ESTRO 36 Abstract Book

S114 ESTRO 36 2017 _______________________________________________________________________________________________

combined to treat large areas) necessary for large H&N and pelvic treatments. Material and Methods As of November 2015, 43 patients were treated on G2 for a total of 74 plans (21 SFUD, 51 IMPT and 3 SIB) and 248 fields (average of 4 fields per plan), of which 26 fields were patched. Parameters recorded during the treatment delivery of these patients (spot positions, MU’s per pencil beam, couch and gantry position) are stored into a log file and used to reconstruct the 3D dose distribution by an in- house developed Independent Dose Calculation software (Meier et al. 2015). A MATLAB script calculates the dose metrics by comparing the reconstructed to the nominal dose distribution. These metrics include the maximum, minimum and mean dose differences as well as the percentage of voxels within +/- 1% of the nominal dose (pass rate). Results Table 1 shows the results of the log file analysis. Interestingly, and despite the typically higher modulation for IMPT, the average pass rate for both SFUD and IMPT is similar, with the 95% percentile actually being a little better for IMPT. In addition, complex plans with steep in- field dose gradients, such as SIB treatments, also had pass rates >99%. Nevertheless, highly modulated plans can have larger local dose differences as seen by the larger max dose deviation in Table 1 and demonstrated for a specific case in Figure 1. Hence, attention should be paid to the location of isolated, highly weighted spots.

Figure 2 shows the comparison between the normalized lateral X and Y profiles of EPID images acquired with and without the B-field (3x3,5x5,8x8,10x10,12x12,15x15,20x20 cm 2 ). More than 99% of the points showed local deviations smaller than 2% for the X and Y profiles.

The 2-D γ-analysis showed that the averaged γ mean was 0.42 ± 0.16 and the % γ≤1 was 96.6 ± 4.5. Conclusion EPID images acquired with and without B-field are virtually identical, indicating that the presence of a small (2.5 mT) magnetic field at the EPID level in the MR-Linac should not become an impediment for the implementation of EPID dosimetry in the MR-Linac. Acknowledgements This research was partly sponsored by Elekta AB, Stockholm, Sweden. The authors would like to thank Robert Spaninks (Elekta) for assistance with the measurements. OC-0230 Treatment log files as a tool to identify inaccuracies in scanned proton beam delivery and planning M. Belosi 1 , R. Van der Meer 1 , P. Garcia de Acilu Laa 2 , A. Bolsi 1 , D. Weber 1 , A. Lomax 1 1 Paul Scherrer Institute, Centre for Proton Therapy, Villigen PSI, Switzerland 2 Hospital Universitario Puerta del Sur Hospitales de Madrid, Radiofisica Hospitalitaria, Madrid, Spain Purpose or Objective Dose distributions delivered at Gantry2 (G2) at the Paul Scherrer Institut (PSI) can be reconstructed on the patient anatomy based on machine log files. These dose reconstructions are a powerful tool in identifying potential issues related to the integrity of the patients’ dose delivery, as has already been demonstrated for a first series of patients treated in G2 for skull base chordomas (Scandurra et al. 2016). Here, such calculations have been extended by investigating their dependency on planning technique (e.g. SFUD vs IMPT, field direction etc) and on couch position. The latter is crucial for quality assurance of the delivery of patched fields (different sub-fields

Finally, the results of the first patched field treatments (2 to 4 patches per field) did not show any evidence of dose deviations at the interface between patches. Conclusion 3D dose reconstruction using treatment log files is a powerful tool to identify delivery problems and trends, and to improve planning robustness. Further effort should be invested in order to predict field robustness to delivery fluctuations before the clinical delivery of the plan as part of the plan’s specific QA. OC-0231 The suitability of radiochromic film in 0.35T magnetic field CO-60 compared with conventional 6MV D.L.J. Barten 1 , L.J. Van Battum 1 , D. Hoffmans 1 , S. Heukelom 1 1 VUMC, Radiotherapie, Amsterdam, The Netherlands