ESTRO 38 Abstract book

S912 ESTRO 38

Material and Methods ARTISCAN Beam QA software is based on EpiDream method (Boutry et al, Med Phys 2017) converting EPID signal in air into absorbed dose in reference conditions. The product is developed to perform dose quality control of photon and electron beams using EPID. Our study was performed on 6 MV and 6 MV FFF photon beams from Elekta linac (Versa HD with iViewGT v3.4.1) and on 6 MV photon beam and 6 MeV electron beam from Varian linac (Clinac 2100C with EPID AS500). Intrinsic characteristics of the solution were first evaluated, including repeatability, reproducibility, dose linearity and dose consistency over the time. Then, we performed tests of our current internal QA program: daily X-Ray and electron output constancy, beam profiles constancy, wedge transmission factor constancy, monitor chamber linearity. Results were compared to those obtained with ionization chamber (0.6 cc, PTW) or with ionization chamber 2D-matrix (StarCheck, PTW). Results The reproducibility of 10 EPID measurements was 0.09% for 6 MV (Elekta). Dose linearity for photon and electron beams for the Varian linac was better than 1% for MUs higher than 20 with EPID solution. From 3 to 15 MUs, dose linearity was between 1.3% and 2.4% for photon beam and 0.7% and 1.9% for electron beam EPID solution. A 4 months analysis showed a dose deviation for a square field between +0.05% and -1.25% for 6 MV beam and between +0.39% and - 1% for 6 MV FFF beam (Elekta). For the daily output constancy on Elekta linac, the mean difference obtained for 32 QAs between EPID dosimetry and ionization chamber was -0.1% (σ=0.3%) for 6 MV and - 0.6% (σ=0.3%) for 6 MV FFF beam. Beam profiles obtained with ARTISCAN Beam QA software and the StarCheck were plotted for 6 MV, 6 MV FFF and 6 MeV beams in Fig 1. The difference on wedge transmission factor obtained with EPID solution and ionization chamber was 0.2% for 6 MV (Varian). The maximal difference on monitor chamber linearity for 6 MV beam (Varian) between EPID solution and ionization chamber was 1.4% for MUs higher than 3, reaching 5% for 1 MU.

1 Addenbrooke's Hospital - Oncology Centre, Medical Physics Box 152, Cambridge, United Kingdom Purpose or Objective When measuring percentage depth doses (PDD) using a radiation detector in a plotting tank, the dose per pulse decreases with depth in water. Since the recombination correction to be applied to the detector varies with dose per pulse, this requires corrections to be made to the PDD. At the small doses per pulse observed in conventional flattened field, these corrections are small (typically less than 0.2%); however with the larger doses per pulse seen in Flattening Filter Free (FFF) beams, corrections of up to 1.3% can be required, depending on the detector used (Budgell et al 2016). The PTW 60019 micro diamond (PTW, Freiburg, Germany) is a commercially available synthetic diamond detector, designed for the dosimetry of small radiotherapy fields. As there is a lack of consistent published data on the dose per pulse dependence of the Micro Diamond the purpose of the measurements described below is to determine whether a dose per pulse correction is needed for this detector. Material and Methods We have performed measurements on two PTW 60019 Micro Diamond detectors to determine whether such a correction is necessary. Depth doses for a 10MV FFF beam were measured with the Micro Diamond, and compared with depth doses measured with an A1SL ion chamber which had been corrected for ion recombination. Results For depths from 30mm to 300mm, the ratio of the depth dose to the corrected A1SL ion chamber depth dose was 1.0005±0.001 for one of the Micro Diamonds, and 1.0001±0.001 for the other. Both of these values are consistent with unity. Conclusion For the measurement of depth doses on FFF beams with large dose per pulse, no correction for the variation with depth of dose per pulse is required when measuring with a PTW 60019 micro diamond. EP-1696 Microdosimetry assessment in cyclotron proton beamline with new 3D-microdetectors J. Prieto-Pena 1 , A. Baratto-Roldán 2 , C. Fleta 3 , M.C. Jiménez-Ramos 4 , J. García López 2 , G. CONSUELO 5 , M. Cortés-Giraldo 6 , J.M. Espino 6 , F. Gómez 1 1 Univ.de Santiago de Compostela, Departamento de Física de Partículas, Santiago de Compostela, Spain ; 2 Centro Nacional de Aceleradores, Univ. Sevilla- CSIC- JA, Sevilla, Spain ; 3 Centro Superior de Investigaciones Científicas CSIC, Instituto de Microelectrónica de Barcelona IMB-CNM, Barcelona, Spain ; 4 Centro Nacional de Aceleradores, Univ.Sevilla- CSIC- JA, Sevilla, Spain ; 5 Centre national de la recherche scientifique, Imagerie et Modélisation en Neurobiologie et Cancérologie, Orsay Ville, France ; 6 Universidad de Sevilla, Dpto. Física Atómica- Molecular y Nuclear, Sevilla, Spain Purpose or Objective Protontherapy achieves very high dose conformity around the target, allowing a better protection of the organs at risk (decreasing radiation side effect) [1]. The determination of the relative biological effectiveness (RBE) of protons depends on several factors, being the particle LET one of them. Therefore the use of devices that help to reduce any uncertainty is essential. Likewise, there is a rising interest in the medical-physics community in placing the enhanced LET of the beam within the tumor or removing it from the most sensitive normal structures around [2]. Nevertheless, there is no instrument to quantify the LET in clinical scenarios, but as the one we show herein. In this work we present direct microdosimetric measurements of 18 MeV proton beams

Conclusion Our results showed that the EPID dosimetry solution proposed by AQUILAB seems to be promising to perform QA programs on Elekta and Varian linacs for both photon and electron beams. Reference A simple algorithm to convert EPID gray values into absorbed dose to water without prior knowledge. Boutry C, Sors A, Fontaine J, Delaby N, Delpon G. Med Phys. 2017 Dec; 44(12):6647-6653. EP-1695 Determining the dose per pulse dependence of a commercial synthetic diamond detector D. O'Doherty 1 , J. Cross 1 , R. Plaistow 1 , K. Fathi 1 , S.J. Thomas 1