ESTRO 2020 Abstract book

S778 ESTRO 2020

PO‐1374 Validation of SRS MapCheck for patient specific QA C. Anson Marcos 1 , M. Roch González 1 , D. Hernández González 1 , P. Castro Tejero 1 , A. Viñals Muñoz 1 , R. Fayos- Solà Capilla 1 , L. Pérez González 1 1 University Hospital La Princesa, Medical Physics, Madrid, Spain Purpose or Objective The administration of high doses requires rigorous quality assurance (QA) procedures to verify the spatial and dosimetric accuracy of the delivery. SRS MapCheck diode array in the StereoPHAN phantom appears as a useful tool for patient specific pre-treatment measurements for plan verification. The objective of this work is the validation of this device for patient specific QA (PSQA). Material and Methods The diode array was firstly calibrated in a Truebeam STx for 6WFF, 6FFF and 10FFF beams and then StereoPHAN correction factors were determined. A CT scan of SRS MapCheck inserted into the StereoPHAN was acquired with 1 mm slice thickness. In treatment planning system (TPS) a QA phantom template was created setting the origin in the intersection of fiducial markers and contouring a structure with the density overridden in four different configurations: (i) forcing the Hounsfield Units (HU) in order to have mass density of 1.2g/cm 3 (QA 1.2 ), (ii) assigning PMMA material (QA PMMA ), (iii) assigning water material (QA Water,1.2 ) and mass density of 1.2 g/cm 3 (iv) assigning water material and mass density of 1.14g/cm 3 (QA Water,1.14 ). AP fields of 5x5cm 2 and 100 MU were used for the absolute dose calibration. Finally, 7 patients treated previously with VMAT technique were evaluated using SRS MapCheck. In order to achieve higher precision on the comparison, the device was aligned using image guidance by acquiring CBCT and a performing a rigid registration, with 6 degrees of freedom, to the planning CT. Treatment plans were calculated over the QA phantom with the Analitycal Anisotropic Algorithm (AAA) and Acuros XB (AXB) algorithm using 1 mm dose grid and the RT Dose DICOM files were then imported into the SNC Patient software and compared to the measured dose distribution (figure 1). Gamma tests applying a 10% threshold were used with evaluation criteria of 1 mm distance to agreement and 3% and 2% absolute dose difference (1mm/3% and 1mm/2%, respectively).

Conclusion The validation of SRS MapCheck in the StereoPHAN phantom for patient specific QA requires TPS-to-phantom dosimetric equivalency to be verified within the accuracy of the dosimetric protocol and dose delivery. PO‐1375 Portal dosimetry prediction using Pinnacle ® TPS for clinical quality assurance applications. M. Fawzi 1 , Y. Cerbah 1 , D. Eid 1 , M. Alabdoaburas Mohamad 1 , M. Sandjon 1 , R. Belshi 1 1 Institut Curie Ensemble Hospitalier, of Radiation Oncology, St Cloud, France Purpose or Objective The aim of this work is to create a model in Pinnacle dedicated to portal pre-treatment dose verification, for image prediction in clinical radiotherapy routine. Amorphous silicon Electronical Portal Image Devices (EPID) are widely used for patient positioning. Several work have been done to develop method to use EPIDs for dose map calculations. This study concern Varian Truebeam (EPID a- Si 1200, 6MV, 6MV FFF) and Novalis Tx (EPID a-Si 1000, 6MV and 6MV SRS) for Volumetric Modulated Arc Therapy and stereotactic treatment technics. Material and Methods The method was developed for the energies and treatment units mentioned below. The EPID was calibrated according to the manufacturer procedure then portal images were acquired for a variety of rectangular fields, and complex field, using the portal device. The approach is to extract in-line and crossline profiles, output factors and implement them in Pinnacle physics tool to create a portal dosimetry machine that models the response of portal imager device. The model of portal dosimetry machine (PinnEPID) was based on the parameters of the clinical machine model. Slight adjustment was done in physics tool to best fit between measured and calculated profiles and output factors. Acquired portal images from simple complex and clinical treatment plans were performed, to evaluate the new model, using local gamma index criteria, (distance to agreement and dose difference). Results A portal dose prediction method based on Pinnacle ® was developed without modifying the commissioned parameters of the model used in clinic. For jaws-only collimated fields, a good accordance between the measured and calculated profiles in central-axis and off- axis areas were obtained, even for the small square fields sizes ( 1cm², 2cm², 3cm²). Processed-measured output factors ratio were between 0, 97-1,021. The passing rates were respectively 99.6% and 98.2% for routine local gamma index of 3%- 3mm and 2%- 2mm, with 10% of maximum dose threshold. For modulated clinical treatment plans ( Prostate, Pelvis, ENT, Lung, Breast) the mean local gamma index results yield respectively 98.7% and 97.3% of points passing for 3%- 3mm and 2,5mm- 2,5% criteria, with 10% of maximum dose threshold. Pass rates were similar for PTW- OCTAVIUS phantom matrix measurements for local gamma 3%- 3mm. Furthermore, the comparison of the predicted images calculated with Pinnacle ® PinnEPID and Eclipse TM portal dosimetry showed a very good agreement. Conclusion This study showed that the use of machine treatment parameters is sufficient with small adjustment to create efficient and accurate predicted images for pre-treatment patient Quality Assurance.

Results Gamma passing rates for each density overriding configuration are shown in Table 1. Plans calculated with AAA and the QA 1.2 phantom showed high passing rates as it was expected from the vendor instructions. On the other hand, for AXB calculated plan average gamma passing rates assigning water material (QA Water,1.2 and QA Water,1.14 ) were higher than for those calculated in PMMA material (QA PMMA ), which showed low agreement between the planned and delivered doses. Meanwhile AAA algorithm uses only HU for the calculation, AXB uses HU and the properties of the assigned material. The highest average gamma passing rate when using AXB is showed for the configuration QA Water,1.14 . Therefore, this configuration was validated for the PSQA workflow.