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size, multileaf collimator (MLC) check and determination of isocentric shift for stereotactic radiosurgery (SRS) were performed and compared with film.
Australasian Physical & Engineering Sciences in Medicine Volume 31 Number 3, 2008

TECHNICAL REPORT

An experimental study on using a diagnostic computed radiography system as a quality assurance tool in radiotherapy T. Peace1, B. Subramanian1,2 and P. Ravindran1

1

Department of Radiotherapy, Christian Medical College, Vellore, India Presently at Physics/Engineering Dept, Radiation Oncology Victoria, East Melbourne, Australia

2

Abstract The advent of improved digital imaging modalities in diagnostic and therapy is fast making conventional films a nonexistent entity. However, several radiotherapy centers still persist with film for performing quality assurance (QA) tests. This paper investigates the feasibility of using a diagnostic computed radiography (CR) system as a QA tool in radiotherapy. QA tests such as light field congruence, field size verification, determination of radiation isocentre size, multileaf collimator (MLC) check and determination of isocentric shift for stereotactic radiosurgery (SRS) were performed and compared with film. The maximum variation observed between CR and film was 0.4 mm for field size verification, -0.13 mm for the radiation isocentre size check, 0.77 for MLC check and -0.1 mm for isocentric shift using the Winston Lutz test tool for SRS QA. From these results obtained with the CR it is concluded that a diagnostic CR system can be an excellent cost-effective digital alternative to therapy film as a tool for QA in radiotherapy.

used for quality assurance in radiotherapy. Films have several disadvantages such as cost and maintenance of developing chemicals and processor, non-reusability and storage, when compared to digital imaging. Megavoltage imaging has poor subject contrast due to the Compton effect1. Unlike analog radiographic films, digital images can be manipulated to improve their contrast. Computed radiography (CR) is a digital imaging system which has been used for diagnostic imaging since 19832. It was introduced into radiotherapy in the late eighties when it was first used for megavoltage portal imaging3,4. The possibility of digital post processing of CR overcomes poor tissue contrast which was the major limitation of conventional radiographic portal film5. CR using photostimulable phosphor (PSP) plates has several advantages. It proves to be very economical as it saves running costs compared to film and it requires a reduced initial investment compared to the electronic portal imaging device (EPID). There have been a few studies in which EPID has been used as a tool for quality assurance6-8 . However, the physical similarity of the PSP plate to film gives it an edge over the EPID9 which is also limited by its radiosensitive electronics. One CR system can be used for several teletherapy units. Its rugged nature allows it to be reused several thousand times. The digital nature of the CR images allows contrast enhancement, easy transfer, archival and viewing through

computed radiography, photostimulable phosphor plate, quality assurance, radiation isocentre size, Winston-Lutz alignment tool

Key words

Introduction Quality assurance (QA) plays a very important role in ensuring that the treatment will be delivered to the patient as intended. Modern treatment delivery techniques such as three-dimensional conformal radiotherapy (3DCRT), intensity modulated radiation therapy (IMRT) and stereotactic radiosurgery (SRS) aim at delivering maximum radiation dose in a highly conformal manner to the tumour and minimum dose to the surrounding normal tissues. Hence various QA tests are to be performed on the linear accelerator in order to ensure accurate treatment delivery. Many of these tests require the use of therapy verification films for routine QA. Though many diagnostic and therapy departments are rapidly switching over to digital imaging options, conventional therapy films continue to be widely Corresponding author: Timothy Peace, Department of Radiotherapy, Christian Medical College, Vellore 632 004, India Tel: +91 – 416 – 2282477, Fax: +91 – 416 - 2235555 Email: [email protected] Received: 17 September 2007; Accepted: 27 July 2008 Copyright © 2008 ACPSEM

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each at isocentre. In this work, the 6 MV beam was used for performing all the experiments. For stereotactic radiosurgery (SRS) QA procedures, micro multileaf collimator (mMLC, BrainLab, USA) consisting of 26 pairs of leaves was used. The central 14 pairs had a leaf-width of 3 mm each, the next 6 outer pairs had a leaf-width of 4.5 mm each while the outermost 6 pairs had a leaf-width of 5.5 mm each at isocentre.

the picture archival communication system (PACS) network, effortless storage of large number of images and accurate image analysis. Though CR has been a well established modality in our institution for diagnostic imaging, only recently successful attempts have been made to use it for megavoltage images which exhibited good soft-tissue contrast. This paper discusses the feasibility of using a diagnostic CR system as a QA tool for radiotherapy.

