IAPM 3rd Annual Scientific Meeting 2012 341

measurement depends upon the accuracy and stability of thearray calibration with respect to the central detector. TheMapCheck device used in this study is calibrated annually usingthe manufacturers’ patented wide-field calibration technique.In our centre each time the response of the central detectoris compared with ion-chamber absolute dose and re-calibratedif the deviation is more than � 0.5%. Constancy of the arraycalibration is verified by measuring the planar response ofa 26� 26 cm2 field size at 5 cm depth and comparing with theplanning system calculated distribution. There are several waysto check the accuracy of the array calibration. In this studywe have analysed 2D measured profiles using water tank andArray rotation techniques. We have checked the accuracy ofthe array calibration with water tank measurement at 2 cm

and 5 cm depth under SAD setup. Using a diode detector,diagonal profiles and cross-line profiles at 1 cm off axisintervals covering a 26� 26 cm2 field were measured witha 6 MV beam. The MapCheck array was calibrated at 2 cm and5 cm SAD depth as per the manufacturers’ method. Then, withthe new array calibration selected, Map-Check was exposed toa 26� 26 cm2 field size at 0, 90, 180 and 270 degrees rotation(array rotation method). The observed average relativeresponse errors of the detector array were (a) Water-tank(0.58% � 0.43%) and (b) Array-rotation methods (0.31% �0.28% for 2 cm depth; 0.13% � 0.44% for 5 cm depth). Theinfluence of the depth of array calibration on the overallaccuracy will be presented.Keywords: IMRT QA, MapCheck, Array calibration

INVITED SPEAKER

Large scale implementation of EPID dosimetry

ANDRE DEKKER, PhDHead of Knowledge Engineering, Maastro Clinic, Maastricht, The Netherlands

Abstract: Since 2000 MAASTRO Clinic has had an EPID based in-vivodosimetry program which is used for every patient treated withcurative intent. Since the early days of point dosimetry usinga camera based system, the program has extended to 2D, 3D and4D dosimetry, to amorphous silicon based EPIDs and to newapplications such as IMRT and VMAT, patient QA and machine QA.

Topics will include:

- Tools and models to make an EPID a dosimetric device- Workflow and supporting information systems necessary forlarge scale applications

- Clinical and QA examples of what EPID dosimetry can do foryour department

PLENARY SESSION 2

YOUNG INVESTIGATOR GRANT 2011 - WINNER 1

The development of a breast ultrasound contrast detail and an anechoic target phantom

S. COURNANE1, A.J. FAGAN1,2 and J.E. BROWNE31Dept. Medical Physics & Bioengineering, St. James’s Hospital, Dublin 8, Ireland, 2Centre for Advanced Medical Imaging, St. James’sHospital / Trinity College Dublin, Ireland, 3Medical Ultrasound Physics and Technology Group, School of Physics & FOCAS Institute,Dublin Institute of Technology, Kevin’s Street, Dublin 8, Ireland

Abstract:The diagnostic accuracy of breast ultrasound imaging isreliant on consistent image quality performance and, indeed, itis the only clinical ultrasound application with formalrequirements for technical performance. Contrast detail andanechoic target detectability are regarded as the most relevantperformance indicators for breast imaging, providing a means ofassessing an ultrasound scanner’s ability to distinguish lesions ofvarying size and contrast from background tissue. Currentcommercially-available phantoms, commonly used for breastimaging quality assurance (QA), typically contain nylon wire andcylindrical grey scale targets embedded in a uniform tissuemimicking background. Cylindrical contrast inserts, which shouldrather be of a spherical geometry, are unsuitable for assessingthe impact of the slice thickness of the ultrasound beam on theultimate image quality while, furthermore, the size andcontrast values of the inserts exceed those necessary forchallenging high frequency ultrasound scanners sensitive tosubtle differences in breast tissue. Thus, these phantoms do notadequately replicate the complex nature of breast tissue and

are considered not to offer a clinically-relevant challenge forthe assessment of breast ultrasound systems.This work proposes a novel clinically-relevant phantom designincorporating anechoic (-30dB) and low contrast (-1–4dB) sphericallesion targets, with a diameter range of 1e4 mm, into an appropriatebreast-mimicking background. The produced test object exhibitedan acoustic velocity and attenuation coefficient of 1560 m/s and0.7dB/cm/MHz, respectively, and displayed a linear responsebetween attenuation and frequency at high frequencies. Thephantom was used to measure the lesion signal-to-noise ratios(LSNR) of a number of modern breast specific ultrasound scanners,calculated with objective analysis software, in order to inform thedevelopment of a fast and clinically reflective approach to BreastUltrasound QA. The phantom thus offers a sensitive means oftracking imaging performance for ultrasound systems where onlya subtle difference between the ultrasonic characterisation ofnormal tissue and malignancy may exist, a difference notchallenged by commercially-available phantoms.Funded by IAPM Young Investigator Grant 2011