An experimental extrapolation technique using the Gafchromic EBT3 film for relative output factor measurements in small x-ray fields

Morales, Johnny, Butson, Martin, Crowe, Scott, Hill, Robin, & Trapp, Jamie (2016) An experimental extrapolation technique using the Gafchromic EBT3 film for relative output factor measurements in small x-ray fields. Medical Physics, 43(8), pp. 4687-4692.

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  • An experimental extrapolation technique is presented which can be used to determine relative output factors for very small x-ray fields using Gafchromic EBT3 film.


  • Relative output factors were measured for Brainlab SRS cones ranging in diameters from 4 to 30 mm2 on a Novalis Trilogy linear accelerator with 6 MV SRS x-rays. The relative output factor was determined from an experimental reducing circular region of interest (ROI) extrapolation technique developed to remove the effects of volume averaging. This was achieved by scanning EBT3 film measurements with high scanning resolution of 1200 DPI. From the high resolution scans, the size of the circular regions of interest was varied to produce a plot of relative output factors versus area of analysis. The plot was then extrapolated to zero to determine the relative output factor corresponding to zero volume.


  • Results have shown that for a 4 mm field size the extrapolated relative output factor was measured at a value of 0.651 ± 0.018 as compared to 0.639 ± 0.019 and 0.633 ± 0.021 for 0.5 mm and 1.0 mm area of analysis values, respectively. This showed a change in relative output factor of 1.8% and 2.8% at these comparative region of interest sizes. In comparison, the 25 mm cone had negligible differences in measured output factor between zero extrapolation, 0.5mm and 1.0 mm circular ROI's respectively.


  • This work shows that for very small fields such as 4.0 mm cone sizes, a measureable difference can be seen in output factor based on the circular ROI and size of the area of analysis using radiochromic film dosimetry. We recommend to scan Gafchromic EBT3 film at a resolution of 1200 DPI for cone sizes less than 7.5mm and to utilize an extrapolation technique for output factor measurements in very small field dosimetry.

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ID Code: 96484
Item Type: Journal Article
Refereed: Yes
Keywords: Small x-ray field dosimetry, Gafchromic EBT3 film, Stereotactic Radiosurgery, extrapolation technique
DOI: 10.1118/1.4958679
ISSN: 0094-2405
Subjects: Australian and New Zealand Standard Research Classification > PHYSICAL SCIENCES (020000) > OTHER PHYSICAL SCIENCES (029900) > Medical Physics (029903)
Divisions: Current > Schools > School of Chemistry, Physics & Mechanical Engineering
Current > QUT Faculties and Divisions > Science & Engineering Faculty
Copyright Owner: Copyright 2016 American Association of Medical Physics
Deposited On: 03 Jul 2016 22:35
Last Modified: 09 Aug 2016 05:55

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