Three-Dimensional Dose Verification for Intensity-Modulated Radiation Therapy in the Radiological Physics Centre Head-and-Neck Phantom Using Optical Computed Tomography Scans of Ferrous Xylenol-Orange Gel Dosimeters

Purpose: To extend the Radiological Physics Centre (RPC) intensity-modulated radiation therapy dose verification protocol to three dimensions using optical computed tomography (CT) scans of ferrous xylenol-orange (FX) gels. Methods and Materials: The dosimetry insert in the RPC head-and-neck phantom was replaced with an FX cylindrical gel dosimeter. Two gels were calibrated, independently irradiated with 6-MV X-ray beams and scanned using laser and cone-beam (VistaTM) optical CT, respectively. For matching dose slices, measured dose distributions were compared with Pinnacle3 computed distributions. Results: Within high-dose regions and low gradients, doses measured using laser CT were 2% to 3% less than the computed dose, whereas with cone-beam CT they were 4% to 5% less. Inside the central 90% of the gel cylinder diameter, the fraction of voxels satisfying the two-dimensional gamma analysis (5% dose difference, 3-mm distance to agreement) with laser-beam- and cone-beam-measured dose distributions were 98.4% and 99.0%, respectively. A three-dimensional gamma analysis with cone-beam data revealed that 96.7% of voxels within the central 90% gel volume satisfied the above criteria. Within the axial and sagittal planes through the primary planning target volume (PTV), computed and measured doses using GAFChromic EBT film (RPC measured) and cone-beam scanned FX gel generally agreed. At equivalent points in the planning target volumes, computed, thermoluminescent dosimeter (RPC-measured), and gel point doses agreed to within 5.1% in absolute dose. Conclusions: Laser and cone-beam CT yield comparable dose distributions in high-dose regions. The RPC head phantom and optical CT-scanned FX gels can be used for accurate intensity-modulated radiation therapy dose verification in three dimensions