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.

Abstract

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. 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 (Vista) optical CT, respectively. For matching dose slices, measured dose distributions were compared with Pinnacle3 computed distributions. 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 GAFChromicEBT 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. 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.