Film dosimetry The therapy films that were used for comparison were X-OMAT V films (Kodak, USA). The films were developed using an XOMAT 2000 automatic film processor (Kodak, USA) and scanned using a Vidar film scanner (Vidar, USA). 30 MU was given per exposure for film.

Theoretical background CR is based on the principle of photostimulable luminescence. It makes use of PSP plates made up of Ba.F.Br doped with Eu2+. When exposed to ionizing radiation, the energy absorbed by the plate is stored in quasi-stable electronic states5. A latent image proportional to the ionizing radiation absorbed is formed. When the plate is scanned within the CR digitizer by a focussed He-Ne laser beam after irradiation, the trapped electrons are locally excited and recombine with the trapped holes causing the emission of light10. The intensity of light emitted is proportional to the locally absorbed radiation dose. This light is collected by a photomultiplier tube (PMT) which converts the light into an electrical signal with a voltage proportional to the amount of light received. Since the read out removes only 80-90 % of the latent image, the plate is exposed to a high intensity white light produced by ten 100 W halogen bulbs kept within the CR digitizer itself.

Image analysis The images obtained with the CR for quality assurance tests were analysed using ImageJ11. This software is a public domain, Java-based image processing program designed with an open architecture that provides extensibility via Java plug-ins and recordable macros. This was used to plot the beam profiles, determine the full width at half maximum (FWHM) and perform other measurements for the different QA tests. Calibration of CR The calibration of CR was performed to relate the change in pixel values with different dose levels. The different dose levels on the detector could be obtained in three ways viz., changing the source to detector distance (SDD), using different monitor units or using partial transmission blocks. Since the CR system used in this preliminary study was a diagnostic one, with low dose latitude, which can only be irradiated to a maximum of 1 MU, the third method was adopted. An in-house, waterfilled, partial transmission, penta step-phantom of perspex, shown in figure 1a, was fabricated. Each step was 5 cm deep. The phantom was positioned on the couch such that the isocentre passed through its centre at a depth of 12.5 cm. The PSP plate was placed below the couch on a trolley at SDD of 150 cm as shown in figure 1b and a CR image was acquired. The next step in the calibration procedure was to determine the dose below each step. For this a 21 borepolystyrene slab with dimensions 26 x 21 x 1.5 cm3 with each bore measuring 12 cm length and 0.8 cm diameter was used to hold the ion chamber in different positions as shown in figure 2a. With the same setup, the PSP plate was replaced by the polystyrene slab as shown in figure 2b. During dose measurement, all the bores were plugged with solid polystyrene cylinders except the one in which the chamber was to be placed. The dose profile below the stepphantom was acquired with a 0.14 cc ion chamber and electrometer (Capintec, USA) by measuring the dose at the 21 positions of the polystyrene slab. In order to accurately obtain the pixel values at the 21 positions where dose was measured, another CR image was acquired in the following manner.With the phantom in the same position, the PSP plate was placed beneath the polystyrene slab. The ion

Materials and methods Computed Radiography system Two CR cassettes (MD40, Agfa - Gaevert, Belgium) of size 14” x 17” were used to perform the experiments in this study. Each PSP plate had a matrix size of 2320 x 2828 pixels with a pixel size of 150 μm. For all CR experiments, the PSP plate was placed inside a black polythene cover and 1 MU was given per exposure. No build up was used in order to avoid the PSP plate getting saturated. Post irradiation, it was replaced into the cassette and fed into a multi-feed CR 75.0 digitizer (Agfa - Gaevert, Belgium). The PMT gain of the digitizer was preset to high sensitivity since it was primarily dedicated for diagnostic imaging and could not be changed. The read out time was approximately 45 seconds. The images which were in DICOM format had a file size of 12,816 kilobytes each and were archived in an exclusive radiotherapy PACS network. This PACS network is also being used to transfer, view and archive digital portal images acquired with PSP plates in order to verify the treatment setup of the patients undergoing 3DCRT and IMRT. Linear accelerator The QA tests were performed on the dual-energy Primus linear accelerator (Siemens, USA) capable of delivering 6 and 15 MV photons and 4 MeV to 21 MeV electrons. The accelerator fitted an MLC consisted of 29 pairs of leaves with 27 pairs having a leaf-width of 1 cm each and the 2 outermost pairs with leaf-width of 6.5 cm 227

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A

B

C

(a)

(b)

Figure 1. (a) Partial transmission penta step-phantom, (b) schematic diagram of setup for acquisition of phantom image with CR. A-Linac head, B- Phantom with cross-wire indicating the isocentre and C-PSP plate.

