ESTRO 38 Abstract book

S946 ESTRO 38

response was calibrated to dose at the machines reference condition. Results Transit dose profiles of the MP, film and TPS are displayed in Fig. 1a) with the dose difference calculated in respect to the MP (Fig. 1b)). The MP measured dose agrees within error to both TPS and film across the inter-umbra region. A drop in dose at the centre of the field, in correspondence with the position of the target is present in all three profiles. The deliberate overdose delivery of 5.2% resulted in an increase in dose measured by the MP at the centre of the field of (5.4 ± 1.8)%. Fig. 2 displays normalised profiles of the EPID and the MP when the target was at isocentre and laterally shifted. In Fig. 2a) the decrease in dose at the centre of the MP profile is aligned with the position of the target observed in the EPID image. The registration of the profiles in Fig. 2b) confirms deviation in the transit dose profile for the misalignment error delivery is due to a shift in the target position.

Conclusion The dual detector system is capable of verifying the transit dose in water, predicted by the TPS for a heterogeneous phantom in real time. The mechanical mounting of an array detector above the EPID would create an easy to implement and low cost in-vivo dosimeter. Development of an array detector with improved spatial resolution would further effectiveness of the system. The dual detector system has been shown to be able to identify potential in-vivo dose errors of incorrect MU delivery and target misalignment with the aid of imaging and dose co-registration. EP-1754 High-resolution assessment of dose calculations in small MV photon beams on and off central axis G. Biasi 1 , N. Hardcastle 2 , M. Petasecca 1 , S. Guatelli 1 , V.L. Perevertaylo 3 , A.B. Rosenfeld 1 , T. Kron 2,4 1 University of Wollongong, Centre for Medical Radiation Physics, Wollongong, Australia ; 2 Peter MacCallum Cancer Centre, Department of Physical Sciences, Melbourne, Australia ; 3 SPA-BIT, n/a, Kiev, Ukraine ; 4 University of Melbourne, Sir Peter MacCallum Cancer Institute, Melbourne, Australia Purpose or Objective There is an increasing use of standard treatment planning systems (TPSs) for stereotactic radiosurgery (SRS), as opposed to dedicated SRS TPSs. In the literature, investigations have assessed their accuracy, and that of dose calculation algorithms, in clinical stereotactic situations with multiple fields combined, but not in small static fields. Small static fields are also not considered for beam modelling in TPS commissioning, since measurement accuracy cannot be guaranteed owing to a mix of uncertainties in effective field size, detector positioning and field-size dependent correction factor. The present work aimed at investigating the accuracy of calculations