7/27/2019 TOWARD ACCURATE SMALL PHOTON FIELD DOSIMETRY USING RADIOCHROMIC FILM

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,2014

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10990/0814

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The purpose of this work is to develop and validate a reliablemethod for characterizing small fields on radiotherapymachines with radiochromic film. A novel multichannelanalysis method is proposed that focuses on correcting filmsystematic errors introduced by manufacturing imperfections.

To validate the technique, a controlled experimentalenvironment is used. Initial results show that the performanceof the method is promising, suggesting potential future forsmall field dosimetry.

Methods:

A. Experimental set up

A Cobalt-60 beamis used to irradiate

radiochromic films

(Gafchromic

EBT-3) homogenously,

allowing the

determination of

parameters for the film

scanner homogeneity

correction,

multichannel analysis

and film dose response

calibration. The setup is shown in Figure 1, while the homogeneity of a

profile across the film at an SSD of 430 cm in Figure 2.

B. Multichannel method

Prior to performing the multichannel analysis, a correction is applied to

the film to account for the scanner light inhomogeneity (see Figure 3). A

multichannel method developed from first principles and based on the

statistical behaviour of optical density for a given dose is then appliedto correct for defects in film emulsion thickness uniformity. This method

allows the film response to be characterised in terms of dose and thickness

perturbation, as shown in Figure 4. By applying the multichannel correction,

as shown in Figure 6, a perturbation and dose index map was extracted, thus

allowing the image to be corrected and the film calibration to dose to be

performed (see Figure 5).

Validation of the method:

The performance of the method is validated against experimental

measurements of dose distributions in a Cobalt-60 beam. Beam profiles

(Figures 7 and 9), as well as percent depth dose distributions (Figures 8 and

10), are measured and used to compare the proposed method with one

using single channel analysis (red only). Results with the new method show

a significant improvement compared to the single channel method, withsystematic errors being reduced by a factor of up to 3.

Conclusion and future work:The proposed method based on multichannel analysis is promising for

applications requiring high accuracy, such as in the characterization of small

fields. Future work requires further validation of the method against other

detectors as well as Monte Carlo calculation.

References:

1) Alfonso et al. A new formalism for reference dosimetry of small and nonstandard

fields. Med. Phys. 35 (2008);

2) Bouchard et al. On the characterization and uncertainty analysis of radiochromic

film dosimetry, 36 (2009);

3) Chung et al. Investigation of three radiation detectors for accurate measurement

of absorbed dose in nonstandard fields, 37 (2010);

4) Micke et al. Multichannel film dosimetry with nonuniformity correction,

Med. Phys. 38 (2011);5) Saur and Frengen, GafChromic EBT film dosimetry with flatbed CCD scanner

Med. Phys. 35 (2008)

Figure 1: Cobalt-60 film irradiation set up.

TOWARD ACCURATE SMALL PHOTON FIELDDOSIMETRY USING RADIOCHROMIC FILM

Billas I, Silvestre I, Bouchard HRadiation Dosimetry Group, National Physical Laboratory, Hampton Road, Teddington, Middlesex, TW11 0LW, UK

ilias.billas@npl.co.uk

Figure 2: Raw signal profile from a homogenously

irradiated film.

Figure 3: Scanner homogeneity correction matrix.

Figure 4: 2D characterization of the film response to dose

and thickness perturbation.

Figure 5: EBT-3 film response curve.

Figure 6 : Image, thickness perturbation and dose index map.

Figure 7: Cobalt-60 beam profile comparison between single and multi channel analysis (horizontal

on the left and vertical on the right).

Figure 8: Cobalt-60 PDD profile comparison between single and multi channel analysis.

Figure 9: A 10 x 10 cm2Cobalt-60 dose distribution. Figure 10: 2D images of a Cobalt-60 PDD.