Quality Assurance of Advanced Treatment Modalities Using Presage® Dosimeters
Computer-controlled therapy machines allow for increasingly complex plans, as there are more variables that can be tuned to produce the ideal result. This makes it increasingly difficult to assure the intended calculated dose is being delivered correctly using current techniques that are 2D-based because the resultant dose distributions can differ markedly in various sections of the target. A measurement of composite dose from the entire plan should be included in patient-specific IMRT QA. A volumetric dosimeter such as PRESAGE® is able to provide a complete 3D measured dosimetry dataset with one treatment plan delivery. It was hypothesized that a PRESAGE® dosimeter would agree with 2D measurements within ±5%/3mm using a gamma index analysis. The PRESAGE® dosimeter will detect dose discrepancies not detected with 2D measurements resulting in a 5% change in the normal tissue complication probability (NTCP). An optical CT scanner was tested for reproducibility and reliability and a standard operating procedure was created. The PRESAGE® dosimeters were extensively tested for dose stability over a range of time for remote dosimetry applications. The effect of temperature changes before, during and after irradiation was investigated. The dosimeter was found to be appropriate for remote dosimetry for relative dose measurements. The IROC-Houston Head and Neck (HN) phantom was imaged with an x-ray CT scanner. One scan used an insert for film and thermoluminescent dosimeter (TLD). A second scan was taken using a PRESAGE® insert. An IMRT treatment plan was created and delivered to the phantom using each insert. The gamma index analysis was performed at ±5%/3mm. The PRESAGE® measurements agreed well with the 2D measurements. Various gamma constraints were applied to the measured data to determine an appropriate passing criterion for 3D gamma analysis. The IMRT treatment plan was modified to induce several different types of treatment and delivery errors. The plans were analyzed using 2D and 3D gamma analysis. Two plans passed a 2D metric while failing the 3D metric with one of the plans also having a 5% change in NTCP. The hypothesis was proven correct and further work should be considered to bring PRESAGE® into a phantom dosimetry program.
radiation dosimetry, 3D dosimetry, optical-CT, PRESAGE, quality assurance, Medicine and Health Sciences, Physics
University of Texas Health Science Center at Houston
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