ESTRO 37 Abstract book

S956

ESTRO 37

beam CT was used for set up correction of translational errors. In vivo three-dimensional dose distributions were computed by Dosimetry Check 4.1. The differences between planned and in vivo doses were evaluated using isocenter dose difference (ΔIso), Gamma Passing Rate (GPR) 3%/3mm (30% Dose threshold), and DVH differences (Δmin, Δmax, Δmean) in CTV. Tolerance for dose differences was set to 5% for mean CTV dose and 10% for maximum and minimum CTV doses. Statistical analysis was performed for evaluate correlations between GPR, DVH-differences and isocenter dose difference. Results 24% of fractions had DVH dose differences out of tolerance levels. In 46% of these fractions causes of discrepancy were identified. 38% of discrepancies were due to the incorrect position of immobilization devices, 31% was due to residual errors after correction of set-up by CBCT, and 31% was due to respiratory induced diaphragm motion. In the remaining fractions out of tolerance levels (54% of total), causes of discrepancy were not identified. Mean dose differences and standard deviation were (-2.6%± 3.5)% for Δmean, (-3.8%± 3.8)% for Δmin and (-3.6%± 3.8)% for Δmax in CTV; (-2.2%± 3.6)% for ΔIso. CTV Δmean, Δmin and Δmax resulted significantly correlated with ΔIso (p<0.01). GPR resulted uncorrelated with DVH and isocenter dose differences (p>0.25).

Conclusion Although Thorax-Low Dose presented the higher dose average variation, was lower than 1.3%. AAA and AcurosXB reproduce similar results while using a CBCT for dose calculation. These results suggest that CBCT-based dose calculation can be made with a reasonable difference in calculated dose. EP-1780 Volumetric EPID transit dosimetry for abdominal and pelvic stereotactic treatments M. Esposito 1 , A. Ghirelli 2 , S. Pini 2 , S. Russo 2 , G. Zatelli 2 , P. Alpi 3 , R. Barca 3 , M. Coppola 3 , S. Fondelli 3 , B. Grilli Leonulli 3 , L. Paoletti 3 , F. Rossi 3 , P. Bastiani 3 1 Esposito Marco, S.C. Fisica Sanitaria, Firenze, Italy 2 Azienda Sanitaria USL centro, S.C. Fisica Sanitaria, Firenze, Italy 3 Azienda Sanitaria USL centro, S. C. Radioterapia, Firenze, Italy Purpose or Objective Stereotactic body radiation therapy (SBRT) is one of the most complex treatment in common clinical use. This complexity, combined with the high dose prescribed, requires high level of dosimetric accuracy. EPID in vivo dosimetry measures the accuracy of the actual delivered treatment and is sensitive to detecting treatments errors that would be missed by pre-treatment QA. In this work we reported findings from routinely measured in vivo doses of pelvic and abdominal SBRT treatments. Different metrics for evaluation of dosimetric accuracy have been compared: punctual dose difference, gamma metric and DVH differences. Material and Methods 83 fractions of 50 stereotactic VMAT treatments with target in the abdomen or pelvis were analyzed (9 liver, 10 adrenal gland, 4 spine, 27 lymph nodes). Daily cone

Conclusion EPID in vivo dosimetry was able to identify treatment errors in abdominal and pelvic SBRT. GPR was not correlated with dose differences in DVH, otherwise in our analysis dose difference in isocenter was strongly correlated with all DVH dose differences analyzed. EP-1781 Towards treatment planning dose calculation for magnetic resonance guided proton therapy F. Padilla 1 , A. Resch 1 , D. Georg 1 , H. Fuchs 1 1 Medizinische Universität Wien, Department of Radiotherapy, Vienna, Austria Purpose or Objective After the successful clinical implementation of Magnetic Resonance Image (MRI) guided X-ray beam therapy, the possibility of MRI guidance for proton therapy (PT) is currently explored in a research setting. A pre-requisite for a successful implementation of MRI-PT is a proper treatment planning system. This work aims to implement and validate a dose calculation algorithm based on pencil beam kernels for proton beams within magnetic field regions.