Three-Dimensional Dosimetric Validation of a Magnetic Resonance Guided Intensity Modulated Radiation Therapy System.
To validate the dosimetric accuracy of a commercially available magnetic resonance guided intensity modulated radiation therapy (MRgIMRT) system using a hybrid approach: 3-dimensional (3D) measurements and Monte Carlo calculations.



We used PRESAGE radiochromic plastic dosimeters with remote optical computed tomography readout to perform 3D high-resolution measurements, following a novel remote dosimetry protocol. We followed the intensity modulated radiation therapy commissioning recommendations of American Association of Physicists in Medicine Task Group 119, adapted to incorporate 3D data. Preliminary tests ("AP" and "3D-Bands") were delivered to 9.5-cm usable diameter cylindrical PRESAGE dosimeters to validate the treatment planning system (TPS) for nonmodulated deliveries; assess the sensitivity, uniformity, and rotational symmetry of the PRESAGE dosimeters; and test the robustness of the remote dosimetry protocol. Following this, 4 clinical MRgIMRT plans ("MultiTarget," "Prostate," "Head/Neck," and "C-Shape") were measured using 13-cm usable diameter PRESAGE dosimeters. For all plans, 3D-γ (3% or 3 mm global, 10% threshold) passing rates were calculated and 3D-γ maps were examined. Point doses were measured with an IBA-CC01 ionization chamber for validation of absolute dose. Finally, by use of an in-house-developed, GPU-accelerated Monte Carlo algorithm (gPENELOPE), we independently calculated dose for all 6 Task Group 119 plans and compared against the TPS.



For PRESAGE measurements, 3D-γ analysis yielded passing rates of 98.7%, 99.2%, 98.5%, 98.0%, 99.2%, and 90.7% for AP, 3D-Bands, MultiTarget, Prostate, Head/Neck, and C-Shape, respectively. Ion chamber measurements were within an average of 0.5% (±1.1%) from the TPS dose. Monte Carlo calculations demonstrated good agreement with the TPS, with a mean 3D-γ passing rate of 98.5% ± 1.9% using a stricter 2%/2-mm criterion.



We have validated the dosimetric accuracy of a commercial MRgIMRT system using high-resolution 3D techniques. We have demonstrated for the first time that hybrid 3D remote dosimetry is a comprehensive and feasible approach to commissioning MRgIMRT. This may provide better sensitivity in error detection compared with standard 2-dimensional measurements and could be used when implementing complex new magnetic resonance guided radiation therapy technologies.
Department of Radiation Oncology, Washington University School of Medicine, St Louis, Missouri; Department of Radiation Oncology, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina. Electronic address: Leith_Rankine@med.unc.edu.
Leith J
LJ
Rankine
Medical Physics Graduate Program, Duke University, Durham, North Carolina.
Stewart
S
Mein
Department of Radiation Oncology, Washington University School of Medicine, St Louis, Missouri.
Bin
B
Cai
Department of Radiation Oncology, Washington University School of Medicine, St Louis, Missouri.
Austen
A
Curcuru
Medical Physics Graduate Program, Duke University, Durham, North Carolina.
Titania
T
Juang
Medical Physics Graduate Program, Duke University, Durham, North Carolina.
Devin
D
Miles
Department of Radiation Oncology, Washington University School of Medicine, St Louis, Missouri.
Sasa
S
Mutic
Department of Radiation Oncology, Washington University School of Medicine, St Louis, Missouri.
Yuhe
Y
Wang
Medical Physics Graduate Program, Duke University, Durham, North Carolina; Department of Radiation Oncology, Duke University Medical Center, Durham, North Carolina.
Mark
M
Oldham
Department of Radiation Oncology, Washington University School of Medicine, St Louis, Missouri. Electronic address: hli@radonc.wustl.edu.
H Harold
HH
Li
D017076:Computer-Aided Design; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000295:instrumentation* / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000295:instrumentation* / Q000379:methods; D009010:Monte Carlo Method; D009369:Neoplasms / Q000532:radiotherapy*; D011874:Radiometry / Q000295:instrumentation* / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation / Q000379:methods; D020266:Radiotherapy, Conformal / Q000295:instrumentation* / Q000379:methods; D061089:Radiotherapy, Image-Guided / Q000295:instrumentation* / Q000379:methods; D015203:Reproducibility of Results; D012542:Scattering, Radiation; D012680:Sensitivity and Specificity