ESTRO 36 Abstract Book

S946 ESTRO 36 2017 _______________________________________________________________________________________________

2 Karolinska Institutet, Department of Oncology and Pathology, Stockholm, Sweden Purpose or Objective For several decades unidirectional photon-grid therapy has been a useful tool in radiation oncology. Its main advantage is to limit the normal tissue toxicity when irradiating the patients with bulky tumors. In this work we use proton grid therapy (PGT). PGT delivered with a crossfiring technique has been used instead of a unidirectional approach. The physical properties of proton beams allow for the protection of risk organs posterior to the target while the crossfiring technique enables a larger separation between the beams, thus better preserving the normal tissue. Here we evaluate the possibility to use PGT as a therapeutic option in certain clinical situations. For example, due to the ability of interlaced proton-beam grids to significantly spare normal tissue, this technique may be useful in re-irradiation cases not otherwise eligible for radiotherapy treatment because of too high doses to organs at risk. Material and Methods CT data from patients previously treated with conventional photon therapy at Karolinska Hospital, Stockholm, were reused in order to create PGT treatment plans with the TPS Eclipse (Varian Medical Systems). Patients that could benefit re-irradiations or palliative care were selected. The aim was to deliver a high and nearly homogeneous target dose, while keeping the grid pattern of the dose distribution, made of peak and valley doses, as close to the target as possible. A low grid dose, with low peak and valley doses, was also preferable to better protect the normal tissue. The dosimetric characteristics of those plans were then evaluated, with a focus on the overall homogeneity of the target dose, as well as dose profiles outside of the target (i.e. evaluation of the grid dose distribution through peak and valley doses analysis). Results All the studied cases presented dose distributions for which the grid pattern was preserved until the direct neighborhood of the targets. When normalizing the minimum target dose to 100%, the valley doses reached around 5%, while the peak doses were approximately 60- 70%, depending on the grid geometry used. Inside the targets, a good dose homogeneity could be achieved (σ= ±10 %). The volumes of organs at risk irradiated with high doses remained small and limited spatially to the dose peaks of the grids. Conclusion PGT produces a combination of nearly homogeneous and high target dose. The grid pattern can be preserved in the normal tissue, from the skin to the direct vicinity of the target, preventing extensive damage to the organs at risk. The PGT approach could present a therapeutic possibility in difficult clinical situations where conventional radiotherapy would fail to provide any suitable option for the patients. EP-1744 Failure modes and effects analysis of Total Skin Electron Irradiation (TSEI) technique B. Ibanez-Rosello 1 , J.A. Bautista-Ballesteros 1 , J. Bonaque 1 , J. Perez-Calatayud 1,2 , A. Gonzalez-Sanchis 3 , J. Lopez-Torrecilla 3 , L. Brualla-Gonzalez 4 , M.T. Garcia- Hernandez 4 , A. Vicedo-Gonzalez 4 , D. Granero 4 , A. Serrano 4 , B. Borderia 4 , J. Rosello 4,5 1 La Fe University and Polytechnic hospital, Radiotherapy, Valencia, Spain 2 Clínica Benidorm, Radiotherapy, Benidorm, Spain 3 General University hospital, Radiation Oncology, Valencia, Spain 4 General University hospital, Medical Physics, Valencia, Spain 5 University of Valencia, Physiology, Valencia, Spain

Purpose or Objective A risk analysis of the Total Skin Electron Irradiation (TSEI) technique was performed. The aim of this study was to evaluate the safety and the quality of the treatment process, as well as to adapt the quality assurance program according to the results. Material and Methods This revision has been executed in a reference center in the TSEI technique, with 80 patients treated following the method Stanford. The risk analysis was made following the methodology proposed by the TG-100 of the AAPM, which is an alternative procedure to the guidelines proposed by the ESTRO in the ACCIDRAD project. To this end, a multidisciplinary team developed the process map, outlining the stages of treatment and steps in which each stage is divided. The potential failure modes (FMs) of each step were proposed and evaluated, according their severity (S), occurrence (O) and detectability (D), with a scale from 1 to 10. The product of this factors resulted in its priority number risk (RPN), which enabled ranking the FMs. Then, the current quality management tools were examined and the FMs were reevaluated taking to account these tools. Finally, the FMs with RPN ≥ 80 were studied and new quality management tools to reduce its RPN were proposed. Results 75 steps contained in a total of 12 stages were observed. 361 FMs were evaluated, initially 103 had a RPN ≥ 80 and 41 had S ≥ 8. After, current management tools were considered, only 30 FMs had RPN ≥ 80 (Figure 1). Thereby, new control tools were derived from the study of these 30 FMs. The riskiest FMs were associated to the patient's position during treatment. For the "general body treatment" stage, the position of the screen and the patient was marked on the floor (Figure 2a) and some templates representing the position of the feet were drawn (Figure 2b). In addition, to facilitate positioning of the patient's limbs during “hands treatment” and “feet treatment” stages, the axes must traverse the lasers and the field size within which should position the extremities were marked on the sheet (Figure 2c). These new management tools have begun to be implemented in the facility.