ESTRO 36 Abstract Book

S770 ESTRO 36 2017 _______________________________________________________________________________________________

patients treated for cancer in the head and neck region showed similar agreement. Conclusion The dosimetric accuracy of the presented method for reconstructing the delivered 3D dose distribution daily CBCT appears very promising. Further studies are however required to study the sensitivity to dosimetric changes from changes in patient anatomy and set-up errors not accounted for by the CBCT based set-up verification procedure. C. Talamonti 1 , S. Russo 2 , M.D. Falco 3 , A. Bartoli 4 , E. Cagni 5 , L. Strigari 6 , M. Esposito 2 , F.R. Giglioli 7 , C. Fiandra 7 , C. Marino 8 , S. Clemente 9 , M. Stasi 10 , E. Villaggi 11 , P. Mancosu 12 1 University of Florence, Dip Scienze Biomediche Sperimantali e Cliniche, Firenze, Italy 2 Azienda Sanitaria Firenze, Fisica Sanitaria, Florence, Italy 3 Policlinico Chieti, Fisica Sanitaria, Chieti, Italy 4 Istituto Nazionale Fisica Nucleare, Sezione di Firenze, Florence, Italy 5 ArciOspedale Santa Maria Nuova, Fisica Sanitaria, Reggio Emilia, Italy 6 IFO, Fisica Sanitaria, Roma, Italy 7 Ospedale Molinette, Fiisica Sanitaria, Torino, Italy 8 Humanitas, Fisica Sanitaria, Catania, Italy 9 IRCCS CROB, Fisica Sanitaria, Potenza, Italy 10 Ospedale ordine Mauriziano, Fisica Sanitaria, Torino, Italy 11 AUSL Piacenza, Fisica Sanitaria, Piacenza, Italy 12 Humanitas, Fisica sanitaria, Milano, Italy Purpose or Objective The aim of this study was to provide Relative Signal Ratio (RSR) values of three Linac models (Elekta, Elekta Synergy BM and Varian) using measurements performed in a multicenter Italian study. An eventual mathematical description of the RSR curve was proposed in order to calculate RSR for arbitrary field sizes with high accuracy. Material and Methods Thirty centres with different LINACs joined this project. All measurements were performed using the new IBA unshielded silicon diode RAZOR and the Stealth flat ionization chamber fixed on the gantry as reference. The RAZOR was positioned at 10cm depth in water and SSD=90cm. RSRs were calculated for square field size ranging 0.6-5.0cm. Data were normalized to the 3x3cm 2 field size. In-plane and cross-plane profile were measured to correct RSR for the effective field size (EFS).The error assigned to the experimental points was estimated taking into account the statistical dispersion of the repeated measurements and the error introduced on RSR by the detector positioning uncertainty. Collected data were clustered by linac model and each group was fitted using the function proposed by Sauer (Med Phys2007). Moreover the obtained curves were compared with the one published by Sauer and Wilbert (SWF) calculated from the fit to the mean values of four solid state detector data series of an ELEKTA linac. Results The experimental data (blue points) with the fit function (black line) and the SWF (pink dotted line) are shown in figure 1 for each linac model. In all cases, It is evident how the proposed analytical functions fit perfectly to the data for all field sizes and for all the three linac models investigated. Deviations from the SWF are shown in the figure 2. The largest difference of the RSR value respect to the SWF ones are evident below 2cm of effective field size. This is mainly due to the different geometry of the head of the linacs and therefore small changes in the energy spectrum EP-1459 Relative Signal Ratios using an unshielded silicon detector: data from 30 centers

Conclusion In general, the adopted methodology removes much of the ambiguity in reporting and interpreting small field dosimetric quantities and facilitates a clear dosimetric comparison across a population of linacs. Moreover it minimizes the influence of measurement uncertainties and it allows accurate determination of values for non- measured field sizes. EP-1460 Detection of forced errors in VMAT plans using EPID and Epiqa dosimetric system D. Granero 1 , L. Brualla 1 , A. Vicedo 1 , M. Trinitat 1 , J. Rosello 1 1 Eresa-Hospital General Universitario, Department of Radiation Physics, Valencia, Spain Purpose or Objective VMAT treatments in radiotherapy are verified usually by an experimental method, such as, measurement in discrete points with an ionization chamber, radiochromic film, EPID or other dosimetric systems. In this study we did an analysis of the error detection capability in treatment plans of aS1200 EPID of TrueBeam 2.0 linac together with the Epiqa v4.0.12 dosimetric software from EPIdos. To do this we have introduced forced errors in the treatment plans of 5 different VMAT patients in order to test if the dosimetric system can detect different forced errors introduced in the treatment plans. Material and Methods 5 different patients have been selected, a hypofractionated prostate+vesicles treatment, a hypofractionated prostate+vesicles+lymph nodes a head and neck patient, a spine SBRT and finally a treatment of the paraaortic lymph nodes. Treatment plans were designed using 1 or 2 complete arcs. We have introduced forced errors in the Pinnacle treatment plans using excel home made macros. A total of 160 different plans have been irradiated with the TrueBeam 2.0 linac and dose has been measured with the EPID. The errors introduced were: