 median times and simulator image
group of pts (20) 3 times PIC
• 4 MU for the setup verification fields were used

9. 1. Results
2.8 / 5
1.8 / 1.8
LR/SI
SI/AP
AP
lateral
3thoracicSimulator
0.9 / 1
0.3 / 0.3
LR/SI
SI/AP
AP
lateral
5thoracicDRR
2.5 / 1.4
1.1 / 1.2
LR/SI
SI/AP
AP
lateral
3abdominalSimulator
1.8 / 1.5
0.3 / 0.2
LR/SI
SI/AP
AP
lateral
4abdominalDRR
1.8 / 2.4
1.9 / 2.2
LR/SI
SI/AP
AP
lateral
11pelvicSimulator
1.7 / 1.7
1.3 / 0.8
LR/SI
SI/AP
AP
lateral
9pelvicDRR
0.9 / 1.9
1.7 / 0.8
LR/SI
SI/AP
AP
lateral
3prostateSimulator
1.8 / 1.6
0.7 / 0.5
LR/SI
SI/AP
AP
lateral
4prostateDRR
Transfer error
(mm)
DirectionFieldNo of pts.Site of tumorDRRs or simulator

10. 2. Results
• A range of transfer errors from anterior field was
0.9 – 1.7mm (LR),
1 – 1.7mm (SI)
and
from lateral field was
0.3 – 1.3mm (AP),
0.2 – 0.8mm (SI) for DRRs group of pts
• For simulator image group of pts range of transfer errors was 0.9 – 2.8mm (LR),
1.9 – 5mm (SI) – anterior field
and
1.1 – 1.9mm (AP),
1.2 – 2.2mm (SI) – lateral field

11. Conclusions
1. There was a trend for the simulator image group of pts to
achieve less precise results than DRRs group of pts
2. The use of DRRs can reduce the risk of transfer error
3. However, the magnitude of the systematic errors for DRRs
and simulation films groups of pts is comparable