title: Heterogeneity phantoms for visualization of 3D dose distributions by MRI-based polymer gel dosimetry.
abstract: Heterogeneity corrections in dose calculations are necessary for radiation therapy treatment plans. Dosimetric measurements of the heterogeneity effects are hampered if the detectors are large and their radiological characteristics are not equivalent to water. Gel dosimetry can solve these problems. Furthermore, it provides three-dimensional (3D) dose distributions. We used a cylindrical phantom filled with BANG-3 polymer gel to measure 3D dose distributions in heterogeneous media. The phantom has a cavity, in which water-equivalent or bone-like solid blocks can be inserted. The irradiated phantom was scanned with an magnetic resonance imaging (MRI) scanner. Dose distributions were obtained by calibrating the polymer gel for a relationship between the absorbed dose and the spin-spin relaxation rate of the magnetic resistance (MR) signal. To study dose distributions we had to analyze MR imaging artifacts. This was done in three ways: comparison of a measured dose distribution in a simulated homogeneous phantom with a reference dose distribution, comparison of a sagittally scanned image with a sagittal image reconstructed from axially scanned data, and coregistration of MR and computed-tomography images. We found that the MRI artifacts cause a geometrical distortion of less than 2 mm and less than 10% change in the dose around solid inserts. With these limitations in mind we could make some qualitative measurements. Particularly we observed clear differences between the measured dose distributions around an air-gap and around bone-like material for a 6 MV photon beam. In conclusion, the gel dosimetry has the potential to qualitatively characterize the dose distributions near heterogeneities in 3D.
authors.affiliation: Department of Radiation Oncology, College of Physicians and Surgeons, Columbia University, 622 West 168th Street, New York, New York 10032, USA. yw155@columbia.edu
authors.forename: Yoichi
authors.initials: Y
authors.lastname: Watanabe
authors.forename: Rob
authors.initials: R
authors.lastname: Mooij
authors.forename: G Mark
authors.initials: GM
authors.lastname: Perera
authors.forename: Marek J
authors.initials: MJ
authors.lastname: Maryanski
mesh_terms: D000465:Algorithms; D016477:Artifacts; D002138:Calibration; D005240:Feasibility Studies; D005782:Gels / Q000032:analysis* / Q000528:radiation effects*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D021621:Imaging, Three-Dimensional / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D019047:Phantoms, Imaging; D011108:Polymers / Q000032:analysis* / Q000528:radiation effects*; D011829:Radiation Dosage; D011874:Radiometry / Q000295:instrumentation / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
chemical_list: C106521:BANG polymer gel; D005782:Gels; D011108:Polymers