ESTRO 36 Abstract Book

S218 ESTRO 36 _______________________________________________________________________________________________

required for establishment in our own lab as well as equipment and reagents that we will have to source to make it possible. Our hosts in Heidelberg also generously arranged to send an experienced lab member to Glasgow to help improve our surgical technique. From this initial visit we established a second collaboration with the lab of Prof. Jim Norman at the Beatson Institute of Cancer Research which was instrumental to allowing introduction of the technique to our home institutes. The ability to talk to our hosts and ask questions in a face to face setting has proved vital in helping us to get a good grasp of all the techniques required. This also allowed us to explore potential for collaborations in the future and for sharing useful reagents such as tumour cell lines between our groups to facilitate our research. SP-0414 Experience with the ESTRO mobility grant; proton irradiation of a 3D dosimeter E.M. Høye 1 , P.S. Skyt 1 , P. Balling 2 , J. Swakon 3 , J.B.B. Petersen 1 , M. Rydygier 3 , G. Mierzwińska 3 , L.P. Muren 1 1 Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark 2 Aarhus University, Department of Physics and Astronomy, Aarhus, Denmark 3 Polish Academy of Sciences, Institute of Nuclear Physics, Krakow, Poland In my visit to the Cyclotron Center Bronowice in Kraków I investigated the performance of a new 3D dosimeter for proton therapy. The aim of the visit was to study the known quenching effects in the Bragg peak of proton beams in our dosimeter. Denmark is currently building its first proton therapy center, and so a collaboration agreement was made with the polish center in order to perform irradiations of the dosimeter. Dosimeter samples were prepared with different chemical compositions, and brought to Kraków. A 1D optical laser scanner was sent to Kraków, in order to allow read out of the proton depth dose curve in the dosimeters few hours after irradiation. Based on preliminary results from our measurements, decisions were made as to which chemical compositions to investigate further. New dosimeters were produced for us in Aarhus, and sent to Kraków to be irradiated in the second week. The experience was logistically challenging, and many people were contributing to the success of the project. The study gave us detailed information as to how the dosimeter can be further optimised for proton therapy. In my presentation I will expand on the challenges we met and on how they were dealt with. I am grateful to ESTRO for allowing me this opportunity, together with all the people in Aarhus and Kraków who helped with the experiments. PV-0415 Verification of pre-treatment DVH measurements for individual plan QA J. Stroom 1 , J. Boita 1 , M. Rodrigues 2 , C. Greco 1 1 Fundação Champalimaud, Radiotherapy, Lisboa, Portugal 2 Mercurius Health, Radiotherapy, Lisbon, Portugal Purpose or Objective Radiotherapy plan QA by measurements is almost mandatory for IMRT and VMAT. Generally, comparison of planned and measured dose is performed using the clinically not so relevant gamma analysis. Recently software has become available to estimate DVHs based on QA measurements. We have validated two such systems. Material and Methods Our new system, 3DVH (v3.3, SunNuclear), converts dose deviations measured with the cylindrical ArcCheck Poster Viewing : Session 9: Dosimetry

phantom to expected dose deviations in the patient, hence enabling calculation of DVHs for targets and OARs. Our existing system, PDAPP (NKI-AVL, Amsterdam), uses back-projection of measured EPID dose images to produce 3D doses in patients or phantoms. These doses and dicom structure files are subsequently read by in-house software ( pDVH ) to calculate DVHs. We performed the following tests: 1. 3DVH : With the new system, we first measured 30 different clinical plans (VMAT/IMRT) with various energies (6MV – 10FFF) on different linacs (Varian/Elekta) and evaluated the measured 3D dose distributions using 3D gamma (3%,3mm). We then compared with the clinical 2D ArcCheck analyses of the same measurements. 3DVH : We subsequently introduced MU errors or systematic MLC errors (all leafs opened or closed) in a subgroup of 6 Elekta plans before measurement and studied the behaviour of 3DVH . 2.

3DVH + pDVH : To compare 3DVH with pDVH , we made 3 conformal plans (AP, AP-PA, 4-field box) and one 4-field IMRT plan on a slab-phantom with PTV and cubic and cylindrical OARs (Fig). The plans with and without errors were measured with the slab-phantom for PDAPP , and with ArcCheck for 3DVH . Mean PTV and OAR doses were compared. 3DVH + pDVH : For the phantom IMRT plan, we predict the effect of an X mm MLC error on the mean PTV dose to be X/, with the average leaf pair distance in the plan. For the cube DVHs of the conformal plans leaf motions should shift the penumbra of the AP/PA beams into or out of the 60mm cube and the resulting DVH up and down by X/60, so X=3mm would yield ΔV D50 ≈ 5% (Fig).

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Results

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Average 3D gamma passing rates of the 30 clinical cases were 97.7±3.5% (1SD), comparable to the 2D rates of 97.0±2.1%. There was no correlation between 2D and 3D results.