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Feasibility Study of MRI-Only Proton Therapy Planning

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M Spadea

M Spadea1*, D Izquierdo2 , C Catana2 , C Collins-Fekete3 , T Bortfeld3 , J Seco3 , (1) ImagEngLab, Magna Graecia University, Catanzaro, Italy (2) Athinoula A. Martinos Center for Biomedical Imaging, Charlestown, MA, (3) Massachusetts General Hospital, Harvard Medical, Boston , MA


SU-E-J-222 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To assess the dosimetric equivalence of MRI based proton planning vs. single energy x-ray CT.

Methods: 8 glioblastoma patients were imaged with CT and MRI after surgical resection. T1-weighted 3D-MPRAGE was used to delineate the GTV, which was subsequently rigidly registered to the CT volume. A pseudoCT was generated from the aligned MRI by combining segmentation and atlas-based approaches. The spatial resolution both for pseudo- and real CT was 0.6x0.6x2.5mm. Three orthogonal proton beams were simulated on the pseudoCT. Two co-planar beams were set on the axial plane. The third one was planned parallel to the cranio-caudal (CC) direction. Each beam was set to cover the GTV at 98% of the nominal dose (18Gy). The proton plan was copied and transferred to the real CT, including aperture/compensator geometry. Dose comparison between pseudoCT and CT plan was performed beam-by-beam by quantifying the range shift of dose profile on each slice of the GTV. The GTV’s V₉₈ was computed for the CT.

Results: For beams in axial plane the median absolute value of the range shift was 0.3mm, with 0.9mm and 1.4mm as 95th percentile and maximum, respectively. Worst scenarios were found for the CC beam, where we measured 1.1mm (median), 2.7mm (95th-percentile) and 5mm (maximum). Regardless the direction, beams passing through the surgical site, where metal (Titanium MRI-compatible) staples were present, were mostly affected by range shift. GTV’s V₉₈ for CT was not lower than 99.3%.

Conclusion: The study showed the clinical feasibility of an MRI-alone proton plan. Advantages include the possibility to rely on better soft tissue contrast for target and organs at risk delineation without the need of further CT scan and image registration. Additional investigation is required in presence of metal implants along the beam path and to account for partial volume effects due to slice thickness

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