ObjectiveOne of the major developments in UK hospitals is a national programme for

integrating IT systems. Part of this, is the full roll-out of PACS/digital imaging to all radiology and radiotherapy departments which involved moving simulator and film-based portal imaging onto a Computed Radiography (CR) platform. The Kodak 2000RT CR system (figure 1) was chosen, since it gives excellent clinical images, but also has an established package for IMRT type dosimetry with small (24 x 30cm) plates[1]. We wanted to gain maximum benefit from the system by examining how it could be used as a simple film substitute for performing standard linear accelerator quality control (QC). However many standard QC fields need a large (> 30 x 40 cm) imaging area[2]. EPIDs, with their smaller active field of view, may be used, but the fields must be subdivided. This is impractical for CR, where plates still require ‘processing’ before the image is available digitally. This paper describes our experiences with commissioning and implementing large (35 x 43 cm) CR plates for this purpose.

Methods and MaterialsWe examined the properties of three (35x43 cm) Agfa CR imaging plates

(model MD10) processed with the Kodak Radiation Oncology Beam Dosimetry package (v1.0) for a variety of QC procedures on our six Elekta (Elekta Ltd, Crawley, UK) linacs. The QC procedures included fields (figure 2) for examining MLC alignment, centring and magnification, X-ray/light field congruence for both leaves and backup collimators, multisegment fields for MLC leaf and backup collimator calibration (for both standard 1 cm and 0.4 cm MLCs). Experiments were conducted as simultaneous dmax, isocentric irradiation of CR plates and film (X-omat-V and EDR2, Kodak, Rochester, NY), for 6MV X-rays. Exposures were made to examine dose density response, uniformity, reproducibility of scaling and signal decay with time. Two scanning resolutions were investigated - 1024 and 2048 pixels, although most scans were performed at the lower resolution. RIT 113 (v4.4) (RIT Inc., Colorado Springs, CO) software was used for analysis using principally its ROI and line profiles functionality (figure 3). Films were analysed using current methods of manual densitometer or RIT software (for the Bayouth test only).

Results and DiscussionAll plates showed a linear response of scanner unit with dose (with no more

than 5% variation between the three). However, a definite inflection point is present and reproducible at approx 1 – 1.2 Gy (figure 4). All further exposures were made in the midrange of these linear portions. The decay rate post irradiation was found to be faster than detailed in the literature, but also varied between the plates studied (table 1). Constant 10 min post-irradiation readout times were consequently used for QC field comparisons. The plates showed considerable non-uniformity of response, although the shape of the response was similar between plates (figure 5). The RIT software does contain a uniformity correction routine that utilises acquired images but the results described below were obtained without uniformity correcting the images. Scaling was examined using a 2D array of ball bearings in a Perspex sheet, and found to be consistent between plates to within 1mm across the full 43 cm length of the plates. Reproducibility of scaling was excellent with variations of just 0.16% (H) and 0.20% (V) (2 SDs). However, scaling factors were not handled correctly for data transfer to the RIT software. For the vast majority of exposures, analysis of line profiles of the CR images matched densitometer based manual measurements of the films to within 1mm (figure 6). Many results were well within 0.5 mm, especially for 2048 resolution Bayouth Test analysis, which used similar, electronic methods of analysis.

ConclusionWe have found that CR imaging with large plates may be used successfully

for a wide range of quality control checks for megavoltage X-ray fields from linear accelerators. The system is easy to use, giving acceptable results even without uniformity correction. Data is easily imported and analysed in commercial dosimetry software such as RIT 113 and shows that CR is an ideal complement to our aSi EPIDs for quality control.

References[1] Olch A 2007 “Evaluation of a computed radiography system for megavoltage

photon beam dosimetry” Med. Phys. 32(5) 2987-2999[2] Kirby MC 2006. Ch 8 in Institute of Physics and Engineering in Medicine

(IPEM) Report 94 Acceptance Testing and Commissioning of Linear Accelerators (IPEM, York, UK) (www.ipem.ac.uk)

Email correspondence: imran.patel@lthtr.nhs.uk

The use of Computed Radiography for Megavoltage Linear Accelerator Quality Control

I Patel, T Natarajan and MC KirbyNorth Western Medical Physics, Radiotherapy Department,

Rosemere Cancer Centre, Royal Preston Hospital, Preston, UK

Figure 1: Photo showing the Kodak 2000RT CR Plus System (courtesy of Carestream Health, Rochester, NY)

Figure 4: Dose response of large Agfa CR plates (log-linear scale) –

response from 3 of the same type of plates shown

Figure 3: Screenshot of RIT113 software used to obtain profiles from QC images

Table 1: Decay rate of scanner unit values with time delay between irradiation

and readout.

Presented at 49th Annual ASTRO Meeting, Los Angeles, 28th Oct 2007 – 1st Nov 2007

Figure 2: QC procedure images for; (a) MLC alignment, (b) Leaf bank centring and magnification, (c) MLC calibration (known as Bayouth test), and, (d) MLC leaf and backup collimator calibration

(c) (d)

(a) (b)

Figure 5: Spatial uniformity of response for CR plates (a) horizontal direction, and,

(b) vertical direction.

(b)

(a)

Figure 6: Histogram plot of spatial differences found analysing line profiles

from CR images and densitometer based measurements from film.

-1.01.4

3.63.73.9

Plate 1

Decay rate (SU/Min)

--

-7.67.4

Plate 2

-0.848-3.960

1.9362.0245.012

Plate 3

Time Delay (Min)