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

1. Background

• The aim of curative radiotherapyTo 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.

2. Background

• A patient set-up error is the difference between the actualand 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 adigitally reconstructed radiograph (DRR)

• The process of simulation may introducea transfer error (a systematic difference between

the patient’s setup position at CT and at simulator)

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).

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)

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

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 thereference 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)

4. Material and methods

• The transfer error was evaluated by calculating the meandifference 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

1. Results

2.8 / 51.8 / 1.8

LR/SISI/AP

APlateral

3thoracicSimulator

0.9 / 10.3 / 0.3

LR/SISI/AP

APlateral

5thoracicDRR

2.5 / 1.41.1 / 1.2

LR/SISI/AP

APlateral

3abdominalSimulator

1.8 / 1.50.3 / 0.2

LR/SISI/AP

APlateral

4abdominalDRR

1.8 / 2.41.9 / 2.2

LR/SISI/AP

APlateral

11pelvicSimulator

1.7 / 1.71.3 / 0.8

LR/SISI/AP

APlateral

9pelvicDRR

0.9 / 1.91.7 / 0.8

LR/SISI/AP

APlateral

3prostateSimulator

1.8 / 1.60.7 / 0.5

LR/SISI/AP

APlateral

4prostateDRR

Transfer error (mm)

DirectionFieldNo of pts.Site of tumorDRRs or simulator

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

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 DRRsand simulation films groups of pts is comparable