A

B C

C

(a)

(b)

Figure 2. (a) Polystyrene slab, (b) Schematic diagram of setup for dose measurement. A-Linac head, B- Phantom with cross-wire indicating the isocentre and C-Polystyrene slab with ion chamber in one bore and the rest plugged.

chamber was not used and all the bores were unplugged to visualize them by improving the contrast on the acquired CR image. Thereby a CR image depicting the phantom overlying the bores was acquired as shown in figure 3.

Using ImageJ, measurement of the pixel grey values (averaged over an ROI of 8 x 10 mm2) corresponding to the 21 ion chamber positions on the step-phantom image acquired was done (as shown in figure 3) after background subtraction. The average dose and pixel values under each 228

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step were determined and the dose-pixel calibration graph was plotted. The calibration graphs were used for the verification of field size and multileaf collimator check as they were dosimetric test. For these tests, the grey values obtained after background subtraction were converted into dose using the dose-pixel calibration graphs for both CR and film. Calibration of film An X-OMAT V film was irradiated with six 5 x 5 cm2 fields with MU settings of 5, 10, 20, 30, 40 and 50 respectively at SDD of 100 cm as shown in figure 4. The doses in each field were determined using a 0.6 cc ion chamber and electrometer (Capintec, USA). The pixel grey values obtained after background subtraction were determined by measuring the average value within an ROI (10 x 10 mm2) and the calibration graph was plotted. Quality Assurance tests Light field congruence The light field congruence test was performed to determine the deviation between the light beam and the xray beam. A QA test tool was designed for this purpose. A 10 x 10 cm2 square was grooved on a 15 x 15 cm2 sheet of 3 mm thick perspex and lead wires of 1 mm diameter were glued inside the grooves. The PSP plate was placed at SDD of 100 cm, centred using the reticle and irradiated to a field size of 10 x 10 cm2. The same procedure was repeated for film.

Figure 3. The CR image of the step phantom shows decrease in attenuation of radiation from the top-most step (thickest) with decrease in thickness. The rectangular outline of the 21 bores and the ROIs (8 x 10 cm2) depicting the active area of the ion chamber have been physically marked on the image.

Verification of field size The radiation field size in both the in-plane and the cross-plane directions was determined from the (FWHM) of the beam profile which was obtained by the following procedure. The PSP plate was placed at a depth of 8.5 cm of perspex (10 cm water equivalent) at source to surface distance (SSD) 100 cm. A CR image was acquired for the 10 x 10 cm2 field and repeated with film. The beam profiles were plotted using ImageJ. Radiation isocentre size check Star shot tests were employed to determine the radiation isocentre wandering due to shifts in three parameters viz., collimator, gantry and couch. The PSP plate was placed perpendicular to the beam for shift measurements of collimator and couch and parallel to the beam for the gantry shift measurements with SDD maintained at 100 cm for the three experiments. The jaws were opened to a field size of 0.2 x 40 cm2. For each test, the measurements were done by varying the parameter of interest through fixed angles of 0o, 45o, 90o and 135o while maintaining the other two parameters at 0o. A straight line was drawn on the resultant image through the centre of each spoke of the magnified star shot as shown in figure 5b and the intersection points were marked. The smallest circle that enclosed all these points of intersection was drawn as shown in figure 6. The radius R of this circle

Figure 4. 6 fields were irradiated with 5, 10, 20, 30, 40 and 50 MU respectively for film calibration.

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represented the radiation isocentre size12. The procedure was repeated for film and the results were compared with that of CR. Multileaf collimator check Feasibity test using CR for checking the positional accuracy of the MLC leaves was carried out. A diamond shaped treatment field test plan was created with Plato treatment planning system (Nucletron, Holland) on the CT images of a head phantom. The leaves were made to align in steps of 1 cm with the maximum opening at the centre measuring 10 cm and tapering to a size of 2 cm on either side. The MLC leaf positions were verified from the widths of the leaf openings which were determined using ImageJ from the FWHM of the beam profiles obtained. The plan was also irradiated on film for comparison.

