338 IAPM 3rd Annual Scientific Meeting 2012

advanced algorithm such as AAA under these conditions remains tobe seen.

Keywords: Carbon fibre couch, dose build-up, PBC, BEAMnrc

Initial Experience of Gafchromic EBT2� film for in vivo dosimetry in electron beam radiotherapy and its practicalapplication in the clinic

AMANDA BARRY, CORA MARSHALL, ALAN PEMBROKE and EIMEAR O’NEILLUPMC Whitfield Cancer Centre, Waterford, Ireland

Abstract:Gafchromic EBT2� film offers a practical solution to in vivodosimetry for electron beam radiotherapy in the clinic. This islargely due to the simplicity and flexibility with which films can beprepared and used for in vivo dose measurements. Unlike the morecommonly used electron diodes, Gafchromic EBT2� film ispractically water equivalent and with a thickness of approximately0.3 mm, it does not give rise to the large perturbation affects thatare typically associated with the use of electron diodes.Furthermore, Gafchromic EBT2� film has been found to showsignificant energy, temperature, dose-rate and angle-independence [1,2] making it a more practical choice of in vivodosimeter than for example diodes and TLDs . The films have goodspatial resolution and show good dose uniformity making themuseful for assessing both the dose at the central axis as well as the2D dose distribution at a given depth across the treatment area.To date, Gafchromic EBT2� film has predominantly been reportedin the literature as a dosimeter used in photon beam radiotherapy[3, 4] with few references to its use as a dosimeter for routine invivo dose verification for electron beam radiotherapy [5, 6]. In thisstudy we report on our initial experience with Gafchromic EBT2�film as a 2D dosimeter for routine in vivo dose verification of

electron beam radiotherapy treatments for a variety of anatomicalsites. Reference to the uncertainty associated with thesemeasurements will be made, in particular with a view to settingaction levels. We also address how a routine in vivo dosimetryservice using Gafchromic EBT2� film has been implemented inpractice in our department.

References

1. B. Arjomandy et al, Med. Phys. 2010, 37(5), 1942 - 19472. F. Su et al, Applied Radiation and Isotopes 2007, 65, 1187 -

119233. T. Cheung et al, Phys. Med. Biol. 2002, 47, N31-N3744. S. Devic et al, Med. Phys. 2008, 33(4), 1116 - 11245. M. Ciocca et al, Radiotherapy and Oncology 2003, 69, 285-28966. L. Gamble et al, Int. J. Radiation Oncology Biol. Phys. 2005,

62(3), 920-924

Keywords: In vivo dosimetry, electron beam radiotherapy

On the use of the g function in a scientific context and the development of an in-house g analysis software tool

DANIEL O’BRIEN1, PATRICK MCCAVANA1, BRENDAN MCCLEAN1 and LUIS LEON-VINTRO2

1St. Luke’s Radiation Oncology Network, Dublin, Ireland, 2UCD, Belfield, Dublin 4, Ireland

Abstract: The g function[1] is a mathematical technique for thecomparison of two dose distributions that is widely used in clinicalsituations such as IMRT verification and implemented by variouscommercial software packages. It is a composite distribution thatcompares both the percentage difference and distance toagreement of two dose distributions relative to a set of acceptabletolerance criteria. This is useful in a clinical context to show thattwo datasets match each other within clinical tolerances. However,from a scientific point of view, the algorithm does not reveal anyinformation that is intrinsic to the data itself. A more desirablefunction from this point of view would be the inverse of the gammafunction e taking two data sets as input and returning theminimum criteria that would produce a 100% pass rate, essentiallythe intrinsic percentage difference and distance to agreementbetween the two datasets. An in-house software tool was createdto perform gamma analysis on dose datasets from both standardand non-standard data formats to assist in small field research and

planning system commissioning work. As part of this software toola feature was developed that would display the “iso-pass-rate”lines allowing the user to see which set of percentage differenceand distance to agreement criteria would produce a desired passrate. Calculating the minimum distance to the origin of these “iso-pass-rate” lines determines the intrinsic agreement between thetwo data sets. It is also useful from a clinical perspective as itshows not just whether two data sets agree within tolerance ofeach other but also reveals at a glace what the minimum criterianecessary to produce a desired pass rate is and so can also beuseful in determining more appropriate tolerance levels.Reference

1. Low et al., Med. Phys. 25 (5), 656, (1998)

Keywords: Gamma analysis, commissioning, dose comparisons

DIAGNOSTIC PHYSICS SESSION 2

A Headband Dosemeter for Measuring Occupational Radiation Dose to the Lens of the Eye in Terms of Hp(3)

PHIL GILVINHealth Protection Agency, UK

Abstract: The forthcoming European Basic Safety Standards directivewill require member states to enforce, within four or five years,

a much-reduced dose limit for the lens of the eye. The limit will bereduced from 150 millisieverts (mSv) to 20 mSv per annum (with some