Abstract Book

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ESTRO 37

effect of build-up on SNR, defined as the ratio of centre to out of field response was evaluated. The TPS Pinnacle TM version 9.0 software (Philips Radiation Oncology Systems, Fitchburg, WI) was used to calculate dose profiles at the array position. To investigate the detector sensitivity of a variation in the dose distribution due to a misalignment of a target, the plan has been delivered to the phantom when the target was at isocentre and laterally shifted by 7 mm. The dose profiles are co-registered to the EPID images taken in the same setup. Results When the target is at isocentre, dose profile comparison between the measured dose and the TPS agrees within 4.5% for inter-umbra region (Fig. 1a). The difference graph (Fig.1b) shows the difference between profile measured at isocentre and the profile measure with the phantom shifted laterally. Fig. 2 shows EPID images taken of the target when at isocentre and laterally shifted. It confirms that the displacement of the dose measured in the shifted configuration is from the misalignment of the target in the treatment field. Similar results were observed with the thinner build-up. The 15 mm build-up resulted in a 65.4% increase in SNR of the array detector, although it results in a 12.5% reduction of the contrast noise ratio (CNR) of the EPID image compared to the 5 mm.

Figure 1. Comparison of transverse profiles obtained with the different models of tissue-assignation. Conclusion Dose planning in water, that is, using water stoichiometry and assigning density from CT numbers to all soft tissues, has been proved inaccurate for breast treatment planning with the INTRABEAM device, as it overestimates the prescribed dose within the planning treatment volume (PTV), and thus treatment plans would yield a dose up to 40% lower than intended. If correct soft tissue assignations in the breast cannot be safely done, adipose tissue should be chosen as main tissue composition of the PTV to avoid under-prescription of dose. Instead, this would overprescribe the intended dose, but only in a few percent. [1] Vaidya J et al 2002 EJSO 28 447-54 [4] Schneider W et al 2000 PMB 45 459-78 [5] Rivard M J et al 2004 Med Phys 31 633-74 [6] Sempau J et al 2011 Med. Phys. 38(11), 5887 [7] Ibáñez P 2017 Implementation and Validation of Ultra- Fast Dosimetric tools for IORT, PhD Thesis EP-1773 Dual detector prototype for on line dose verification during patient radiotherapy treatment O. Brace 1 , S. Alhujaili 1 , S. Deshpande 2 , P. Vial 2 , P. Metcalfe 1,3 , M.L.F. Lerch 1,3 , M. Petasecca 1,3 , A.B. Rosenfeld 1,3 1 University of Wolongong, Centre for Medical Radiation Physics, Wollongong, Australia 2 Department of Medical Physics, Liverpool and Macarthur Cancer Therapy Centre, Sydney, Australia 3 Illawarra Health and Medical Research Institute, IHMRI, Wollongong, Australia feasibility of performing in-vivo dosimetry and image co- registration with a dual detector. A 2D array detector based on silicon diodes is placed above an EPID. Requiring no adjustment to the EPID, near tissue equivalent dosimetry could be performed while the EPID can still function as an imaging device, aiding in patient positioning error detection. Material and Methods In order to validate the dual detector approach, a lung phantom embedded with a 2cm diameter spherical water equivalent target was positioned on the patient couch at isocentre. A LINAC was used to deliver 250 MU for a field of 5x5 cm 2 with 6 MV photon beam. The EPID was set up at SSD of 150 cm directly beneath the LINAC couch. The 2D array detector (MP) composed of 11×11 small 1.5x1.5 mm 2 epitaxial diodes with 10 mm pitch, embedded in a 0.6 mm thick kapton carrier is placed above the EPID. The array detector had 5 mm thick water equivalent material underneath and build-up of 5 mm or 15 mm. The [2] White S et al 2014 Med Phys 41 (6) 061701 [3] Beaulieu L et al 2012 Med Phys 39 6208-36 Purpose or Objective The aim of this work is to demonstrate the