(a)

Isocentric shift for stereotactic radiosurgery QA is performed before every SRS procedure due to its increased complexity of delivery technique. The WinstonLutz Alignment QA test tool consists of a lead ball of diameter 6 mm, fixed to the centre of a localizer box. It was positioned at the isocentre of the linear accelerator. The field size was opened to 1.2 x 1.2 cm2. The test was performed for gantry angles 0°, 90° and 270° with couch angle 0°. Then with the gantry angle 0°, the test was performed for two couch angles 90° and 270°. Film placement beneath the test tool was achieved by placing a rectangular strip of film within the box below the lead ball for the gantry at 0° and securing it on to the sides of the localiser box for lateral gantry angles of 90° and 270°. This test was repeated using CR without changing the setup. The PSP plate being larger in size, was placed on a trolley and positioned beneath the localizer box for the gantry angle 0° and was fixed on a stand for lateral gantry angles. The resulting images were corrected for magnification and analysed using ImageJ. The images had a white circular area corresponding to the position of the lead ball and a darkened area corresponding to the irradiated square field. Two squares were drawn on the images in such a way that one enclosed the irradiated square and the other enclosed the image of the lead ball and diagonals were marked as shown in the magnified image in figure 7a. The shift in radiation isocentre was determined by measuring the distance between the centre of the square field and the lead ball centre as shown in figure 7b.

(b) Figure 5. (a) CR image of the spoke shot obtained to assess the radiation isocentre size during collimator rotation, (b) Magnified CR image of the collimator rotation spoke shot with lines drawn at the centre of each spoke.

R

Results and discussion Calibration of CR and film The calibration graphs for CR and film are shown in figures 8a and 8b respectively. Both the graphs were found to be linear in nature. Unknown doses could be determined using the corresponding equations obtained from the calibration graphs.

Figure 6. A schematic diagram of the smallest circle enclosing all the points of intersection and showing the circle of radiation isocentre wandering.

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(a)

(b)

Figure 7. (a) Magnified CR image with two squares drawn with diagonals, (b) Magnified view showing the distance between the centres of the two squares signifying the shift in isocentre.

Pixel - Dose Relation

1600 1400 1200 1000 800 600 400 200 0

25000

y = 422.7x + 1710

y = 4024.3x - 94.763

20000 pixel Value

Pixel Value

Pixel - Dose Relation

15000 10000 5000 0

0

0.1

0.2

0.3

0.4

0.5

0

Dose (cGy)

10

20

30

40

50

Dose (cGy)

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Figure 8. (a), (b) Pixel-dose calibration graphs for CR and film.

performing a double exposure, the diagnostic setting (high gain of the photomultiplier tube) of the CR scanner caused a saturation of the inner field which received 2 MU. A CR scanner with wider dose latitude would help overcome this issue. The main advantage of using CR for field congruence is the ability to store images online and have a permanent record. The versatility of using CR will also enable to compare the field congruence with a baseline image online.

Quality Assurance tests Light field congruence Figure 9 shows the image obtained with CR for light field-radiation field congruence. While the light field and radiation field matched in the y direction (inplane), a variation of 1.35 mm was observed between the light field and radiation field in the x direction (crossplane). This was verified with the film and the result obtained with the film was found to be consistent with the measurements obtained from CR. From table 1, it is seen that the measured field size (in the cross-plane direction) is smaller than the set field size. Hence, the lead wires enclosing the field (in the cross-plane direction) could not be distinguished very clearly from the unexposed part of the image making it impossible to determine the shift using the FWHM method. While the contrast of the lead wire in the image could be improved by

Field size verification The field size in both the cross-plane and the in-plane directions determined using the FWHM of the beam profiles for the 10 x 10 cm2 field obtained using CR and film are tabulated in table 1. The variation between CR and film in the measured field size was found to be 0.4 mm in the cross-plane direction and 0.1 mm in the in-plane direction at a depth of 8.5 cm. 231

