ESTRO 36 Abstract Book

S784 ESTRO 36 _______________________________________________________________________________________________

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 head and neck (H&N), brain and sarcoma in tomotherapy; breasts at Linac and skin tumors in Plesio-Röntgen therapy. For each patient films were located in 1 to 3 reproducible points (see figure 1 and 2) and measurements were repeated on average in three consecutive fractions. EBT3 films were read with a flatbed scanner Epson

10000XL and images were analyzed using the red channel calibration. In vivo dose evaluations were compared with measurements performed on Cheese phantom both with and without thermoplastic mask at Linac and in Tomotherapy.

Results A total of 117 film measurements were performed on 21 patients. The absolute value of the mean difference between measured and TPS-calculated dose and its standard deviation was 11.3% ± 6.5% for all treatments. A mean absolute difference of 17.7% for Linac plans, 11.6% in Tomotherapy and 4.6% in Plesio-Röntgen therapy were achieved. Both at Linac and in Plesio-Röntgen therapy there was not a clear trend of overestimation of the TPS with respect to measurements. Instead in Tomotherapy there was an underestimation of the TPS (-9.1%) for H&N and brain treatments (in these case measurements were performed with thermoplastic mask) and an overestimation for the sarcoma (9.2%). This trend was confirmed by the measurements made on the Cheese phantom in Tomotherapy, where there was an overestimation of the TPS without mask (28.6% vs -0.7% with mask). Moreover, an improvement of the agreement between EBT3 measurements and Pinnacle and Tomotherapy dose estimation was shown in presence of mask (28.6% to -0.7% in Tomotherapy and -20.7% to -16.3% at Linac). Conclusion Gafchromic films are suitable detectors for skin dose measurements in radiotherapy. In vivo surface dose measurements with EBT3 are a useful tool for quality assurance in radiotherapy, since the TPS does not give accurate dose values in the first millimiters of skin.