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Investigating the Robustness of MR-Guided IMPT for Prostate Cancer in the Presence of Anatomical Changes and Positioning Errors

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C Kurz

C Kurz*1,2, G Landry3, A Resch3,4, G Dedes3, F Kamp1, U Ganswindt1, C Belka1, B Raaymakers2, K Parodi3, (1) LMU Munich, Munich, Germany, (2) University Medical Center Utrecht, Utrecht, the Netherlands, (3) Ludwig-Maximilians-Universitaet Muenchen, Munich, Germany, (4) Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Vienna, Austria


MO-RAM-GePD-JT-4 (Monday, July 31, 2017) 9:30 AM - 10:00 AM Room: Joint Imaging-Therapy ePoster Theater

Purpose: To characterize the robustness of IMPT for prostate cancer against inter-fractional anatomical changes and positioning errors in the presence of a 1.5T magnetic field.

Methods: Five prostate cancer patients with three repeated CTs were included in this study. A Monte-Carlo (MC)-based research treatment planning tool was adapted to enable IMPT optimization in a magnetic field. Single-field uniform dose IMPT plans were generated at 90° gantry angle without B-field ('no B', used as reference), with ±1.5T B-field ('B' and 'minus B') and at 81° gantry angle with +1.5T B-field ('B G81'). Plans were optimized on CT1 and re-calculated for comparison on CT2/3 using Geant4. The impact of inter-fractional motion was characterized by studying changes in clinically relevant target and OAR DVH parameters between CT1 and CT2/3. Additionally, Gamma-index pass-rates (2%, 2mm, 60% dose-cut) and proton range (80% distal dose fall-off) agreement were evaluated. The impact of ±5mm positioning shifts of CT1 on the considered DVH parameter was studied.

Results: With respect to 'no B' a substantially reduced robustness was found for the 'B' scenario due to beam curvature and more pronounced changes in the patient outline closer to the treatment table. Sensitivity to changes in rectum filling was also higher. Considering all patients, median gamma pass-rates decreased from 96% to 92% and percentage of proton ranges within 3mm decreased from 80% to 48%. PTV V95% was significantly reduced on CT2/3 compared to 'no B'. Improved robustness was found for 'minus B' and 'B G81', the latter showing no clinically relevant differences to 'no B'. Dosimetric changes due to positioning errors were not worsened by the B-field.

Conclusion: Robustness equivalent to IMPT without B-field can be achieved after careful adaptation of the treatment parameters, particularly the gantry angle. MRI-guidance might thus be implemented without compromising robustness with respect to CT-guided IMPT.

Funding Support, Disclosures, and Conflict of Interest: This work was supported by the German Cancer Aid (Deutsche Krebshilfe) and the German Research Foundation (DFG) Cluster of Excellence Munich-Centre for Advanced Photonics (MAP). There are no conflicts of interest.

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