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Radiation isocentre verification The spoke shot obtained with the CR for the verification of the radiation isocentre size is shown in figure 5a. Table 2 lists the radii representing the radiation isocentre sizes for rotation of collimator, gantry and couch for CR and film alongwith the deviation between the two modalities. The radiation isocentre size determined using CR was found to be smallest for collimator rotation (0.30 mm) and largest for couch rotation (0.80 mm). These were found to be comparable with the 0.30 mm for collimator rotation and the 0.73 mm for couch rotation obtained with film. While the collimator rotation isocentre size obtained with CR matched exactly with that of film, the couch rotation isocentre size had a difference of 0.07 mm. The maximum variation between the film and CR was found to be 0.13 mm for the isocentre walk out during gantry rotation proving that the CR and film demonstrate similar variation. Since film was not as sensitive as CR, more radiation had to be delivered to obtain useful images. This QA procedure performed using film consumed more time as each spoke required 30 MU when compared to that of 1 MU required for the CR.

Figure 9. CR image of the radiation field –light field congruence test. Field size irradiated 10 x 10 cm2, SDD = 100 cm, 6 MV photons.

Table 1. Cross-plane and in-plane field sizes determined from the beam profiles obtained at a depth of 8.5 cm of Perspex with SSD 100 cm.

Measured field size ( mm) Field Size (At depth of 8.5 cm) cm2 108.5 x 108.5

Cross - plane

In - plane

CR

Film

Difference

CR

Film

Difference

107.1

106.7

0.4

109.2

109.1

0.1

Table 2. Comparison of radiation isocentre size using CR and film.

Radius representing radiation isocentre size (mm)

Rotation of

CR

Film

Difference

Collimator Gantry Couch

0.30 0.50 0.80

0.30 0.63 0.73

0.00 - 0.13 0.07

Multileaf collimator check Figure 10 shows the feasibility of using CR for verifying MLC leaf positions using MLC. The width of the leaf openings measured from the FWHM of the beam profiles which were plotted using ImageJ software and the variations between the CR and the film values are tabulated in table 3. As in the case of the field size measurement, all the nine leaf openings measured with CR were found to be more than the measurements made with film and the difference ranged from 0.33 to 0.77 mm. The maximum difference between the CR and film was found to be 0.77 mm. The maximum limit for the difference between the CR and film was set at 1 mm as this was a preliminary work to study the feasibility of using the CR as a quality assurance tool using the diagnostic CR scanner already available in the institution.

Figure 10. CR image of the multileaf collimator check test.

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Table 3. Comparison of measured leaf openings for MLC check using CR and film.

Measured width (mm)

Planned width (mm) 20 40 60 80 100 80 60 40 20

CR

Film

19.92 38.78 59.31 79.32 99.31 80.26 58.47 38.99 19.30

19.37 38.26 58.98 78.66 98.79 79.52 58.8 38.22 18.64

Difference between CR and Film (mm) 0.55 0.52 0.33 0.66 0.52 0.74 0.33 0.77 0.66

Table 4. Isocentre check using Winston-Lutz Alignment test tool for Stereotactic Radiosurgery.

S. No 1 2 3 4 5

Couch 0

0 00 00 900 2700

Isocentric shift (mm)

Gantry 0

0 900 2700 00 00

CR

Film

Difference

0.34 0.52 0.36 0.43 0.54

0.28 0.57 0.46 0.37 0.48

0.06 -0.05 -0.10 0.06 0.06

Figure 11. (a), (b) CR and X-OMAT V film images of the WinstonLutz Alignment test tool to determine isocentric shift for stereotactic radiosurgery.

and 270° with gantry angle 0°, the shift determined using CR varied from 0.43 mm to 0.54 mm and 0.37 mm to 0.48 mm using film. The variation between the CR and the film measurement was found to be ≤ 0.1 mm. In this method, there exists a possibility of an error being introduced due to variation in the mMLC field size or shape which could be determined using a light field congruence test for mMLC. This was not possible with the type of CR scanner with high PMT gain used in the study as it gave saturated images for small fields set with mMLC. The other reason for the saturation could be the low energy photons from the mMLC reaching the phosphor plate that has an increased sensitivity due to the presence of barium which has a higher atomic number compared to the silver in film 9. Using CR as a QA tool for the SRS procedure was found to be advantageous over using film as it saved film processing time. It also avoided the need for repeat images due to technical hitches in film processing thereby minimising the waiting time for the patient.

