3D dose verification using tomotherapy CT detector array.

PURPOSE: To evaluate a three-dimensional dose verification method based on the exit dose using the onboard detector of tomotherapy. METHODS AND MATERIALS: The study included 347 treatment fractions from 24 patients, including 10 prostate, 5 head and neck (HN), and 9 spinal stereotactic body radiation therapy (SBRT) cases. Detector sonograms were retrieved and back-projected to calculate entrance fluence, which was then forward-projected on the CT images to calculate the verification dose, which was compared with ion chamber and film measurement in the QA plans and with the planning dose in patient plans. RESULTS: Root mean square (RMS) errors of 2.0%, 2.2%, and 2.0% were observed comparing the dose verification (DV) and the ion chamber measured point dose in the phantom plans for HN, prostate, and spinal SBRT patients, respectively. When cumulative dose in the entire treatment is considered, for HN patients, the error of the mean dose to the planning target volume (PTV) varied from 1.47% to 5.62% with a RMS error of 3.55%. For prostate patients, the error of the mean dose to the prostate target volume varied from -5.11% to 3.29%, with a RMS error of 2.49%. The RMS error of maximum doses to the bladder and the rectum were 2.34% (-4.17% to 2.61%) and 2.64% (-4.54% to 3.94%), respectively. For the nine spinal SBRT patients, the RMS error of the minimum dose to the PTV was 2.43% (-5.39% to 2.48%). The RMS error of maximum dose to the spinal cord was 1.05% (-2.86% to 0.89%). CONCLUSIONS: An excellent agreement was observed between the measurement and the verification dose. In the patient treatments, the agreement in doses to the majority of PTVs and organs at risk is within 5% for the cumulative treatment course doses. The dosimetric error strongly depends on the error in multileaf collimator leaf opening time with a sensitivity correlating to the gantry rotation period.

Quality assurance of an image guided intracranial stereotactic positioning system.

This work reports on the development and testing of an intracranial stereotactic patient positioning system (ISPPS) for Tomotherapy. The ISPPS consists of the combination of a head frame, head frame couch interface (HCI), megavoltage CT (MVCT), and optical tracking camera system. Three quality assurance tests were designed to quantify the positioning system's ability to localize an intracranial target. The first two of these tests were designed to determine (a) the ability of the MVCT to detect a known shift applied to an anthropomorphic phantom and (b) the precision of fixing the phantom to the treatment couch via a head frame and specially designed head frame couch interface. A system verification test, using a phantom and EDR2 film, was used to determine the overall delivery precision through comparison of a measured dose distribution on film to calculated dose. The average net translational difference between a known shift applied to a phantom and that detected by MVCT image fusion was 0.62 mm. Setup reproducibility of the head frame was measured with both MVCT and optical tracking. The frame setup precision was found to be well within 1 mm for translations as well as rotations. A system delivery verification test in phantom using film showed spatial agreement between planned and delivered dose distributions to within 1 mm.