ESTRO 36 Abstract Book

S783 ESTRO 36 2017 _______________________________________________________________________________________________

The calculation models the spatial response of the IQM chamber and the fluence transmitted through the individual collimating elements. The chamber response is modelled as a 2D map. The fluence from the machine is divided into 2 components: a point source at the target and an extended source at the flattening filter, referred to as the primary and extended source, respectively. The primary source is characterized by a radial intensity profile and is attenuated through the jaws and multileaf collimator. Transmission is calculated for a 2D array matching the chamber response map, and area averaged fluence is calculated for moving collimating elements during beam delivery. The extended source is modeled as a Gaussian distributed source with a Compton angular intensity distribution. The contribution of the Gaussian source to each element in the fluence array is raytraced through the collimation to obtain the area averaged fluence. An element-wise multiplication of the chamber response map with the primary and extended source fluence is summed to generate the predicted signal, modified by factors reflecting the chamber volume, the intensity of the primary and extended sources and change in machine output with field aperture. The model has been implemented for Varian and Elekta treatment units, with calculations and measurements compared for clinically relevant fields. Results Parameters for the model were determined from a series of rectangular field measurements with the IQM chamber combined with ion chamber measurements. Iterative optimization of parameter values to match rectangular field IQM measurement were performed. Similar techniques were used to extract normalization parameters. The agreement between the calculated and measured signals on a Varian TrueBeam unit for over 300 different IMRT field segments from Prostate and Head & Neck plans show 99% of segments agree within ±5%; 95% within ±3%. Similar results were seen for an Elekta Agility unit in a sample of over 400 different IMRT field segments, with 97% of segments agreeing within ±5% and 91% within ±3%. Conclusion A 2-source calculation model has been implemented for an area-fluence monitor designed for on-line patient QA. EP-1483 Pre-Treatment QA of MLC plans on a CyberKnife M6 using a liquid ion chamber array. L. Masi 1 , R. Doro 1 , O. Blanck 2 , S. Calusi 3 , I. Bonucci 4 , S. Cipressi 4 , V. Di Cataldo 4 , L. Livi 5 1 IFCA, Medical Physics, Firenze, Italy 2 Saphir Radiosurgery Center, Medical Physics, Frankfurt/ Gustrow, Germany 3 University of Florence, Department of Clinical and Experimental Biomedical Sciences "Mario Serio", Firenze, Italy Purpose or Objective CyberKnife MLC plans require accurate patient-specific QA. The purpose of this study is to validate the use of a liquid ion chamber array for Delivery Quality Assurance (DQA) of robotic MLC plans, using several test scenarios and routine patient plans and comparing results to film dosimetry. Material and Methods Five preliminary sensitivity test scenarios were created from a baseline plan modifying each MLC segment by introducing increasing shifts in leaves positions (0.5 mm - 2 mm). The baseline and test plans were delivered to an Octavius 1000SRS array (PTW) as well as to EBT3 films. An average correction was applied to 1000SRS results to account for the response dependence on source-detector- distance (SDD) [O. Blanck et.al. Phys Med 2016]. The same 4 IFCA, Radiation Oncology, Firenze, Italy 5 Azienda Ospedaliera Universitaria Careggi, Radiotherapy Unit, Firenze, Italy

five test plans were delivered a second time to the 1000SRS re-orienting all beams perpendicularly to the array (nominal position) to eliminate SDD and angle dependence. As a second step 40 clinical MLC plans optimized for various treatment sites (liver, spine, prostate) were delivered to the liquid ion chamber array for patient-specific QA using both the clinical beam orientations and the beam “nominal position”. For the latter only a subset of segments(18-21) was selected. Finally, for 15 out of 40 clinical plans a film-based DQA was also performed. All results were analyzed using (2%, 2mm),( 3%, 1mm) and (2%, 1mm) gamma index criteria [O. Blanck et.al. Phys Med 2016]. Results The pass-rate reductions from the baseline,obtained delivering the five test plans, are shown in fig.1 for (3%, 1mm) gamma criteria. The Octavius 1000SRS showed a good sensitivity to simulated delivery errors with pass-rate reductions increasing from 1.7% to a maximum of 43% with increasing leaves shifts (0.5 mm - 2 mm). Similar sensitivity was observed when the beams were re-oriented in the nominal position geometry. The pass-rate reductions observed with films showed a more irregular trend, and the maximum reduction was 16%. The average pass-rates obtained over clinical plans are shown in fig.2, for the three gamma index criteria. The mean values obtained by the 1000 SRS array, using both the clinical and nominal beam geometry, and by film-dosimetry are all above 92%, when using 3%, 1mm criteria. Differences among the mean pass-rates observed for the three measurement modalities were not statistically significant (p> 0.1, t-test)

Conclusion The results confirm that the 1000SRS array is reliable for pre-treatment QA of CyberKnife MLC plans. The test scenarios highlighted a higher sensitivity to small leaves shifts than what observed by film dosimetry. The gamma