Isocentric shift for stereotactic radiosurgery Figures 11a and 11b show the images obtained with CR and film respectively. The results obtained for the WinstonLutz QA test for verification of isocentre for SRS procedure are tabulated in table 4. The isocentric shifts determined using CR ranged from 0.34 mm to 0.52 mm for the three gantry angles with couch angle 0° and from 0.28 mm to 0.57 mm using film for the same. For the couch angles 90°

Limitation of this diagnostic CR system The inability to reduce the PMT gain sensitivity proved to be a limitation in using this diagnostic CR system for more dosimetric applications. A diagnostic CR scanner with the capacity to modify the PMT gain or commercially available high dose therapy PSP plates with wider dose latitude could help overcome this hurdle. With the confidence gained from this feasibility study, further investigations are currently being carried out with a therapy PSP plate.

(a)

(b)

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Conclusions

References

The feasibility of using a diagnostic CR system as a quality assurance tool for radiotherapy was successfully investigated. The QA results obtained with CR were found to be comparable to those obtained with film. The main advantage of using CR as a quality assurance tool was its versatility in acquiring the images quickly and quantifying the variations accurately. The QA procedures were found to consume less time making it possible to perform routine QA without affecting valuable clinical time. In addition, the problem of storage of films was overcome and a permanent digital record was made available. This facility enabled easy comparison of the routine QA results with baseline QA data. A single cassette-feed diagnostic CR digitizer is only slightly more expensive than a film processor and incurs very minimal running costs compared to film. The major limitation observed in this study was the inability to modify the PMT gain and the use of PSP plates with narrow dose latitude. At a higher cost, CR systems meant for therapy applications such as the Kodak 2000RT CR Plus System can be considered for high dose applications. Hence it is concluded that a diagnostic CR system serve as a costeffective digital alternative to therapy film as a tool for QA in radiotherapy.

1. Langmack, K.A., Portal imaging. Br J Radiol., 74(885): p. 789-804, 2001. 2. Sonoda, M., Takano, M., Miyahara, J. and Kato, H., Computed radiography utilizing scanning laser stimulated luminescence. Radiology, 148(3): p. 833-8, 1983. 3. Wilenzick, R.M., C.R. Merritt, and S. Balter, Megavoltage portal films using computed radiographic imaging with photostimulable phosphors. Med Phys, 14(3): p. 389-92, 1987. 4. Gur, D., Deutsch, M., Fuhrman, C.R., Clayton, P.A., Weiser, J.C., Rosenthal, M.S. and Bukovitz, A.G., The use of storage phosphors for portal imaging in radiation therapy: therapists' perception of image quality. Med Phys, 16(1): p. 132-6, 1989. 5. Whittington, R., Bloch, P., Hutchinson, D. and Bjarngard, B.E., Verification of prostate treatment setup using computed radiography for portal imaging. J Appl Clin Med Phys, 3(2): p. 88-96, 2002. 6. Curtin-Savard, A.a.P., E.B., An electronic portal imaging device as a physics tool, Med Dosim, 22(2): p. 101-5, 1997. 7. Dunscombe, P., Humphreys, S. and Leszczynski, K., A test tool for the visual verification of light and radiation fields using film or an electronic portal imaging device. Med Phys, 26(2): p. 239-43, 1999. 8. Liu, G., van Doorn, T., and E. Bezak, The linear accelerator mechanical and radiation isocentre assessment with an electronic portal imaging device (EPID). Australas Phys Eng Sci Med., 27(3): p. 111-117, 2004. 9. Olch, A.J., Evaluation of a computed radiography system for megavoltage photon beam dosimetry. Med Phys, 32(9): p. 2987-99, 2005. 10. Seggern, H.V., Photostimulable x-ray storage phosphors: a review of present understanding. Braz. J. Phys., 29(2): p. 254268, 1999. 11. Rasband, W.S., ImageJ, U.S.N.I.o. Health, Editor, http://rsb.info.nih.gov/ij/: Bethesda, Maryland, USA, 2005. 12. Gonzalez, A., Castro, I. and Martinez, J.A., A procedure to determine the radiation isocenter size in a linear accelerator. Med Phys, 31(6): p. 1489-93, 2004.

Acknowledgements The authors would like to thank the Atomic Energy Regulatory Board, India for funding this project. We acknowledge Dr Joe Fleming and Ms Hannah Mary Thomas, for their help in fine-tuning the manuscript and Ms Anurupa Mahata for assisting with the experiments involving film.

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