Abstract Book

S1208

ESTRO 37

Purpose or Objective At our institute, we have more than 20 years of experience of treating patients with Pencil Beam Scanned (PBS) proton therapy, all of which have been planned using in-house developed treatment planning systems and delivery. Clinical outcomes have been excellent. The purpose of this work is to evaluate the quality of plans produced by the Varian Eclipse treatment planning and ProBeam systems in comparison to historically applied plans. Our aim was to assess whether Eclipse calculated plans, delivered on the ProBeam system, would provide the same standard of care as has been provided by our in- house systems. Material and Methods 28 patients, all treated using PBS proton therapy in 2016 and 2017, have been included in this study, covering CSI, Cranial, head and neck, naso-pharynx, ocular and extra- cranial tumour sites. For each site, at least two patients have been included. All patients have been re-planned using Eclipse (with the NUPO optimization algorithm) and using the same treatment geometries (i.e. number and orientation of beams), but with beam data and delivery characteristics of the Varian ProBeam system. For all Eclipse plans, the manufacturers recommended settings have been used. PTV coverage, doses to OAR’s and the number of planned pencil beams have all been compared to corresponding metrics of the clinically applied plans. Results Figure 1 shows a comparison for an example case (meningioma). The minimum doses calculated to the PTV by Eclipse/ProBeam have been found to be systematically higher than those calculated by the in-house systems (median +1.07%), whilst plans optimized in Eclipse also demonstrated improved coverage at the V95 level (median +2.62%, 25% percentile +0.24%, 75% percentile 7.59%) (figure 2a). In addition, no significant differences were found for D2 or mean doses to any OAR’s between systems. Interestingly, for the Eclipse plans, only 37% (median over all fields and cases) of pencil beams used in clinical settings of the in-house planning system have been required to obtain clinically acceptable plans (figure 2b).

Conclusion Eclipse calculated PBS proton therapy plans, using beam data from the Varian ProBeam proton therapy system, have been shown, over many patients and treatment sites, to provide clinically similar plans to those delivered to patients at our institute, but using a factor three fewer pencil beams per field. For a similar sparing of OAR’s, target coverage has slightly improved using Eclipse. This could be due to differences in the used dose calculation algorithms (pencil beam de-convolution for Eclipse, ray-casting for our in-house system), but also due to a systematic difference in the definition of target/OAR volumes in both systems, with Eclipse calculated volumes being on average 10% smaller than those calculated in the in-house system. In summary however, Eclipse has been shown to provide plans of similar or better quality to the historically applied treatments at our institute. EP-2187 Do we need bolus for planning superficial sarcomas on the MR-Linac? E. Holden 1 , J. Webb 2 , A. McWilliam 3 , R. Chuter 4 1 Guy's and St.Thomas' Hospital NHS Foundation Trust, Medical Physics, London, United Kingdom 2 The Christie NHS Foundation Trust, Radiotherapy, Manchester, United Kingdom 3 University of Manchester, Manchester Cancer Research Centre, Manchester, United Kingdom 4 The Christie NHS Foundation Trust, Christie Medical Physics and Engineering, Manchester, United Kingdom Purpose or Objective Treatments on an MR-Linac result in increased superficial dose at tissue-air interfaces as a result of the electron return effect (ERE). This work investigates whether the ERE can be utilised to provide increased coverage of superficial disease. Peripheral chest wall sarcoma treatment plans are typically treated with bolus to ensure acceptable target coverage. If the ERE can compensate for bolus, patient workflows would be reduced and uncertainties introduced by unintentional air gaps between the patient and bolus would be removed. Material and Methods Five patients previously treated for chest wall sarcomas were replanned on Monaco Research Version 5.19.02, modelling the 1.5T MR-Linac (Elekta Unity, Elekta AB, Stockholm, Sweden) using an MR-Linac specific beam model. For each patient a 5-7 beam step and shoot IMRT plan was created. Each patient had four plans optimised (calculated to 1% statistical uncertainty); 2 plans were created without the magnetic field (0T), with and without 0.5cm bolus and 2 plans with the magnetic field (1.5T), with and without 0.5cm bolus to cover the PTV. Plans were normalised to give a mean dose of 60Gy in 30# to an edited PTV clipped back from the external (patient surface or bolus) by 5mm. To evaluate the superficial dose to the PTV, a 5mm thick superficial surface of the PTV (PTVSurface) was created. Results Optimising without bolus at 1.5T increased the mean dose delivered to PTVsurface by 4.3% on average (range: -4.5% to 9.2%). However, including the magnetic field did not increase the mean dose to this region to higher than 91% of the prescription dose, indicating that the majority of the PTVSurface received below the ICRU minimum requirement. Optimising using bolus increased the mean dose delivered to PTVSurface regardless of whether the magnetic field was included. The increase in mean dose to PTVSurface when adding bolus was 11.3% and 7.4% at 0T and 1.5T respectively.