Abstract Book

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ESTRO 37

profiles are shown in Fig. 2. They represent the treatment plan well, considering that the planned fluence maps neglect the distribution of the photon source, scattering within the material, as well as delivery uncertainties.

Conclusion Alignment of an open MR scanner’s FOV with a horizontal fixed proton beam has been realized taking into account the scanner’s B 0 field induced beam deflection. Sequence-dependent gradient fields do not affect the beam profile. OC-0606 µIMRT – Bringing small animal irradiations closer to clinical standards A.M. Reinhart 1 , S. Nill 1 , U. Oelfke 1 1 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Joint Department of Physics, London, United Kingdom Purpose or Objective Pre-clinical studies are essential to advance our radiobiological understanding of current radiotherapy. However, the current standard in small animal irradiations differs greatly from clinical treatments. In recent years, dedicated high precision irradiators for small animals enabled more accurate and translatable pre-clinical studies. Still, these devices do not yet support IMRT. This is not caused by hardware constraints, but by a lack of sophisticated treatment planning solutions. The Small Animal Radiation Research Platform (SARRP) is equipped with a 225 kVp x-ray tube, a rotating gantry, a variable rectangular collimator, a robotic couch, and CBCT image guidance. However, the current control system only supports forward-planned irradiations with fixed beam sizes. We present a µIMRT framework based on our in-house dose engine, a jaw-only direct aperture optimisation, and intensity modulated beam delivery with the variable The workflow within our µIMRT framework is as follows: We acquire CBCT images on the SARRP, and calculate beam influence matrices (Dijs) with our in-house superposition-convolution kV dose engine [Reinhart et al, BJR 90, 2016]. The treatment plan optimisation is integrated into our in-house treatment planning system DynaPlan. We developed a gradient-based jaw-only direct aperture optimisation, based on [Wild et al, MedPhys 42(5), 2015]. After an initial fluence optimisation and sequencing step, the objective function gradients are calculated in terms of aperture weights and jaw positions. The sequencer decomposes the fluence map into rectangular shapes in a largest rectangle approach. We present a proof-of-concept study on a cylindrical, mouse-sized solid water phantom with radius r=1cm and height h=7cm, irradiated with 7 equidistant, co-planar beams. An artificial, horseshoe-shaped target consisting of a bent tube with 1.5mm radius, to be irradiated with 65Gy, wraps around an artificial cylindrical organ at risk of radius r=1.5mm and height h=3cm whose dose should not exceed 30Gy. We chose beamlet sizes of 0.5mmx0.5mm and evaluated the individual fluence profiles with Gafchromic EBT3 films. Results The optimised dose distribution and individual fluence maps are shown in Fig. 1. The optimised treatment plan achieved D 98 target =60.2Gy and D 5 OAR =39.9Gy, with the number of shapes limited to 70. The measured fluence rectangular collimator. Material and Methods

Conclusion We successfully demonstrated a first proof-of-concept for preclinical IMRT, from dose calculation to beam delivery. The next steps will focus on the absolute dosimetry, as well as the 3D evaluation. The µIMRT framework brings preclinical irradiations closer to clinical standards and is an essential step to bridge the gap between preclinical studies and clinical implementations. OC-0607 Demonstration of x-ray acoustic computed tomography for non-standard field dosimetry applications S. Hickling 1 , M. Hobson 1 , I. El Naqa 2 1 McGill University Health Center- Cedars Cancer Centre, Medical Physics, Montreal, Canada 2 University of Michigan, Radiation Oncology, Ann Arbor, USA Purpose or Objective X-ray acoustic tomography (XACT) is an emerging imaging technique that forms images related to the dose deposited within an object by detecting the acoustic waves induced via the photoacoustic effect during radiation therapy. With the advancement of radiotherapy delivery techniques, there is a need for novel volumetric dosimetry methods that will allow for the rapid measurement of non-standard complex fields for QA purposes. This study demonstrates the ability of XACT to be used as a dose mapping technique for volumetric modulated arc therapy (VMAT) deliveries. Material and Methods A portion of a representative prostate VMAT plan was delivered to a water tank. An immersion ultrasound transducer was placed inside the water tank to detect