ESTRO 36 Abstract Book

S774 ESTRO 36 2017 _______________________________________________________________________________________________

study is transition from 2D QA to 3D dose reconstruction in a patient CT scan which could be achieved using dose reconstruction method from 2D detector array in the Compass system. The first step in the clinical introduction of this system, instead of currently used 2D QA in OmniPro system, is to test reliability of dose reconstructions. In this work we investigated the validation of the method with OmniPro results as a reference. We check whether the Compass QA measurements of VMAT plans fulfill the QA requirements. Material and Methods 50 different treatments according to VMAT plans were selected from our database; 20 prostate, 20 gynecology and 10 brain. The QA results were divided based on the mean gamma index and the 3%/3mm and 1%/1mm criteria. Results from OmiPro were compared with Compass and TPS. Additionally, recalculation plan from TPS (Monte Carlo) in Compass system based on the different algorithm (Collapse Cone Convolution) were performed. MLC tests (3ABUT, 7SegA, FOURL plan) were implemented before each set of measurements for evaluation of interleaf leakage, tongue and groove effect. Results Mann-Whitney test showed good agreement between Compass 3D-reconstructed dose and OmniPro results (mean gamma 0.23 ±0.03 for 3%/3mm and 0.53±0.06 for 1%/1mm criteria). Scatter plot of results from TPS vs. Compass against TPS vs. OmniPro showed small differences in the region of gamma between 0.2 and 0.4. Comparison TPS vs. Compass mean dose in PTV and OAR did not reveal significant differences for prostate 50.04 Gy±0.4, 50.35Gy±0.33, bladder 32.04 Gy±0.41, 32.45 Gy±0.23; gynecology 45.07 Gy±0.34, 45.02 Gy±0.25, bladder 35.04 Gy±0.74, 35.75 Gy±0.49; brain 60.07 Gy±0.53, 60.02 Gy±0.71, brain stem d max 40.04 Gy±0.83, 39.08 Gy±0.33 respectively. Conclusion Agreement between results obtained from Compass and OmniPro was reached. 3D dose reconstructions in CT patient allowed to evaluate the dosimetric errors and their clinical relevance. Compass reconstruction offers good opportunities to examine dynamic plans and check characteristics of MLC. EP-1467 IPEM Code of Practice for proton and ion beam dosimetry: update on work in progress S. Green 1 , R. Amos 2 , F. Van den Heuvel 3 , A. Kacperek 4 , R.I. MacKay 5 , H. Palmans 6 , D. D'Souza 2 , R. Thomas 6 1 Hall-Edwards Radiotherapy Research Group- Queen Elizabeth Hospital, Medical Physics, Birmingham, United Kingdom 2 University College London Hospitals, Radiotherapy Physics, London, United Kingdom 3 Churchill Hospital, Radiotherapy Physics, Oxford, United Kingdom 4 Clatterbridge Cancer centre, Physics Department, Wirral, United Kingdom 5 The Christie NHS Foundation Trust, Medical Physics, Manchester, United Kingdom 6 National Physical Laboratory, Radiation Dosimetry Group, London, United Kingdom Purpose or Objective Current standard methods for reference dosimetry of proton and ion beams typically involve the use of an ionization chamber calibrated in a cobalt-60 beam, with a beam quality correction factor applied to account for the difference between the chamber response in the proton and the calibration beams. This approach gives rise to uncertainties (at 68% confidence level) on the reference dosimetry of 2.4% for proton beams and 3.4% for carbon ion beams when using a plane-parallel ionization chamber. This poster provides an update on the development of a new Code of Practice for reference dosimetry of proton and ion beams, applicable to both scanned and scattered

beam configurations. It is aimed to deliver an uncertainty on reference dosimetry for protons of approximately 2% and will utilise a primary standard graphite calorimeter that is robust and portable enough to be used in the end- user facility. Material and Methods This project involves a core team (authors on this submission) plus a group of experts in the field to provide peer-review. Proposed key elements of the protocol are the use of the NPL portable graphite calorimeter, calibration in a composite field defined to cover what is termed a Standard Test Volume (STV) of delivered dose for scanned beams and calibration in a broad field spread- out Bragg Peak to cover the STV for scattered beams. Results While for scattered beams the recommendations will be largely in line with those already published, the key steps for scanned beams are proposed to be as follows: Step 1: Derive the curve which defines the number of particles per Monitor Unit (MU) for a range of incident proton/ion energies. This is the Hartmann method and utilises a plane-parallel ionization chamber at a shallow depth in pristine Bragg peaks. Step 2 : Input the curve above into the treatment planning system (TPS) for the centre/treatment room and then use the TPS to plan a prescribed dose to the STV in water and deliver this treatment to the calorimeter with its core at the centre of the STV. Step 3: Re-normalise the data obtained in step 1 if necessary, to ensure that the calibration in terms of the number or particles per MU results in the measured dose to the STV. Step 4. Test against alternative STVs to quantify uncertainties in the dose delivered. In addition, ionisation chambers belonging to the clinical centre will be cross-calibrated against the standard calorimeter at the time of beam commissioning and at regular intervals (to be defined) thereafter. Conclusion A proton and ion beam dosimetry protocol will be developed which involves direct use of a primary standard level calorimeter in clinical ion beams. This may provide a model to be followed elsewhere, ultimately reducing dose uncertainty for patient treatments worldwide. The code is under development and due for completion at the end of 2017. This will coincide with beam commissioning at the UK centres during 2018. This poster will describe the proposed methodology with the aim of stimulating wider debate and comments on this approach. EP-1468 Skin dose in radiotherapy: results of in vivo measurements with gafchromic EBT3 films A. Giuliano 1 , V. Ravaglia 2 1 Istituto Nazionale di Fisica Nucleare INFN, Pisa, Pisa, Italy 2 San Luca Hospital, Medical Physics, Lucca, Italy Purpose or Objective Clinical side effects to skin are a major concern with radiotherapy patients during the treatment of malignant disease by radiation. As a consequence, it becomes important to accurately determine the dose delivered to a patient skin during radiotherapy owing to complications that can arise. However, the Treatment Planning Systems (TPS) do not accurately model skin dose. The aim of this study is to report the results of surface dose measurements performed during treatments in tomotherapy, at Linac both with 3D-CRT (TPS Pinnacle) and VMAT (TPS Monaco) and in Plesio-Röntgen therapy using EBT3 Gafchromic films. Material and Methods In vivo measurements were performed with the application of EBT3 film pieces of 2x2 cm 2 directly on the skin of patients or in the inner side of thermoplastic mask, if used during the treatment. The target sites included