340 IAPM 3rd Annual Scientific Meeting 2012

licence amendments required. The paper also reviews which testswere found to have most impact on radiation safety. Issues arisingout of measurement technique are also discussed. In the majorityof cases the review showed some improvement in radiation safety

in dental practices. In some cases high exposures were avoided byonsite testing.Keywords: Radiation protection, dental, RPA, diagnostic radiology

RADIOTHERAPY PHYSICS SESSION 2

A source occlusion independent method for reporting small field output factors

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

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

Abstract: The quantification of accurate output factors for smallfields has been an area of active research over the last few years.Recently papers [1,2] have started to report output factors forcertain field sizes along with correction factors for a variety ofdosimeters based on the new formalism [3] proposed by the jointIAEA/AAPM task force. These correction factors were calculatedbased on Monte Carlo models that where calibrated by matchingthe focal spot size to that of the particular Linac used to performthe measurements. However, it is known that for very small fieldsizes (where the field size is small enough to cause an overlap ofgeometric penumbra) source occlusion can significantly affect theoutput factor [4]. The reported output factors for these field sizesare thus only valid for the spot sizes they were modelled for. Spotsize is machine dependent as evidenced by the different spot sizesreported in the literature. For this reason it is still necessary andimportant for those wishing to use such fields in a clinical settingto determine their own output factors. The differences in on-axiscollision kerma caused by source occlusion [4] make it unclearwhether the correction factors reported would be valid underdifferent circumstances and more work is needed to verify these

factors. This makes determination of small field output factorsfrom measurements alone more difficult and modelling may still benecessary on a machine-by-machine basis. A method is proposedhere for determining spot-size independent output factors withcorresponding spot size correction factors. An Elekta Precise modelis implemented in Geant4 to produce a point source beam for eachfield size, a ‘universal’ output factor can be calculated. Then bymodelling various spot sizes a relative output correction factor canbe determined allowing the output factor to be scaled to anindividual machine’s source properties.

References

1. P. Francescon et al., Med. Phys. 38 (12), 6513, (2011)2. G. Cranmer-Sargison et al., Med. Phys. 38, 6592, (2011)3. R. Alfonso et al., Med. Phys. 35, 5179, (2008)4. Scott et al., Med. Phys. 36, 3132 (2009)

Keywords: Small fields, output factors, source occlusion

Patient specific QA for RapidArc

SINEAD KEOGH and ELAINE TYNERSt. Luke’s Radiation Oncology Network, Dublin, Ireland

Abstract: RapidArc is a volumetric arc therapy which was recentlyintroduced to Ireland by St. Lukes Radiation Oncology Network.RapidArc is characterised by continuous MLC and gantry motion incombination with varying gantry speed, dose rate and MLC speed.This approach leads to significantly shortened treatment times anddose conformity which is comparable to that achieved by staticIMRT. Because of the complexity of RapidArc treatment plans, it isvital to implement an efficient and accurate protocol for routinepatient specific QA. We decided to use the ArcCHECK phantom forthis purpose. ArcCHECK is a commercial diode array designedspecifically for rotational measurements. Our ArcCHECKmeasurements were validated by comparison with Gafchromic filmmeasurements as this is a standard high resolution patientverification method. Specifically, our work involved deliveringa number of prostate RapidArc treatment plans to ArcCHECK.Independent validation of ArcCHECK measurements with Gafchromicfilm, in two orientations was performed for a subset of these plans.

Gafchromic film analysis was performed using the ‘triple channeldosimetry’ approach. This is a new method of evaluatingradiochromic film dosimetry data which improves the integrity of thedose information in the film by removing disturbances in scannedimages. As a result of following this method, a high level dosimetricaccuracy was evident in our film measurements. Dose distributionswere analysed using gamma criteria of 3% dose and 3 mm distance toagreement. A pass rate of at least 95% was achieved using data fromArcCHECK and Gafchromic film respectively. This work combines theconstruction of an efficient protocol for using ArcCHECK for patientspecific QA of RapidArc treatment plans with an implementation ofthe ‘triple channel dosimetry’ approach for Gafchromic filmanalysis. Thus we have formulated a protocol for patient specific QAof RapidArc treatment plans and incorporated Gafchromic film asa suitable backup system.Keywords: RapidArc, Volumetric arc therapy, Patient specific QA,ArcCHECK, Gafchromic film, Triple channel dosimetry

Effect of calibration depth on a 2D array IMRT QA system

RAJASEKAR DAVID, CHRISTOPH KLEEFELD, GORDON SANDS and SINEAD CLEARYDept. of Medical Physics & Bioengineering, University Hospital Galway, Ireland

Abstract: 2D array detector systems are widely used inverification of IMRT plans since they are easy to setup and

provide 2D fluence distribution and direct electronic datawithout post processing. The accuracy of the planar