ESTRO 2020 Abstract book

S924 ESTRO 2020

had no treatment fractions with the shifts larger than 5 mm in Z axis direction. Conclusion The presented method allows a simple assessment of the effects of rotational movements. The obtained translations of the point A approximate the shifts of CTV points resulting from roll and pitch rotations. For significant number of patients, the pitch rotational errors caused the shifts larger than 5 mm in superior-inferior and anterior-posterior direction. Setup variations caused by rotations cannot be ignored for prostate patients treated with whole-pelvic radiation therapy. The impact of rotational errors should be taken into consideration in calculations of the CTV-PTV margin. Replacing the point A coordinates with the coordinates of selected CTV points in the mentioned method allows to calculate the shifts of these points as well as the CTV-PTV margin. It is recommended that margin increases with distance from the isocenter in order to take rotational errors into account. PO-1606 Measurement of dose distribution of cardio- synchronous brain motion in microbeam radiation therapy M. Petasecca 1 , M. Duncan 1 , M. Donzelli 2 , P. Pellicioli 3 , E. Brauer-Krisch 4 , J. Davis 1 , A. Rosenfeld 1 , M.L.F. Lerch 1 1 University of Wollongong, Centre for Medical Radiation Physics, Wollongong, Australia ; 2 The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, Joint Department of Physics, London, United Kingdom ; 3 Swansea University- Singleton Park, Medical School, Swansea, United Kingdom ; 4 European Synchrotron Radiation Facility, Biomedical beamline ID17, Grenoble, France Purpose or Objective Microbeam radiation therapy (MRT) is an emerging radiation oncology modality ideal for treating inoperable brain tumours. MRT employs quasi-parallel beams of low energy x-rays produced from modern synchrotrons. The peak to valley dose ratio (PVDR) is of critical importance to the efficacy of MRT. The underlying radiobiological advantage of MRT relies on high peak dose (50um FWHM) for tumour control and low valley dose for healthy tissue sparing (400um spacing between peaks). The brain is known to exhibit cardio synchronous pulsating motion of the order of 100-200 mm, which is comparable to the microbeam width and spacing. This can result in dose blurring through overlap of microbeams and a reduced PVDR which will compromise the radiobiological effectiveness. This work presents the first experimental measurement of the effect of brain motion on MRT dose distribution. We present microbeam profiles measured under simulated brain motion conditions using a silicon strip detector and Gafchromic film.These are compared to a dataset calculated using GEANT4 Monte Carlo toolkit. Material and Methods The Centre for Medical Radiation Physics has developed a real time dosimetry system for MRT based on a silicon strip detector (SSD). Measurements were performed at the ID17 beamline at the European Synchrotron Radiation Facility (Grenoble – France). The detector was mounted in a 15cm x 15cm x 15cm solid water phantom and the brain motion was realised by a stepper motor. Profiles at different scanning speed, with/out motion, were reconstructed by stepping the detector horizontally through the width of the radiation field while the dose is delivered by shifting the target vertically in front of the beam apertures.Gafchromic HD-V2 film was used under the same conditions. PDVR and FWHM was calculated for each of the profiles and for each detector dataset. Comparison to Montecarlo simulations were performed using the Geant4

correlation of GE to clinical and/or planning parameters were observed.

Conclusion Even though based on a small sample, quantitative analysis of TPT of the RGMRgRT system answers to the aims of the study. BH inspiration gating technique could be optimised in order to reduce the TPT. Once the analysis on the completed dataset of 210 patients will be completed, the results will more clearly indicate any correlations between the GE and some clinical/physical/mechanical parameters and if further gating techniques should be considered to improve and optimize the use of RGMRgRT. PO-1605 Can we ignore rotational errors in the positioning of patients with high-risk prostate cancer? M. Poncyljusz 1 , M. Piziorska 1 , B. Czyzew 1 , N. Piotrkowicz 1 1 The Central Clinical Hospital of the Ministry of The Interior and Administration in Warsaw, The Radiotherapy Center, Warsaw, Poland Purpose or Objective Image guided radiotherapy provides an excellent solution to quantitative assessment and correct for patient set-up errors in the modern radiotherapy. However, the majority of linear accelerators are equipped with conventional couches that can be moved in three translational directions and perform only yaw rotation. Uncorrected roll and pitch result in rotational set-up errors, particularly when the distance from the isocenter to CTV border is large. This study reports on the residual rotational set-up error after using daily cone beam computed tomography to position high-risk prostate cancer patients for radiotherapy treatment. The aim of this study was to analyse the shifts resulting from uncorrected roll and pitch rotations. Material and Methods 22 patients with prostate cancer treated with VMAT technique had daily CBCT scans (570 CBCTs in total) prior to treatment delivery. The rotational errors remaining after on-line correction were retrospectively analysed. The shifts caused by uncorrected rotations were calculated with the use of the rotation matrix. The point A was a hypothetical point. The (x y z) coordinates of the point A were equal to the maximum distance from the isocenter to the CTV border in the X, Y and Z axis directions, respectively (X axis was defined as superior- inferior, Y as left-right and Z as anterior-posterior direction). The segment connecting the isocenter to the point A was the radius of the sphere that covered CTV. This implies that the shift of each point of CTV was smaller than the shift of the point A. Results 139 and 36 of the 570 rotational errors were larger than 1,5 degrees in pitch and roll direction respectively. The angle of rotation was from -4.2 to 3.9 degrees and from - 3.8 to 1.6 degrees in pitch and roll directions respectively. 17 of the 22 patients had no treatment fractions with the shifts of the point A larger than 5 mm in X axis direction due to residual rotational error. The shifts larger than 5 mm was not noted in Y axis direction. 15 of 22 patients